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ref:ymodem [2011/07/13 20:50]
digitalman
ref:ymodem [2011/07/13 22:02] (current)
digitalman
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-====== YMODEM ====== +====== ​XMODEM/YMODEM ​Protocol Reference ​======
-XMODEM/​YMODEM Protocol Reference+
  
 A compendium of documents describing the XMODEM and YMODEM File Transfer Protocols. A compendium of documents describing the XMODEM and YMODEM File Transfer Protocols.
  
-This document was originally edited and formatted (by Chuck Forsberg) 10-27-87. +This document was originally edited and formatted (by [[http://​omen.com|Chuck Forsberg]]) 10-27-87.
- +
-This document ported to Wiki syntax by Rob Swindell July-13-2011. +
- +
- +
- +
-Please distribute as widely as possible. +
-Questions to Chuck Forsberg +
- +
- +
- +
- +
- +
-                               Omen Technology Inc +
-                          The High Reliability Software +
-                            17505-V Sauvie Island Road +
-                              Portland Oregon 97231 +
-                            VOICE: 503-621-3406 :VOICE +
-    Modem (TeleGodzilla):​ 503-621-3746 Speed 19200(Telebit PEP),​2400,​1200,​300 +
-                              CompuServe: 70007,​2304 +
-                                    GEnie: CAF +
-                        UUCP: ...!tektronix!reed!omen!caf+
  
 +This document was formatted for Wiki syntax by [[person:​digital man|Rob Swindell]] on July-13-2011.
  
 ===== Tower of Babel ===== ===== Tower of Babel =====
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 Jeff Garbers (Crosstalk package development director) said it all: "With Jeff Garbers (Crosstalk package development director) said it all: "With
 protocols in the public domain, anyone who wants to dink around with them protocols in the public domain, anyone who wants to dink around with them
-can go ahead." ​[1]+can go ahead." ​((Page C/12, PC-WEEK July 12, 1987))
  
 Documents containing altered examples derived from YMODEM.DOC have added Documents containing altered examples derived from YMODEM.DOC have added
Line 62: Line 41:
 ==== Definitions ==== ==== Definitions ====
  
-    ​ARC     ​ARC is a program that compresses one or more files into an archive +=== ARC === 
-            and extracts files from such archives.+ARC is a program that compresses one or more files into an archive 
 +and extracts files from such archives. 
 + 
 +=== XMODEM ===
  
-    ​XMODEM ​ refers to the file transfer etiquette introduced by Ward +XMODEM refers to the file transfer etiquette introduced by Ward 
-            Christensen'​s 1977 MODEM.ASM program. ​ The name XMODEM comes from +Christensen'​s 1977 MODEM.ASM program. ​ The name XMODEM comes from 
-            Keith Petersen'​s XMODEM.ASM program, an adaptation of MODEM.ASM +Keith Petersen'​s XMODEM.ASM program, an adaptation of MODEM.ASM 
-            for Remote CP/M (RCPM) systems. ​ It's also called the MODEM or +for Remote CP/M (RCPM) systems. ​ It's also called the MODEM or 
-            MODEM2 protocol. ​ Some who are unaware of MODEM7'​s unusual batch +MODEM2 protocol. ​ Some who are unaware of MODEM7'​s unusual batch 
-            file mode call it MODEM7. ​ Other aliases include "CP/M Users'​ +file mode call it MODEM7. ​ Other aliases include "CP/M Users'​ 
-            Group" and "TERM II FTP 3"​. ​ The name XMODEM caught on partly +Group" and "TERM II FTP 3"​. ​ The name XMODEM caught on partly 
-            because it is distinctive and partly because of media interest in +because it is distinctive and partly because of media interest in 
-            bulletin board and RCPM systems where it was accessed with an +bulletin board and RCPM systems where it was accessed with an 
-            "​XMODEM"​ command. ​ This protocol is supported by every serious +"​XMODEM"​ command. ​ This protocol is supported by every serious 
-            communications program because of its universality,​ simplicity,​ +communications program because of its universality,​ simplicity,​ 
-            and reasonable performance.+and reasonable performance.
  
-    ​XMODEM/CRC replaces XMODEM'​s 1 byte checksum with a two byte Cyclical +=== XMODEM/CRC === 
-            Redundancy Check (CRC-16), giving modern error detection +XMODEM/CRC replaces XMODEM'​s 1 byte checksum with a two byte Cyclical 
-            protection.+Redundancy Check (CRC-16), giving modern error detection 
 +protection.
  
-    ​XMODEM-1k ​Refers ​to the XMODEM/CRC protocol with 1024 byte data blocks.+=== XMODEM-1k ​=== 
 +XMODEM-1k refers ​to the XMODEM/CRC protocol with 1024 byte data blocks.
  
-    ​YMODEM ​ ​Refers ​to the XMODEM/CRC (optional 1k blocks) protocol with batch +=== YMODEM ​=== 
-            transmission as described below. ​ In a nutshell, YMODEM means +YMODEM refers ​to the XMODEM/CRC (optional 1k blocks) protocol with batch 
-            BATCH.+transmission as described below. ​ In a nutshell, YMODEM means 
 +BATCH.
  
-    ​YMODEM-g ​Refers ​to the streaming YMODEM variation described below.+=== YMODEM-g ​=== 
 +YMODEM-g refers ​to the streaming YMODEM variation described below.
  
-    ​True YMODEM(TM) In an attempt to sort out the YMODEM Tower of Babel, Omen +=== True YMODEM(TM)=== 
-            Technology has trademarked the term True YMODEM(TM) to represent +In an attempt to sort out the YMODEM Tower of Babel, Omen 
-            the complete YMODEM protocol described in this document, including +Technology has trademarked the term True YMODEM(TM) to represent 
-            pathname, length, and modification date transmitted in block 0. +the complete YMODEM protocol described in this document, including 
-            Please contact Omen Technology about certifying programs for True +pathname, length, and modification date transmitted in block 0. 
-            YMODEM(TM) compliance.+Please contact Omen Technology about certifying programs for True 
 +YMODEM(TM) compliance.
  
-    ​ZMODEM ​ uses familiar XMODEM/CRC and YMODEM technology in a new protocol +=== ZMODEM === 
-            that provides reliability,​ throughput, file management, and user +ZMODEM uses familiar XMODEM/CRC and YMODEM technology in a new protocol 
-            amenities appropriate to contemporary data communications.+that provides reliability,​ throughput, file management, and user 
 +amenities appropriate to contemporary data communications.
  
-    ​ZOO     ​Like ARC, ZOO is a program that compresses one or more files into +=== ZOO === 
-            a "zoo archive"​. ​ ZOO supports many different operating systems +Like ARC, ZOO is a program that compresses one or more files into 
-            including Unix and VMS.+a "zoo archive"​. ​ ZOO supports many different operating systems 
 +including Unix and VMS.
             ​             ​
 ===== YMODEM Minimum Requirements ===== ===== YMODEM Minimum Requirements =====
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 ==== Some Messages from the Pioneer ==== ==== Some Messages from the Pioneer ====
  
 +<​code>​
     #: 130940 S0/​Communications 25-Apr-85 ​ 18:38:47     #: 130940 S0/​Communications 25-Apr-85 ​ 18:38:47
     Sb: my protocol     Sb: my protocol
-    Fm: Ward Christensen 76703,​302 ​((Edited for typesetting appearance))+    Fm: Ward Christensen 76703,302
     To: all     To: all
  
-    Be aware the article((Infoworld April 29 p. 16)) DID quote me correctly in terms of the phrases +    Be aware the article (Infoworld April 29 p. 16) DID quote me correctly 
-    ​like "not robust",​ etc.+    ​in terms of the phrases like "not robust",​ etc.
  
