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ref:ymodem [2010/03/04 17:16]
digitalman created
ref:ymodem [2011/07/13 22:02] (current)
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-====== YMODEM ====== +====== ​XMODEM/YMODEM ​Protocol Reference ​====== 
-FIXME import ​ymodem.doc+ 
 +A compendium of documents describing the XMODEM and YMODEM File Transfer Protocols. 
 + 
 +This document was originally edited and formatted (by [[http://​omen.com|Chuck Forsberg]]) 10-27-87. 
 + 
 +This document was formatted for Wiki syntax by [[person:​digital man|Rob Swindell]] on July-13-2011. 
 + 
 +===== Tower of Babel ===== 
 + 
 +A "​YMODEM Tower of Babel" has descended on the microcomputing community 
 +bringing with it confusion, frustration,​ bloated phone bills, and wasted 
 +man hours. ​ Sadly, I (Chuck Forsberg) am partly to blame for this mess. 
 + 
 +As author of the early 1980s batch and 1k XMODEM extensions, I assumed 
 +readers of earlier versions of this document would implement as much of 
 +the YMODEM protocol as their programming skills and computing environments 
 +would permit. ​ This proved a rather naive assumption as programmers 
 +motivated by competitive pressure implemented as little of YMODEM as 
 +possible. ​ Some have taken whatever parts of YMODEM that appealed to them, 
 +applied them to MODEM7 Batch, Telink, XMODEM or whatever, and called the 
 +result YMODEM. 
 + 
 +Jeff Garbers (Crosstalk package development director) said it all: "​With 
 +protocols in the public domain, anyone who wants to dink around with them 
 +can go ahead."​ ((Page C/12, PC-WEEK July 12, 1987)) 
 + 
 +Documents containing altered examples derived from YMODEM.DOC have added 
 +to the confusion. ​ In one instance, the heading in YMODEM.DOC'​s Figure 1 
 +has mutated from "1024 byte Packets"​ to "​YMODEM/​CRC File Transfer 
 +Protocol"​. ​ None of the XMODEM and YMODEM examples shown in that document 
 +were correct. 
 + 
 +To put an end to this confusion, we must make "​perfectly clear" what 
 +YMODEM stands for, as Ward Christensen defined it in his 1985 coining of 
 +the term. 
 + 
 +To the majority of you who read, understood, and respected Ward'​s 
 +definition of YMODEM, I apologize for the inconvenience. 
 + 
 +==== Definitions ==== 
 + 
 +=== ARC === 
 +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 
 +Christensen'​s 1977 MODEM.ASM program. ​ The name XMODEM comes from 
 +Keith Petersen'​s XMODEM.ASM program, an adaptation of MODEM.ASM 
 +for Remote CP/M (RCPM) systems. ​ It's also called the MODEM or 
 +MODEM2 protocol. ​ Some who are unaware of MODEM7'​s unusual batch 
 +file mode call it MODEM7. ​ Other aliases include "CP/M Users'​ 
 +Group" and "TERM II FTP 3"​. ​ The name XMODEM caught on partly 
 +because it is distinctive and partly because of media interest in 
 +bulletin board and RCPM systems where it was accessed with an 
 +"​XMODEM"​ command. ​ This protocol is supported by every serious 
 +communications program because of its universality,​ simplicity,​ 
 +and reasonable performance. 
 + 
 +=== XMODEM/CRC === 
 +XMODEM/CRC replaces XMODEM'​s 1 byte checksum with a two byte Cyclical 
 +Redundancy Check (CRC-16), giving modern error detection 
 +protection. 
 + 
 +=== XMODEM-1k === 
 +XMODEM-1k refers to the XMODEM/CRC protocol with 1024 byte data blocks. 
 + 
 +=== YMODEM === 
 +YMODEM refers to the XMODEM/CRC (optional 1k blocks) protocol with batch 
 +transmission as described below. ​ In a nutshell, YMODEM means 
 +BATCH. 
 + 
 +=== 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 
 +Technology has trademarked the term True YMODEM(TM) to represent 
 +the complete YMODEM protocol described in this document, including 
 +pathname, length, and modification date transmitted in block 0. 
 +Please contact Omen Technology about certifying programs for True 
 +YMODEM(TM) compliance. 
 + 
 +=== ZMODEM === 
 +ZMODEM uses familiar XMODEM/CRC and YMODEM technology in a new protocol 
 +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 
 +a "zoo archive"​. ​ ZOO supports many different operating systems 
 +including Unix and VMS. 