     It was a quick hack I threw together, very unplanned (like everything I     It was a quick hack I threw together, very unplanned (like everything I
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     (2) propose an "​incremental extension"​ to it, which might take "​exactly"​     (2) propose an "​incremental extension"​ to it, which might take "​exactly"​
     the form of Chuck Forsberg'​s YAM protocol. ​ He wrote YAM in C for CP/M and     the form of Chuck Forsberg'​s YAM protocol. ​ He wrote YAM in C for CP/M and
-    put it in the public domain, and wrote a batch protocol for Unix((VAX/VMS versions of these programs are also available)) ​called+    put it in the public domain, and wrote a batch protocol for Unix called
     rb and sb (receive batch, send batch), which was basically XMODEM with     rb and sb (receive batch, send batch), which was basically XMODEM with
        (a) a record 0 containing filename date time and size        (a) a record 0 containing filename date time and size
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     called rb and sb - receive batch and send batch. ​ They cleverly send a     called rb and sb - receive batch and send batch. ​ They cleverly send a
     "block 0" which contains the filename, date, time, and size.     "block 0" which contains the filename, date, time, and size.
-    Unfortunately,​ its UNIX style, and is a bit weird((The file length, time, and file mode are optional. ​ The pathname and file length may be sent alone if desired.)) ​- octal numbers, etc.+    Unfortunately,​ its UNIX style, and is a bit weird - octal numbers, etc.
     BUT, it is a nice way to overcome the kludgy "echo the chars of the name"     BUT, it is a nice way to overcome the kludgy "echo the chars of the name"
     introduced with MODEM7. ​ Further, chuck uses CRC-16 and optional 1K     introduced with MODEM7. ​ Further, chuck uses CRC-16 and optional 1K
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     CIS it detects the "​busy"​ string back from the modem and substitutes a     CIS it detects the "​busy"​ string back from the modem and substitutes a
     diff phone # into the dialing string and branches back to try it.     diff phone # into the dialing string and branches back to try it.
 +</​code>​
  
 ===== XMODEM Protocol Enhancements ===== ===== XMODEM Protocol Enhancements =====
Line 366: Line 357:
 receiver commands CRC-16. receiver commands CRC-16.
  
-              ​Figure 1.  ​XMODEM-1k Blocks+=== Figure 1XMODEM-1k Blocks ​===
  
-              ​SENDER ​                                 RECEIVER +      ​SENDER ​                                 RECEIVER 
-                                                      "s -k foo.bar"​ +                                              "s -k foo.bar"​ 
-              "​foo.bar open x.x minutes"​ +      "​foo.bar open x.x minutes"​ 
-                                                      +                                              
-              STX 01 FE Data[1024] CRC CRC +      STX 01 FE Data[1024] CRC CRC 
-                                                      ACK +                                              ACK 
-              STX 02 FD Data[1024] CRC CRC +      STX 02 FD Data[1024] CRC CRC 
-                                                      ACK +                                              ACK 
-              STX 03 FC Data[1000] CPMEOF[24] CRC CRC +      STX 03 FC Data[1000] CPMEOF[24] CRC CRC 
-                                                      ACK +                                              ACK 
-              EOT +      EOT 
-                                                      ACK+                                              ACK
  
-              ​Figure ​2.  ​Mixed 1024 and 128 byte Blocks+=== Figure ​3: Mixed 1024 and 128 byte Blocks ​===
  
-              ​SENDER ​                                 RECEIVER +      ​SENDER ​                                 RECEIVER 
-                                                      "s -k foo.bar"​ +                                              "s -k foo.bar"​ 
-              "​foo.bar open x.x minutes"​ +      "​foo.bar open x.x minutes"​ 
-                                                      +                                              
-              STX 01 FE Data[1024] CRC CRC +      STX 01 FE Data[1024] CRC CRC 
-                                                      ACK +                                              ACK 
-              STX 02 FD Data[1024] CRC CRC +      STX 02 FD Data[1024] CRC CRC 
-                                                      ACK +                                              ACK 
-              SOH 03 FC Data[128] CRC CRC +      SOH 03 FC Data[128] CRC CRC 
-                                                      ACK +                                              ACK 
-              SOH 04 FB Data[100] CPMEOF[28] CRC CRC +      SOH 04 FB Data[100] CPMEOF[28] CRC CRC 
-                                                      ACK +                                              ACK 
-              EOT +      EOT 
-                                                      ACK+                                              ACK
  
 ===== YMODEM Batch File Transmission ===== ===== YMODEM Batch File Transmission =====
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 Only the pathname (file name) part is required for batch transfers. Only the pathname (file name) part is required for batch transfers.
  
-    ​To maintain upwards compatibility,​ all unused bytes in block 0 must be set +To maintain upwards compatibility,​ all unused bytes in block 0 must be set to null.
-    ​to null.+
  
-    ​Pathname The pathname (conventionally,​ the file name) is sent as a null +=== Pathname ​=== 
-         ​terminated ASCII string. ​ This is the filename format used by the +The pathname (conventionally,​ the file name) is sent as a null 
-         ​handle oriented MSDOS(TM) functions and C library fopen functions. +terminated ASCII string. ​ This is the filename format used by the 
-         ​An assembly language example follows: +handle oriented MSDOS(TM) functions and C library fopen functions. 
-                                  DB      '​foo.bar',​0 +An assembly language example follows: 
-         ​No spaces are included in the pathname. ​ Normally only the file name +    DB      '​foo.bar',​0 
-         ​stem (no directory prefix) is transmitted unless the sender has +No spaces are included in the pathname. ​ Normally only the file name 
-         ​selected YAM's f option to send the full pathname. ​ The source drive +stem (no directory prefix) is transmitted unless the sender has 
-         ​(A:, B:, etc.) is not sent. +selected YAM's f option to send the full pathname. ​ The source drive 
- +(A:, B:, etc.) is not sent.
-Filename Considerations:​+
  
 +== Filename Considerations ==
  
   * File names are forced to lower case unless the sending system supports upper/lower case file names. ​ This is a convenience for       users of systems (such as Unix) which store filenames in upper and lower case.   * File names are forced to lower case unless the sending system supports upper/lower case file names. ​ This is a convenience for       users of systems (such as Unix) which store filenames in upper and lower case.
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   *  If directories are included, they are delimited by /; i.e., "​subdir/​foo"​ is acceptable, "​subdir\foo"​ is not.   *  If directories are included, they are delimited by /; i.e., "​subdir/​foo"​ is acceptable, "​subdir\foo"​ is not.
  
-==== Length ​====+=== Length ===
 The file length and each of the succeeding fields are optional.((Fields may not be skipped.)) The file length and each of the succeeding fields are optional.((Fields may not be skipped.))
 The length field is stored in the block as a decimal string counting The length field is stored in the block as a decimal string counting
Line 461: Line 451:
 any padding added by the sender to fill up the last block. any padding added by the sender to fill up the last block.
  
-==== Modification Date ====+=== Modification Date ===
 The mod date is optional, and the filename and length The mod date is optional, and the filename and length
 may be sent without requiring the mod date to be sent. may be sent without requiring the mod date to be sent.
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-==== Mode ====+=== Mode ===
 If the file mode is sent, a single space separates the file mode If the file mode is sent, a single space separates the file mode
 from the modification date.  The file mode is stored as an octal from the modification date.  The file mode is stored as an octal
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-==== Serial Number ​====+=== Serial Number ===
 If the serial number is sent, a single space separates the If the serial number is sent, a single space separates the
 serial number from the file mode.  The serial number of the serial number from the file mode.  The serial number of the
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-==== Other Fields ​====+=== Other Fields ===
 YMODEM was designed to allow additional header fields to be YMODEM was designed to allow additional header fields to be
 added as above without creating compatibility problems with older added as above without creating compatibility problems with older
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 RZSZ.ZOO should answer other questions about YMODEM batch protocol. RZSZ.ZOO should answer other questions about YMODEM batch protocol.
  