 +             
 +===== YMODEM Minimum Requirements ===== 
 + 
 +All programs claiming to support YMODEM must meet the following minimum requirements:​ 
 + 
 +  * The sending program shall send the pathname (file name) in block 0. 
 + 
 +  * The pathname shall be a null terminated ASCII string as described below. 
 + 
 +  * The receiving program shall use this pathname for the received file name, unless explicitly overridden. 
 + 
 +  * The sending program shall use CRC-16 in response to a "​C"​ pathname nak, otherwise use 8 bit checksum. 
 + 
 +  * The receiving program must accept any mixture of 128 and 1024 byte blocks within each file it receives. ​ Sending programs may switch ​ between 1024 and 128 byte blocks at the end of file(s), and when the frequency of retransmissions so suggests. 
 + 
 +  * The sending program must not change the length of an unacknowledged block. 
 + 
 +  * At the end of each file, the sending program shall send EOT up to ten times until it receives an ACK character. ​ (This is part of the  XMODEM spec.) 
 + 
 +  * The end of a transfer session shall be signified by a null (empty) pathname. 
 + 
 +Programs not meeting all of these requirements are not YMODEM compatible, and shall not be described as supporting YMODEM. 
 + 
 +Meeting these MINIMUM requirements does not guarantee reliable file transfers under stress. ​ Particular attention is called to XMODEM'​s single character supervisory messages that are easily corrupted by transmission errors. 
 + 
 + 
 +===== Why YMODEM? ===== 
 + 
 + 
 +Since its development half a decade ago, the Ward Christensen modem 
 +protocol has enabled a wide variety of computer systems to interchange 
 +data.  There is hardly a communications program that doesn'​t at least 
 +claim to support this protocol. 
 + 
 +Advances in computing, modems and networking have revealed a number of 
 +weaknesses in the original protocol: 
 + 
 +  * The short block length caused throughput to suffer when used with timesharing systems, packet switched networks, satellite circuits, ​ and buffered (error correcting) modems. 
 + 
 +  * The 8 bit arithmetic checksum and other aspects allowed line impairments to interfere with dependable, accurate transfers. 
 + 
 +  * Only one file could be sent per command. ​ The file name had to be given twice, first to the sending program and then again to the   ​receiving program. 
 + 
 +  * The transmitted file could accumulate as many as 127 extraneous bytes. 
 + 
 +  * The modification date of the file was lost. 
 + 
 +A number of other protocols have been developed over the years, but none 
 +have displaced XMODEM to date: 
 + 
 +  * Lack of public domain documentation and example programs have kept proprietary protocols such as Blast, Relay, and others tightly bound to the fortunes of their suppliers. 
 + 
 +  * Complexity discourages the widespread application of BISYNC, SDLC, HDLC, X.25, and X.PC protocols. 
 + 
 +  * Performance compromises and complexity have limited the popularity of the Kermit protocol, which was developed to allow file transfers in environments hostile to XMODEM. 
 + 
 +The XMODEM protocol extensions and YMODEM Batch address some of these 
 +weaknesses while maintaining most of XMODEM'​s simplicity. 
 + 
 +YMODEM is supported by the public domain programs YAM (CP/M), YAM(CP/​M-86),​ YAM(CCPM-86),​ IMP (CP/M), KMD (CP/M), rz/sz (Unix, Xenix, ​   VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus). ​ Commercial implementations include MIRROR, and Professional-YAM.((Available for IBM PC,XT,AT, Unix and Xenix))  
 + 
 +Communications programs supporting these extensions have been in use since 1981. 
 + 
 +The 1k block length (XMODEM-1k) described below may be used in conjunction 
 +with YMODEM Batch Protocol, or with single file transfers identical to the 
 +XMODEM/CRC protocol except for minimal changes to support 1k blocks. 
 + 
 +Another extension is the YMODEM-g protocol. ​ YMODEM-g provides batch 
 +transfers with maximum throughput when used with end to end error 
 +correcting media, such as X.PC and error correcting modems, including 9600 
 +bps units by TeleBit, U.S.Robotics,​ Hayes, Electronic Vaults, Data Race, 
 +and others. 
 + 
 +To complete this tome, edited versions of Ward Christensen'​s original 
 +protocol document and John Byrns'​s CRC-16 document are included for 
 +reference. 