-              ​Figure 3.  ​YMODEM Batch Transmission Session+=== Figure 3YMODEM Batch Transmission Session ​===
  
-              ​SENDER ​                                 RECEIVER +      ​SENDER ​                                 RECEIVER 
-                                                      "sb foo.*<​CR>"​ +                                              "sb foo.*<​CR>"​ 
-              "​sending in batch mode etc."​ +      "​sending in batch mode etc."​ 
-                                                      C (command:​rb) +                                              C (command:​rb) 
-              SOH 00 FF foo.c NUL[123] CRC CRC +      SOH 00 FF foo.c NUL[123] CRC CRC 
-                                                      ACK +                                              ACK 
-                                                      +                                              
-              SOH 01 FE Data[128] CRC CRC +      SOH 01 FE Data[128] CRC CRC 
-                                                      ACK +                                              ACK 
-              SOH 03 FC Data[128] CRC CRC +      SOH 03 FC Data[128] CRC CRC 
-                                                      ACK +                                              ACK 
-              SOH 04 FB Data[100] CPMEOF[28] CRC CRC +      SOH 04 FB Data[100] CPMEOF[28] CRC CRC 
-                                                      ACK +                                              ACK 
-              EOT +      EOT 
-                                                      NAK +                                              NAK 
-              EOT +      EOT 
-                                                      ACK +                                              ACK 
-                                                      +                                              
-              SOH 00 FF NUL[128] CRC CRC +      SOH 00 FF NUL[128] CRC CRC 
-                                                      ACK+                                              ACK
  
-            ​Figure 4.  ​YMODEM Batch Transmission Session-1k Blocks+=== Figure 4YMODEM Batch Transmission Session-1k Blocks ​===
  
-            ​SENDER ​                                 RECEIVER +      ​SENDER ​                                 RECEIVER 
-                                                    "sb -k foo.*<​CR>"​ +                                              "sb -k foo.*<​CR>"​ 
-            "​sending in batch mode etc."​ +      "​sending in batch mode etc."​ 
-                                                    C (command:​rb) +                                              C (command:​rb) 
-            SOH 00 FF foo.c NUL[123] CRC CRC +      SOH 00 FF foo.c NUL[123] CRC CRC 
-                                                    ACK +                                              ACK 
-                                                    +                                              
-            STX 02 FD Data[1024] CRC CRC +      STX 02 FD Data[1024] CRC CRC 
-                                                    ACK +                                              ACK 
-            SOH 03 FC Data[128] CRC CRC +      SOH 03 FC Data[128] CRC CRC 
-                                                    ACK +                                              ACK 
-            SOH 04 FB Data[100] CPMEOF[28] CRC CRC +      SOH 04 FB Data[100] CPMEOF[28] CRC CRC 
-                                                    ACK +                                              ACK 
-            EOT +      EOT 
-                                                    NAK +                                              NAK 
-            EOT +      EOT 
-                                                    ACK +                                              ACK 
-                                                    +                                              
-            SOH 00 FF NUL[128] CRC CRC +      SOH 00 FF NUL[128] CRC CRC 
-                                                    ACK+                                              ACK
  
  
  
-           Figure 5.  ​YMODEM Filename block transmitted by sz+=== Figure 5YMODEM Filename block transmitted by sz === 
 +<​code>​ 
 +      -rw-r--r-- ​ 6347 Jun 17 1984 20:34 bbcsched.txt
  
-           ​-rw-r--r-- ​ 6347 Jun 17 1984 20:34 bbcsched.txt +      ​00 0100FF62 62637363 6865642E 74787400 ​  ​|...bbcsched.txt.| 
- +      10 36333437 20333331 34373432 35313320 ​  |6347 3314742513 | 
-           00 0100FF62 62637363 6865642E 74787400 ​  ​|...bbcsched.txt.| +      20 31303036 34340000 00000000 00000000 ​  ​|100644..........| 
-           ​10 36333437 20333331 34373432 35313320 ​  |6347 3314742513 | +      30 00000000 00000000 00000000 00000000 
-           ​20 31303036 34340000 00000000 00000000 ​  ​|100644..........| +      40 00000000 00000000 00000000 00000000 
-           ​30 00000000 00000000 00000000 00000000 +      50 00000000 00000000 00000000 00000000 
-           ​40 00000000 00000000 00000000 00000000 +      60 00000000 00000000 00000000 00000000 
-           ​50 00000000 00000000 00000000 00000000 +      70 00000000 00000000 00000000 00000000 
-           ​60 00000000 00000000 00000000 00000000 +      80 000000CA 56 
-           ​70 00000000 00000000 00000000 00000000 +</​code>​ 
-           ​80 000000CA 56 +=== Figure 6YMODEM Header Information and Features ​====
- +
-                Figure 6.  ​YMODEM Header Information and Features+
  
     _____________________________________________________________     _____________________________________________________________
Line 606: Line 596:
     |___________|________|______|______|_____|________|__________|     |___________|________|______|______|_____|________|__________|
  
-=== KMD/IMP Exceptions to YMODEM ===+==== KMD/IMP Exceptions to YMODEM ​====
  
-    ​KMD and IMP use a "​CK"​ character sequence emitted by the receiver to +KMD and IMP use a "​CK"​ character sequence emitted by the receiver to 
-    trigger the use of 1024 byte blocks as an alternative to specifying this +trigger the use of 1024 byte blocks as an alternative to specifying this 
-    option to the sending program. ​ Although this two character sequence works +option to the sending program. ​ Although this two character sequence works 
-    well on single process micros in direct communication,​ timesharing systems +well on single process micros in direct communication,​ timesharing systems 
-    and packet switched networks can separate the successive characters by +and packet switched networks can separate the successive characters by 
-    several seconds, rendering this method unreliable.+several seconds, rendering this method unreliable.
  
-    ​Sending programs may detect the CK sequence if the operating enviornment +Sending programs may detect the CK sequence if the operating enviornment 
-    does not preclude reliable implementation.+does not preclude reliable implementation.
  
-    ​Instead of the standard YMODEM file length, KMD and IMP transmit the CP/M +Instead of the standard YMODEM file length, KMD and IMP transmit the CP/M 
-    record count in the last two bytes of the header block.+record count in the last two bytes of the header block.
  
 +===== YMODEM-g File Transmission =====
  
 +Developing technology is providing phone line data transmission at ever
 +higher speeds using very specialized techniques. ​ These high speed modems,
 +as well as session protocols such as X.PC, provide high speed, nearly
 +error free communications at the expense of considerably increased delay
 +time.
  
 +This delay time is moderate compared to human interactions,​ but it
 +cripples the throughput of most error correcting protocols.
  
 +The g option to YMODEM has proven effective under these circumstances.
 +The g option is driven by the receiver, which initiates the batch transfer
 +by transmitting a G instead of C.  When the sender recognizes the G, it
 +bypasses the usual wait for an ACK to each transmitted block, sending
 +succeeding blocks at full speed, subject to XOFF/XON or other flow control
 +exerted by the medium.
  
 +The sender expects an inital G to initiate the transmission of a
 +particular file, and also expects an ACK on the EOT sent at the end of
 +each file.  This synchronization allows the receiver time to open and
 +close files as necessary.
  
 +If an error is detected in a YMODEM-g transfer, the receiver aborts the
 +transfer with the multiple CAN abort sequence. ​ The ZMODEM protocol should
 +be used in applications that require both streaming throughput and error
 +recovery.
  
 +=== Figure 7: YMODEM-g Transmission Session ====
  
 +      SENDER ​                                 RECEIVER
 +                                              "sb foo.*<​CR>"​
 +      "​sending in batch mode etc..."​
 +                                              G (command:rb -g)
 +      SOH 00 FF foo.c NUL[123] CRC CRC
 +                                              G
 +      SOH 01 FE Data[128] CRC CRC
 +      STX 02 FD Data[1024] CRC CRC
 +      SOH 03 FC Data[128] CRC CRC
 +      SOH 04 FB Data[100] CPMEOF[28] CRC CRC
 +      EOT
 +                                              ACK
 +                                              G
 +      SOH 00 FF NUL[128] CRC CRC
  
  
-    Chapter 6                                     XMODEM Protocol ​Enhancements+===== XMODEM Protocol ​Overview =====
  
 +8/9/82 by Ward Christensen.
  
 +I will maintain a master copy of this.  Please pass on changes or
 +suggestions via CBBS/​Chicago at (312) 545-8086, CBBS/CPMUG (312) 849-1132
 +or by voice at (312) 849-6279.
  
 +==== Definitions ====
  
 +  <soh> 01H
 +  <eot> 04H
 +  <ack> 06H
 +  <nak> 15H
 +  <can> 18H
 +  <​C> ​  43H
  
  
 +==== Transmission Medium Level Protocol ====
  
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              17+Asynchronous,​ 8 data bits, no parity, one stop bit.
  
 +The protocol imposes no restrictions on the contents of the data being
 +transmitted. ​ No control characters are looked for in the 128-byte data
 +messages. ​ Absolutely any kind of data may be sent - binary, ASCII, etc.
 +The protocol has not formally been adopted to a 7-bit environment for the
 +transmission of ASCII-only (or unpacked-hex) data , although it could be
 +simply by having both ends agree to AND the protocol-dependent data with
 +7F hex before validating it.  I specifically am referring to the checksum,
 +and the block numbers and their ones- complement.
  
 +Those wishing to maintain compatibility of the CP/M file structure, i.e.
 +to allow modemming ASCII files to or from CP/M systems should follow this
 +data format:
  
-    6.  YMODEM-g File Transmission +  ​* ASCII tabs used (09H); tabs set every 8. 
- +  ​* ​Lines terminated by CR/LF (0DH 0AH) 
-    Developing technology is providing phone line data transmission at ever +  ​* ​End-of-file indicated by ^Z, 1AH.  (one or more) 
-    higher speeds using very specialized techniques. ​ These high speed modems, +  ​* ​Data is variable length, i.e. should be considered a continuous stream of data bytes, broken into 128-byte chunks purely for the      purpose of transmission. 
-    as well as session protocols such as X.PC, provide high speed, nearly +  ​* ​A CP/M "​peculiarity":​ If the data ends exactly on a 128-byte boundary, i.e. CR in 127, and LF in 128, a subsequent sector ​        ​containing the ^Z EOF character(s) is optional, but is preferred. Some utilities or user programs still do not handle EOF without ^Zs. 
-    error free communications at the expense of considerably increased delay +  ​* ​The last block sent is no different from others, i.e.  there is no "short block"​. 
-    time. +  
- +=== Figure 8XMODEM Message Block Level Protocol ​===
-    This delay time is moderate compared to human interactions,​ but it +
-    cripples the throughput of most error correcting protocols. +
- +
-    The g option to YMODEM has proven effective under these circumstances. +
-    The g option is driven by the receiver, which initiates the batch transfer +
-    by transmitting a G instead of C.  When the sender recognizes the G, it +
-    bypasses the usual wait for an ACK to each transmitted block, sending +
-    succeeding blocks at full speed, subject to XOFF/XON or other flow control +
-    exerted by the medium. +
- +
-    The sender expects an inital G to initiate the transmission of a +
-    particular file, and also expects an ACK on the EOT sent at the end of +
-    each file.  This synchronization allows the receiver time to open and +
-    close files as necessary. +
- +
-    If an error is detected in a YMODEM-g transfer, the receiver aborts the +
-    transfer with the multiple CAN abort sequence. ​ The ZMODEM protocol should +
-    be used in applications that require both streaming throughput and error +
-    recovery. +
- +
-            Figure 7.  YMODEM-g Transmission Session +
- +
-            SENDER ​                                 RECEIVER +
-                                                    "sb foo.*<​CR>"​ +
-            "​sending in batch mode etc..."​ +
-                                                    G (command:rb -g) +
-            SOH 00 FF foo.c NUL[123] CRC CRC +
-                                                    G +
-            SOH 01 FE Data[128] CRC CRC +
-            STX 02 FD Data[1024] CRC CRC +
-            SOH 03 FC Data[128] CRC CRC +
-            SOH 04 FB Data[100] CPMEOF[28] CRC CRC +
-            EOT +
-                                                    ACK +
-                                                    G +
-            SOH 00 FF NUL[128] CRC CRC +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
-    Chapter 6                                     ​XMODEM Protocol Enhancements +
- +
- +
- +
- +
- +
- +
- +
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              18 +
- +
- +
- +
-    7.  XMODEM PROTOCOL OVERVIEW +
- +
-    8/9/82 by Ward Christensen. +
- +
-    I will maintain a master copy of this.  Please pass on changes or +
-    suggestions via CBBS/​Chicago at (312) 545-8086, CBBS/CPMUG (312) 849-1132 +
-    or by voice at (312) 849-6279. +
- +
-    7.1  Definitions +
- +
-      <soh> 01H +
-      <eot> 04H +
-      <ack> 06H +
-      <nak> 15H +
-      <can> 18H +
-      <​C> ​  43H +
- +
- +
-    7.2  Transmission Medium Level Protocol +
- +
-    Asynchronous,​ 8 data bits, no parity, one stop bit. +
- +
-    The protocol imposes no restrictions on the contents of the data being +
-    transmitted. ​ No control characters are looked for in the 128-byte data +
-    messages. ​ Absolutely any kind of data may be sent - binary, ASCII, etc. +
-    The protocol has not formally been adopted to a 7-bit environment for the +
-    transmission of ASCII-only (or unpacked-hex) data , although it could be +
-    simply by having both ends agree to AND the protocol-dependent data with +
-    7F hex before validating it.  I specifically am referring to the checksum, +
-    and the block numbers and their ones- complement. +
- +
-    Those wishing to maintain compatibility of the CP/M file structure, i.e. +
-    to allow modemming ASCII files to or from CP/M systems should follow this +
-    data format: +
- +
-       ​+ ​ASCII tabs used (09H); tabs set every 8. +
- +
-       ​+ ​Lines terminated by CR/LF (0DH 0AH) +
- +
-       ​+ ​End-of-file indicated by ^Z, 1AH.  (one or more) +
- +
-       ​+ ​Data is variable length, i.e. should be considered a continuous +
-         stream of data bytes, broken into 128-byte chunks purely for the +
-         purpose of transmission. +
- +
-       ​+ ​A CP/M "​peculiarity":​ If the data ends exactly on a 128-byte +
-         boundary, i.e. CR in 127, and LF in 128, a subsequent sector +
-         containing the ^Z EOF character(s) is optional, but is preferred. +
-         Some utilities or user programs still do not handle EOF without ^Zs. +
- +
- +
- +
- +
- +
-    Chapter 7                                         ​Xmodem Protocol Overview +
- +
- +
- +
- +
- +
- +
- +
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              19 +
- +
- +
- +
-       ​+ ​The last block sent is no different from others, i.e.  there is no +
-         "short block"​. +
-                  Figure 8.  ​XMODEM Message Block Level Protocol+
  
     Each block of the transfer looks like:     Each block of the transfer looks like:
Line 789: Line 712:
     <​cksum> ​      = the sum of the data bytes only.  Toss any carry.     <​cksum> ​      = the sum of the data bytes only.  Toss any carry.
  
-    7.3  ​File Level Protocol +==== File Level Protocol====
- +
-    7.3.1  Common_to_Both_Sender_and_Receiver +
-    All errors are retried 10 times. ​ For versions running with an operator +
-    (i.e. NOT with XMODEM), a message is typed after 10 errors asking the +
-    operator whether to "retry or quit"​. +
- +
-    Some versions of the protocol use <​can>,​ ASCII ^X, to cancel transmission. +
-    This was never adopted as a standard, as having a single "​abort"​ character +
-    makes the transmission susceptible to false termination due to an <​ack>​ +
-    <nak> or <soh> being corrupted into a <can> and aborting transmission. +
- +
-    The protocol may be considered "​receiver driven",​ that is, the sender need +
-    not automatically re-transmit,​ although it does in the current +
-    implementations. +
- +
- +
-    7.3.2  Receive_Program_Considerations +
-    The receiver has a 10-second timeout. ​ It sends a <nak> every time it +
-    times out.  The receiver'​s first timeout, which sends a <​nak>,​ signals the +
-    transmitter to start. ​ Optionally, the receiver could send a <​nak>​ +
-    immediately,​ in case the sender was ready. ​ This would save the initial 10 +
-    second timeout. ​ However, the receiver MUST continue to timeout every 10 +
-    seconds in case the sender wasn't ready. +
- +
-    Once into a receiving a block, the receiver goes into a one-second timeout +
-    for each character and the checksum. ​ If the receiver wishes to <nak> a +
-    block for any reason (invalid header, timeout receiving data), it must +
-    wait for the line to clear. ​ See "​programming tips" for ideas +
- +
-    Synchronizing: ​ If a valid block number is received, it will be: 1) the +
-    expected one, in which case everything is fine; or 2) a repeat of the +
-    previously received block. ​ This should be considered OK, and only +
-    indicates that the receivers <ack> got glitched, and the sender re- +
-    transmitted;​ 3) any other block number indicates a fatal loss of +
-    synchronization,​ such as the rare case of the sender getting a line-glitch +
- +
- +
- +
-    Chapter 7                                         ​Xmodem Protocol Overview +
- +
- +
  
 +=== Common_to_Both_Sender_and_Receiver ===
 +All errors are retried 10 times. ​ For versions running with an operator
 +(i.e. NOT with XMODEM), a message is typed after 10 errors asking the
 +operator whether to "retry or quit".
  
 +Some versions of the protocol use <​can>,​ ASCII ^X, to cancel transmission.
 +This was never adopted as a standard, as having a single "​abort"​ character
 +makes the transmission susceptible to false termination due to an <ack>
 +<nak> or <soh> being corrupted into a <can> and aborting transmission.
  
 +The protocol may be considered "​receiver driven",​ that is, the sender need
 +not automatically re-transmit,​ although it does in the current
 +implementations.
  
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              20 
  
 +=== Receive_Program_Considerations ===
 +The receiver has a 10-second timeout. ​ It sends a <nak> every time it
 +times out.  The receiver'​s first timeout, which sends a <​nak>,​ signals the
 +transmitter to start. ​ Optionally, the receiver could send a <nak>
 +immediately,​ in case the sender was ready. ​ This would save the initial 10
 +second timeout. ​ However, the receiver MUST continue to timeout every 10
 +seconds in case the sender wasn't ready.
  
 +Once into a receiving a block, the receiver goes into a one-second timeout
 +for each character and the checksum. ​ If the receiver wishes to <nak> a
 +block for any reason (invalid header, timeout receiving data), it must
 +wait for the line to clear. ​ See "​programming tips" for ideas
  
-    ​that looked like an <​ack>​. ​ Abort the transmission,​ sending a <can>+== Synchronizing == 
 +If a valid block number is received, it will be: 1) the 
 +expected one, in which case everything is fine; or 2) a repeat of the 
 +previously received block. ​ This should be considered OK, and only 
 +indicates that the receivers <ack> got glitched, and the sender re- 
 +transmitted;​ 3) any other block number indicates a fatal loss of 
 +synchronization,​ such as the rare case of the sender getting a line-glitch 
 +that looked like an <​ack>​. ​ Abort the transmission,​ sending a <can>
  
  
-    7.3.3  ​Sending_program_considerations +=== Sending_program_considerations ​=== 
-    While waiting for transmission to begin, the sender has only a single very +While waiting for transmission to begin, the sender has only a single very 
-    long timeout, say one minute. ​ In the current protocol, the sender has a +long timeout, say one minute. ​ In the current protocol, the sender has a 
-    10 second timeout before retrying. ​ I suggest NOT doing this, and letting +10 second timeout before retrying. ​ I suggest NOT doing this, and letting 
-    the protocol be completely receiver-driven. ​ This will be compatible with +the protocol be completely receiver-driven. ​ This will be compatible with 
-    existing programs.+existing programs.
  
-    ​When the sender has no more data, it sends an <​eot>,​ and awaits an <​ack>,​ +When the sender has no more data, it sends an <​eot>,​ and awaits an <​ack>,​ 
-    resending the <eot> if it doesn'​t get one.  Again, the protocol could be +resending the <eot> if it doesn'​t get one.  Again, the protocol could be 
-    receiver-driven,​ with the sender only having the high-level 1-minute +receiver-driven,​ with the sender only having the high-level 1-minute 
-    timeout to abort.+timeout to abort.
  
  
-    ​Here is a sample of the data flow, sending a 3-block message. ​ It includes +Here is a sample of the data flow, sending a 3-block message. ​ It includes 
-    the two most common line hits - a garbaged block, and an <ack> reply +the two most common line hits - a garbaged block, and an <ack> reply 
-    getting garbaged. ​ <xx> represents the checksum byte.+getting garbaged. ​ <xx> represents the checksum byte.
  
-                  ​Figure 9.  ​Data flow including Error Recovery+=== Figure 9Data flow including Error Recovery ​===
  
     SENDER ​                                 RECEIVER     SENDER ​                                 RECEIVER
Line 880: Line 789:
     (finished)     (finished)
  
-    7.4  ​Programming Tips+==== Programming Tips ====
  
-       ​+ ​The character-receive subroutine should be called with a parameter +1. The character-receive subroutine should be called with a parameter specifying the number of seconds to wait.  The receiver should first call it with a time of 10, then <nak> and try again, 10 times.
-         specifying the number of seconds to wait.  The receiver should first +
-         call it with a time of 10, then <nak> and try again, 10 times.+
  
-         After receiving the <​soh>,​ the receiver should call the character +After receiving the <​soh>,​ the receiver should call the character 
-         ​receive subroutine with a 1-second timeout, for the remainder of the +receive subroutine with a 1-second timeout, for the remainder of the 
-         ​message and the <​cksum>​. ​ Since they are sent as a continuous stream, +message and the <​cksum>​. ​ Since they are sent as a continuous stream, 
-         ​timing out of this implies a serious like glitch that caused, say, +timing out of this implies a serious like glitch that caused, say, 
-         ​127 characters to be seen instead of 128.+127 characters to be seen instead of 128.
  
 +2. When the receiver wishes to <​nak>,​ it should call a "​PURGE"​ subroutine, to wait for the line to clear. ​ Recall the sender tosses ​  any characters in its UART buffer immediately upon completing sending a block, to ensure no glitches were mis- interpreted.
  
 +The most common technique is for "​PURGE"​ to call the character
 +receive subroutine, specifying a 1-second timeout,​((These times should be adjusted for use with timesharing systems.
 +)) and looping
 +back to PURGE until a timeout occurs. ​ The <nak> is then sent,
 +ensuring the other end will see it.
  
-    Chapter 7                                         ​Xmodem Protocol Overview+3. You may wish to add code recommended by John Mahr to your character receive routine - to set an error flag if the UART shows framing ​        ​error,​ or overrun. ​ This will help catch a few more glitches - the         most common of which is a hit in the high bits of the byte in two         ​consecutive bytes. ​ The <​cksum>​ comes out OK since counting in 1-byte ​        ​produces the same result of adding 80H + 80H as with adding 00H + 00H.
  
 +===== XMODEM/CRC Overview =====
  
 +Original 1/13/85 by John Byrns -- CRC option.
  
 +Please pass on any reports of errors in this document or suggestions for
 +improvement to me via Ward'​s/​CBBS at (312) 849-1132, or by voice at (312) 885-1105.
  
 +The CRC used in the Modem Protocol is an alternate form of block check
 +which provides more robust error detection than the original checksum.
 +Andrew S. Tanenbaum says in his book, Computer Networks, that the CRC-
 +CCITT used by the Modem Protocol will detect all single and double bit
 +errors, all errors with an odd number of bits, all burst errors of length
 +16 or less, 99.997% of 17-bit error bursts, and 99.998% of 18-bit and
 +longer bursts.((This reliability figure is misleading because XMODEM'​s critical supervisory functions are not protected by this CRC.))
  
  
 +The changes to the Modem Protocol to replace the checksum with the CRC are
 +straight forward. If that were all that we did we would not be able to
 +communicate between a program using the old checksum protocol and one
 +using the new CRC protocol. An initial handshake was added to solve this
 +problem. The handshake allows a receiving program with CRC capability to
 +determine whether the sending program supports the CRC option, and to
 +switch it to CRC mode if it does. This handshake is designed so that it
 +will work properly with programs which implement only the original
 +protocol. A description of this handshake is presented in section 10.
  
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              21 +=== Figure 10Message Block Level Protocol, CRC mode ===
- +
- +
- +
-       + When the receiver wishes to <​nak>,​ it should call a "​PURGE"​ +
-         ​subroutine,​ to wait for the line to clear. ​ Recall the sender tosses +
-         any characters in its UART buffer immediately upon completing sending +
-         a block, to ensure no glitches were mis- interpreted. +
- +
-         The most common technique is for "​PURGE"​ to call the character +
-         ​receive subroutine, specifying a 1-second timeout,[1] and looping +
-         back to PURGE until a timeout occurs. ​ The <nak> is then sent, +
-         ​ensuring the other end will see it. +
- +
-       + You may wish to add code recommended by John Mahr to your character +
-         ​receive routine - to set an error flag if the UART shows framing +
-         ​error,​ or overrun. ​ This will help catch a few more glitches - the +
-         most common of which is a hit in the high bits of the byte in two +
-         ​consecutive bytes. ​ The <​cksum>​ comes out OK since counting in 1-byte +
-         ​produces the same result of adding 80H + 80H as with adding 00H + +
-         ​00H. +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
-    __________ +
- +
-     1. These times should be adjusted for use with timesharing systems. +
- +
- +
- +
- +
-    Chapter 7                                         ​Xmodem Protocol Overview +
- +
- +
- +
- +
- +
- +
- +
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              22 +
- +
- +
- +
-    8.  XMODEM/CRC Overview +
- +
-    Original 1/13/85 by John Byrns -- CRC option. +
- +
-    Please pass on any reports of errors in this document or suggestions for +
-    improvement to me via Ward'​s/​CBBS at (312) 849-1132, or by voice at (312) +
-    885-1105. +
- +
-    The CRC used in the Modem Protocol is an alternate form of block check +
-    which provides more robust error detection than the original checksum. +
-    Andrew S. Tanenbaum says in his book, Computer Networks, that the CRC- +
-    CCITT used by the Modem Protocol will detect all single and double bit +
-    errors, all errors with an odd number of bits, all burst errors of length +
-    16 or less, 99.997% of 17-bit error bursts, and 99.998% of 18-bit and +
-    longer bursts.[1] +
- +
-    The changes to the Modem Protocol to replace the checksum with the CRC are +
-    straight forward. If that were all that we did we would not be able to +
-    communicate between a program using the old checksum protocol and one +
-    using the new CRC protocol. An initial handshake was added to solve this +
-    problem. The handshake allows a receiving program with CRC capability to +
-    determine whether the sending program supports the CRC option, and to +
-    switch it to CRC mode if it does. This handshake is designed so that it +
-    will work properly with programs which implement only the original +
-    protocol. A description of this handshake is presented in section 10. +
- +
-                ​Figure 10.  ​Message Block Level Protocol, CRC mode+
  
     Each block of the transfer in CRC mode looks like:     Each block of the transfer in CRC mode looks like:
Line 1010: Line 847:
     <CRC lo> ​    = byte containing the 8 lo order coefficients of the CRC.     <CRC lo> ​    = byte containing the 8 lo order coefficients of the CRC.
  
-    8.1  ​CRC Calculation+==== CRC Calculation ​====
  
-    8.1.1  ​Formal_Definition +=== Formal_Definition ​=== 
-    To calculate the 16 bit CRC the message bits are considered to be the +To calculate the 16 bit CRC the message bits are considered to be the 
-    coefficients of a polynomial. This message polynomial is first multiplied +coefficients of a polynomial. This message polynomial is first multiplied 
-    by X^16 and then divided by the generator polynomial (X^16 + X^12 + X^5 ++by X^16 and then divided by the generator polynomial (X^16 + X^12 + X^5 + 
 +1) using modulo two arithmetic. The remainder left after the division is 
 +the desired CRC. Since a message block in the Modem Protocol is 128 bytes 
 +or 1024 bits, the message polynomial will be of order X^1023. The hi order 
 +bit of the first byte of the message block is the coefficient of X^1023 in 
 +the message polynomial. ​ The lo order bit of the last byte of the message 
 +block is the coefficient of X^0 in the message polynomial.
  
 +===Figure 11: Example of CRC Calculation written in C ===
  
-    __________ +The following XMODEM crc routine is taken from "​rbsb.c"​. ​ Please refer to 
- +the source code for these programs (contained in RZSZ.ZOO) for usage. ​ A 
-     1. This reliability figure is misleading because XMODEM'​s critical +fast table driven version is also included in this file.
-        supervisory functions are not protected by this CRC. +
- +
- +
- +
- +
-    Chapter 8                                         ​Xmodem Protocol Overview +
- +
- +
- +
- +
- +
- +
- +
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              23 +
- +
- +
- +
-    1) using modulo two arithmetic. The remainder left after the division is +
-    the desired CRC. Since a message block in the Modem Protocol is 128 bytes +
-    or 1024 bits, the message polynomial will be of order X^1023. The hi order +
-    bit of the first byte of the message block is the coefficient of X^1023 in +
-    the message polynomial. ​ The lo order bit of the last byte of the message +
-    block is the coefficient of X^0 in the message polynomial. +
- +
-               ​Figure 11.  Example of CRC Calculation written in C +
- +
-    ​The following XMODEM crc routine is taken from "​rbsb.c"​. ​ Please refer to +
-    the source code for these programs (contained in RZSZ.ZOO) for usage. ​ A +
-    fast table driven version is also included in this file.+
  
 +<​code>​
     /* update CRC */     /* update CRC */
     unsigned short     unsigned short
Line 1072: Line 888:
             return crc;             return crc;
     }     }
 +</​code>​
 +==== CRC File Level Protocol Changes ====
  
-    8.2  CRC File Level Protocol Changes +=== Common_to_Both_Sender_and_Receiver ​=== 
- +The only change to the File Level Protocol for the CRC option is the 
-    8.2.1  ​Common_to_Both_Sender_and_Receiver +initial handshake which is used to determine if both the sending and the 
-    The only change to the File Level Protocol for the CRC option is the +receiving programs support the CRC mode. All Modem Programs should support 
-    initial handshake which is used to determine if both the sending and the +the checksum mode for compatibility with older versions. ​ A receiving 
-    receiving programs support the CRC mode. All Modem Programs should support +program that wishes to receive in CRC mode implements the mode setting 
-    the checksum mode for compatibility with older versions. ​ A receiving +handshake by sending a <C> in place of the initial <​nak>​. ​ If the sending 
-    program that wishes to receive in CRC mode implements the mode setting +program supports CRC mode it will recognize the <C> and will set itself 
-    handshake by sending a <C> in place of the initial <​nak>​. ​ If the sending +into CRC mode, and respond by sending the first block as if a <nak> had 
-    program supports CRC mode it will recognize the <C> and will set itself +been received. If the sending program does not support CRC mode it will 
-    into CRC mode, and respond by sending the first block as if a <nak> had +not respond to the <C> at all. After the receiver has sent the <C> it will 
-    been received. If the sending program does not support CRC mode it will +wait up to 3 seconds for the <soh> that starts the first block. If it 
-    not respond to the <C> at all. After the receiver has sent the <C> it will +receives a <soh> within 3 seconds it will assume the sender supports CRC 
-    wait up to 3 seconds for the <soh> that starts the first block. If it +mode and will proceed with the file exchange in CRC mode. If no <soh> is 
-    receives a <soh> within 3 seconds it will assume the sender supports CRC +received within 3 seconds the receiver will switch to checksum mode, send 
-    mode and will proceed with the file exchange in CRC mode. If no <soh> is +a <​nak>,​ and proceed in checksum mode. If the receiver wishes to use 
- +checksum mode it should send an initial <nak> and the sending program 
- +should respond to the <nak> as defined in the original Modem Protocol. 
- +After the mode has been set by the initial <C> or <nak> the protocol 
-    Chapter 8                                         ​Xmodem Protocol Overview +follows the original Modem Protocol and is identical whether the checksum 
- +or CRC is being used.
- +
- +
- +
- +
- +
- +
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              24 +
- +
- +
- +
-    ​received within 3 seconds the receiver will switch to checksum mode, send +
-    a <​nak>,​ and proceed in checksum mode. If the receiver wishes to use +
-    checksum mode it should send an initial <nak> and the sending program +
-    should respond to the <nak> as defined in the original Modem Protocol. +
-    After the mode has been set by the initial <C> or <nak> the protocol +
-    follows the original Modem Protocol and is identical whether the checksum +
-    or CRC is being used. +
- +
- +
-    8.2.2  Receive_Program_Considerations +
-    There are at least 4 things that can go wrong with the mode setting +
-    handshake. +
- +
-      1.  the initial <C> can be garbled or lost. +
- +
-      2.  the initial <soh> can be garbled. +
- +
-      3.  the initial <C> can be changed to a <​nak>​. +
- +
-      4.  the initial <nak> from a receiver which wants to receive in checksum +
-          can be changed to a <​C>​. +
- +
-    The first problem can be solved if the receiver sends a second <C> after +
-    it times out the first time. This process can be repeated several times. +
-    It must not be repeated too many times before sending a <nak> and +
-    switching to checksum mode or a sending program without CRC support may +
-    time out and abort. Repeating the <C> will also fix the second problem if +
-    the sending program cooperates by responding as if a <nak> were received +
-    instead of ignoring the extra <​C>​. +
- +
-    It is possible to fix problems 3 and 4 but probably not worth the trouble +
-    since they will occur very infrequently. They could be fixed by switching +
-    modes in either the sending or the receiving program after a large number +
-    of successive <​nak>​s. This solution would risk other problems however. +
- +
- +
-    8.2.3  Sending_Program_Considerations +
-    The sending program should start in the checksum mode. This will insure +
-    compatibility with checksum only receiving programs. Anytime a <C> is +
-    received before the first <nak> or <ack> the sending program should set +
-    itself into CRC mode and respond as if a <nak> were received. The sender +
-    should respond to additional <C>s as if they were <​nak>​s until the first +
-    <ack> is received. This will assist the receiving program in determining +
-    the correct mode when the <soh> is lost or garbled. After the first <​ack>​ +
-    is received the sending program should ignore <​C>​s. +
- +
- +
- +
- +
- +
- +
- +
- +
- +
-    Chapter 8                                         ​Xmodem Protocol Overview +
- +
  
  
 +=== Receive_Program_Considerations ===
 +There are at least 4 things that can go wrong with the mode setting
 +handshake.
  
 +  -  the initial <C> can be garbled or lost.
 +  -  the initial <soh> can be garbled.
 +  -  the initial <C> can be changed to a <​nak>​.
 +  -  the initial <nak> from a receiver which wants to receive in checksum ​     can be changed to a <C>.
  
 +The first problem can be solved if the receiver sends a second <C> after
 +it times out the first time. This process can be repeated several times.
 +It must not be repeated too many times before sending a <nak> and
 +switching to checksum mode or a sending program without CRC support may
 +time out and abort. Repeating the <C> will also fix the second problem if
 +the sending program cooperates by responding as if a <nak> were received
 +instead of ignoring the extra <C>.
  
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              25+It is possible to fix problems 3 and 4 but probably not worth the trouble 
 +since they will occur very infrequently. They could be fixed by switching 
 +modes in either the sending or the receiving program after a large number 
 +of successive <​nak>​s. This solution would risk other problems however.
  
  
 +=== Sending_Program_Considerations ===
 +The sending program should start in the checksum mode. This will insure
 +compatibility with checksum only receiving programs. Anytime a <C> is
 +received before the first <nak> or <ack> the sending program should set
 +itself into CRC mode and respond as if a <nak> were received. The sender
 +should respond to additional <C>s as if they were <​nak>​s until the first
 +<ack> is received. This will assist the receiving program in determining
 +the correct mode when the <soh> is lost or garbled. After the first <ack>
 +is received the sending program should ignore <C>s.
  
-    8.3  Data Flow Examples with CRC Option 
  
-    ​Here is a data flow example for the case where the receiver requests +==== Data Flow Examples with CRC Option ==== 
-    transmission in the CRC mode but the sender does not support the CRC +Here is a data flow example for the case where the receiver requests 
-    option. This example also includes various transmission errors. ​ <​xx>​ +transmission in the CRC mode but the sender does not support the CRC 
-    represents the checksum byte.+option. This example also includes various transmission errors. ​ <​xx>​ 
 +represents the checksum byte.
  
-          ​Figure 12.  ​Data Flow: Receiver has CRC Option, Sender Doesn'​t+=== Figure 12Data Flow: Receiver has CRC Option, Sender Doesn'​t ​===
  
     SENDER ​                                       RECEIVER     SENDER ​                                       RECEIVER
Line 1204: Line 981:
                             <​--- ​               <ack>                             <​--- ​               <ack>
  
-    ​Here is a data flow example for the case where the receiver requests +Here is a data flow example for the case where the receiver requests 
-    transmission in the CRC mode and the sender supports the CRC option. ​ This +transmission in the CRC mode and the sender supports the CRC option. ​ This 
-    example also includes various transmission errors. ​ <​xxxx>​ represents the +example also includes various transmission errors. ​ <​xxxx>​ represents the 
-    2 CRC bytes.+2 CRC bytes.
  
- +=== Figure 13Receiver and Sender Both have CRC Option ​===
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
-    Chapter 8                                         ​Xmodem Protocol Overview +
- +
- +
- +
- +
- +
- +
- +
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              26 +
- +
- +
- +
-               Figure 13.  ​Receiver and Sender Both have CRC Option+
  
     SENDER ​                                      ​RECEIVER     SENDER ​                                      ​RECEIVER
Line 1256: Line 1006:
  
  
 +===== More Information =====
  
 +Please contact Omen Technology for troff source files and typeset copies
 +of this document.
  
  
 +==== TeleGodzilla Bulletin Board ====
 +More information may be obtained by calling TeleGodzilla at 503-621-3746.
 +Speed detection is automatic for 1200, 2400 and 19200(Telebit PEP) bps.
 +TrailBlazer modem users may issue the TeleGodzilla trailblazer command to
 +swith to 19200 bps once they have logged in.
  
 +Interesting files include RZSZ.ZOO (C source code), YZMODEM.ZOO (Official
 +XMODEM, YMODEM, and ZMODEM protocol descriptions),​ ZCOMMEXE.ARC,​
 +ZCOMMDOC.ARC,​ and ZCOMMHLP.ARC (PC-DOS shareware comm program with XMODEM,
 +True YMODEM(TM), ZMODEM, Kermit Sliding Windows, Telink, MODEM7 Batch,
 +script language, etc.).
  
  
 +==== Unix UUCP Access ====
  
 +UUCP sites can obtain the current version of this file with
 +    uucp omen!/​u/​caf/​public/​ymodem.doc /tmp
 +A continually updated list of available files is stored in
 +    /​usr/​spool/​uucppublic/​FILES.
 +When retrieving these files with uucp,
 +remember that the destination directory on your system must be writeable
 +by anyone, or the UUCP transfer will fail.
  
 +The following L.sys line calls TeleGodzilla (Pro-YAM in host operation).
 +TeleGodzilla determines the incoming speed automatically.
  
 +In response to "Name Please:"​ uucico gives the Pro-YAM "​link"​ command as a
 +user name.  The password (Giznoid) controls access to the Xenix system
 +connected to the IBM PC's other serial port.  Communications between
 +Pro-YAM and Xenix use 9600 bps; YAM converts this to the caller'​s speed.
  
- +Finally, the calling uucico logs in as uucp.
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
- +
-    Chapter 8                                         ​Xmodem Protocol Overview +
- +
- +
- +
- +
- +
- +
- +
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              27 +
- +
- +
- +
-    9.  MORE INFORMATION +
- +
-    Please contact Omen Technology for troff source files and typeset copies +
-    of this document. +
- +
- +
-    9.1  TeleGodzilla Bulletin Board +
- +
-    More information may be obtained by calling TeleGodzilla at 503-621-3746. +
-    Speed detection is automatic for 1200, 2400 and 19200(Telebit PEP) bps. +
-    TrailBlazer modem users may issue the TeleGodzilla trailblazer command to +
-    swith to 19200 bps once they have logged in. +
- +
-    Interesting files include RZSZ.ZOO (C source code), YZMODEM.ZOO (Official +
-    XMODEM, YMODEM, and ZMODEM protocol descriptions),​ ZCOMMEXE.ARC,​ +
-    ZCOMMDOC.ARC,​ and ZCOMMHLP.ARC (PC-DOS shareware comm program with XMODEM, +
-    True YMODEM(TM), ZMODEM, Kermit Sliding Windows, Telink, MODEM7 Batch, +
-    script language, etc.). +
- +
- +
-    9.2  Unix UUCP Access +
- +
-    UUCP sites can obtain the current version of this file with +
-                     uucp omen!/​u/​caf/​public/​ymodem.doc /tmp +
-    A continually updated list of available files is stored in +
-    /​usr/​spool/​uucppublic/​FILES. ​ When retrieving these files with uucp, +
-    remember that the destination directory on your system must be writeable +
-    by anyone, or the UUCP transfer will fail. +
- +
-    The following L.sys line calls TeleGodzilla (Pro-YAM in host operation). +
-    TeleGodzilla determines the incoming speed automatically. +
- +
-    In response to "Name Please:"​ uucico gives the Pro-YAM "​link"​ command as a +
-    user name.  The password (Giznoid) controls access to the Xenix system +
-    connected to the IBM PC's other serial port.  Communications between +
-    Pro-YAM and Xenix use 9600 bps; YAM converts this to the caller'​s speed. +
- +
-    ​Finally, the calling uucico logs in as uucp. +
     omen Any ACU 2400 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp     omen Any ACU 2400 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp
  
  
  
-    10.  REVISIONS+===== Revisions =====
  
-    ​10-27-87 Optional fields added for number of files remaining to be sent +=== 10-27-87 ​=== 
-    ​and total number of bytes remaining to be sent. +Optional fields added for number of files remaining to be sent and total number of bytes remaining to be sent. 
-    10-18-87 Flow control discussion added to 1024 byte block descritpion,​ +=== 10-18-87 ​=== 
-    ​minor revisions for clarity per user comments. +Flow control discussion added to 1024 byte block descritpion,​ minor revisions for clarity per user comments. 
-    8-03-87 Revised for clarity. +=== 8-03-87 ​=== 
-    5-31-1987 emphasizes minimum requirements for YMODEM, and updates+Revised for clarity. 
 +=== 5-31-1987 ​=== 
 +emphasizes minimum requirements for YMODEM, and updates ​information on accessing files. 
 +=== 9-11-1986 === 
 +clarifies nomenclature and some minor points. 
 +=== 4-15-1986 === 
 +clarifies some points concerning CRC calculations and spaces in the header.
  
  
 +===== YMODEM Programs =====
  
-    Chapter 10                                        Xmodem Protocol Overview 
  
 +ZCOMM, A shareware little brother to Professional-YAM,​ is available as
 +ZCOMMEXE.ARC on TeleGodzilla and other bulletin board systems. ​ ZCOMM may
 +be used to test YMODEM amd ZMODEM implementations.
  
 +Unix programs supporting YMODEM are available on TeleGodzilla in RZSZ.ZOO.
 +This ZOO archive includes a ZCOMM/​Pro-YAM/​PowerCom script ZUPL.T to upload
 +a bootstrap program MINIRB.C, compile it, and then upload the rest of the
 +files using the compiled MINIRB. ​ Most Unix like systems are supported,
 +including V7, Xenix, Sys III, 4.2 BSD, SYS V, Idris, Coherent, and
 +Regulus.
  
 +A version for VAX-VMS is available in VRBSB.SHQ.
  
 +Irv Hoff has added 1k blocks and basic YMODEM batch transfers to the KMD
 +and IMP series programs, which replace the XMODEM and MODEM7/​MDM7xx series
 +respectively. ​ Overlays are available for a wide variety of CP/M systems.
  
- +Questions about Professional-YAM communications software may be directed to:
- +
-    X/YMODEM Protocol Reference ​     10-27-87 ​                              28 +
- +
- +
- +
-    information on accessing files. +
-    9-11-1986 clarifies nomenclature and some minor points. +
-    The April 15 1986 edition clarifies some points concerning CRC +
-    calculations and spaces in the header. +
- +
- +
-    11.  YMODEM Programs +
- +
-    ZCOMM, A shareware little brother to Professional-YAM,​ is available as +
-    ZCOMMEXE.ARC on TeleGodzilla and other bulletin board systems. ​ ZCOMM may +
-    be used to test YMODEM amd ZMODEM implementations. +
- +
-    Unix programs supporting YMODEM are available on TeleGodzilla in RZSZ.ZOO. +
-    This ZOO archive includes a ZCOMM/​Pro-YAM/​PowerCom script ZUPL.T to upload +
-    a bootstrap program MINIRB.C, compile it, and then upload the rest of the +
-    files using the compiled MINIRB. ​ Most Unix like systems are supported,​ +
-    including V7, Xenix, Sys III, 4.2 BSD, SYS V, Idris, Coherent, and +
-    Regulus. +
- +
-    A version for VAX-VMS is available in VRBSB.SHQ. +
- +
-    Irv Hoff has added 1k blocks and basic YMODEM batch transfers to the KMD +
-    and IMP series programs, which replace the XMODEM and MODEM7/​MDM7xx series +
-    respectively. ​ Overlays are available for a wide variety of CP/M systems. +
- +
-    ​Questions about Professional-YAM communications software may be directed +
-    ​to:+
          Chuck Forsberg          Chuck Forsberg
          Omen Technology Inc          Omen Technology Inc
Line 1405: Line 1095:
          ​GEnie:​ CAF          ​GEnie:​ CAF
  
-    ​Unlike ZMODEM and Kermit, XMODEM and YMODEM place obstacles in the path of +Unlike ZMODEM and Kermit, XMODEM and YMODEM place obstacles in the path of 
-    a reliable high performance implementation,​ evidenced by poor reliability +a reliable high performance implementation,​ evidenced by poor reliability 
-    under stress of the industry leaders'​ XMODEM and YMODEM programs. ​ Omen +under stress of the industry leaders'​ XMODEM and YMODEM programs. ​ Omen 
-    Technology provides consulting and other services to those wishing to +Technology provides consulting and other services to those wishing to 
-    implement XMODEM, YMODEM, and ZMODEM with state of the art features and +implement XMODEM, YMODEM, and ZMODEM with state of the art features and 
-    reliability. +reliability.
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-    Chapter 11                                        Xmodem Protocol Overview +
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-                                     ​CONTENTS +
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-     ​1. ​ TOWER OF BABEL................................................... ​  2 +
-         ​1.1 ​ Definitions................................................. ​  2 +
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-     ​2. ​ YMODEM MINIMUM REQUIREMENTS...................................... ​  4 +
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-     ​3. ​ WHY YMODEM?​...................................................... ​  5 +
-         ​3.1 ​ Some Messages from the Pioneer.............................. ​  6 +
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-     ​4. ​ XMODEM PROTOCOL ENHANCEMENTS..................................... ​  9 +
-         ​4.1 ​ Graceful Abort.............................................. ​  9 +
-         ​4.2 ​ CRC-16 Option............................................... ​  9 +
-         ​4.3 ​ XMODEM-1k 1024 Byte Block................................... ​ 10 +
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-     ​5. ​ YMODEM Batch File Transmission................................... ​ 12 +
-         ​5.1 ​ KMD/IMP Exceptions to YMODEM................................ ​ 16 +
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-     ​6. ​ YMODEM-g File Transmission....................................... ​ 17 +
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-     ​7. ​ XMODEM PROTOCOL OVERVIEW......................................... ​ 18 +
-         ​7.1 ​ Definitions................................................. ​ 18 +
-         ​7.2 ​ Transmission Medium Level Protocol.......................... ​ 18 +
-         ​7.3 ​ File Level Protocol......................................... ​ 19 +
-         ​7.4 ​ Programming Tips............................................ ​ 20 +
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-     ​8. ​ XMODEM/CRC Overview.............................................. ​ 22 +
-         ​8.1 ​ CRC Calculation............................................. ​ 22 +
-         ​8.2 ​ CRC File Level Protocol Changes............................. ​ 23 +
-         ​8.3 ​ Data Flow Examples with CRC Option.......................... ​ 25 +
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-     ​9. ​ MORE INFORMATION................................................. ​ 27 +
-         ​9.1 ​ TeleGodzilla Bulletin Board................................. ​ 27 +
-         ​9.2 ​ Unix UUCP Access............................................ ​ 27 +
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-    10.  REVISIONS........................................................ ​ 27 +
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-    11.  YMODEM Programs.................................................. ​ 28 +
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-                                      - i - +
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-                                 LIST OF FIGURES +
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-     ​Figure 1.  XMODEM-1k Blocks.......................................... ​ 11 +
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-     ​Figure 2.  Mixed 1024 and 128 byte Blocks............................ ​ 11 +
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-     ​Figure 3.  YMODEM Batch Transmission Session......................... ​ 15 +
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-     ​Figure 4.  YMODEM Batch Transmission Session-1k Blocks............... ​ 15 +
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-     ​Figure 5.  YMODEM Filename block transmitted by sz................... ​ 16 +
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-     ​Figure 6.  YMODEM Header Information and Features.................... ​ 16 +
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-     ​Figure 7.  YMODEM-g Transmission Session............................. ​ 17 +
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-     ​Figure 8.  XMODEM Message Block Level Protocol....................... ​ 19 +
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-     ​Figure 9.  Data flow including Error Recovery........................ ​ 20 +
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-    Figure 10.  Message Block Level Protocol, CRC mode.................... ​ 22 +
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-    Figure 11.  Example of CRC Calculation written in C................... ​ 23 +
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-    Figure 12.  Data Flow: Receiver has CRC Option, Sender Doesn'​t........ ​ 25 +
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-    Figure 13.  Receiver and Sender Both have CRC Option.................. ​ 26 +
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-                                      - ii - +
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 ===== See Also ===== ===== See Also =====
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