 + 
 +References to the MODEM or MODEM7 protocol have been changed to XMODEM to 
 +accommodate the vernacular. ​ In Australia, it is properly called the 
 +Christensen Protocol. 
 + 
 +==== Some Messages from the Pioneer ==== 
 + 
 +<​code>​ 
 +    #: 130940 S0/​Communications 25-Apr-85 ​ 18:38:47 
 +    Sb: my protocol 
 +    Fm: Ward Christensen 76703,302 
 +    To: all 
 + 
 +    Be aware the article (Infoworld April 29 p. 16) DID quote me correctly 
 +    in terms of the phrases like "not robust",​ etc. 
 + 
 +    It was a quick hack I threw together, very unplanned (like everything I 
 +    do), to satisfy a personal need to communicate with "some other" people. 
 + 
 +    ONLY the fact that it was done in 8/77, and that I put it in the public 
 +    domain immediately,​ made it become the standard that it is. 
 + 
 +    I think its time for me to 
 + 
 +    (1) document it; (people call me and say "my product is going to include 
 +    it - what can I '​reference'",​ or "​I'​m writing a paper on it, what do I put 
 +    in the bibliography"​) and 
 + 
 +    (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 
 +    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 
 +       (a) a record 0 containing filename date time and size 
 +       (b) a 1K block size option 
 +       (c) CRC-16. 
 + 
 +    He did some clever programming to detect false ACK or EOT, but basically 
 +    left them the same. 
 + 
 +    People who suggest I make SIGNIFICANT changes to the protocol, such as 
 +    "full duplex",​ "​multiple outstanding blocks",​ "​multiple destinations",​ etc 
 +    etc don't understand that the incredible simplicity of the protocol is one 
 +    of the reasons it survived to this day in as many machines and programs as 
 +    it may be found in! 
 + 
 +    Consider the PC-NET group back in '77 or so - documenting to beat the band 
 +    - THEY had a protocol, but it was "​extremely complex",​ because it tried to 
 +    be "all things to all people"​ - i.e. send binary files on a 7-bit system, 
 +    etc.  I was not that "​benevolent"​. I (emphasize > I < ) had an 8-bit UART, 
 +    so "my protocol was an 8-bit protocol",​ and I would just say "​sorry"​ to 
 +    people who were held back by 7-bit limitations. ​ ... 
 + 
 +    Block size: Chuck Forsberg created an extension of my protocol, called 
 +    YAM, which is also supported via his public domain programs for UNIX 
 +    called rb and sb - receive batch and send batch. ​ They cleverly send a 
 +    "block 0" which contains the filename, date, time, and size. 
 +    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"​ 
 +    introduced with MODEM7. ​ Further, chuck uses CRC-16 and optional 1K 
 +    blocks. ​ Thus the record 0, 1K, and CRC, make it a "​pretty slick new 
 +    protocol"​ which is not significantly different from my own. 
 + 
 +    Also, there is a catchy name - YMODEM. ​ That means to some that it is the 
 +    "next thing after XMODEM",​ and to others that it is the Y(am)MODEM 
 +    protocol. ​ I don't want to emphasize that too much - out of fear that 
 +    other mfgrs might think it is a "​competitive"​ protocol, rather than an 
 +    "​unaffiliated"​ protocol. ​ Chuck is currently selling a much-enhanced 
 +    version of his CP/M-80 C program YAM, calling it Professional Yam, and its 
 +    for the PC - I'm using it right now.  VERY slick! ​ 32K capture buffer, 
 +    script, scrolling, previously captured text search, plus built-in commands 
 +    for just about everything - directory (sorted every which way), XMODEM, 
 +    YMODEM, KERMIT, and ASCII file upload/​download,​ etc.  You can program it 
 +    to "​behave"​ with most any system - for example when trying a number for 
 +    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. 
 +</​code>​ 
 + 
 +===== XMODEM Protocol Enhancements ===== 
 + 
 +This chapter discusses the protocol extensions to Ward Christensen'​s 1982 
 +XMODEM protocol description document. 
 + 
 +The original document recommends the user be asked whether to continue 
 +trying or abort after 10 retries. ​ Most programs no longer ask the 
 +operator whether he wishes to keep retrying. ​ Virtually all correctable 
 +errors are corrected within the first few retransmissions. ​ If the line is 
 +so bad that ten attempts are insufficient,​ there is a significant danger 
 +of undetected errors. ​ If the connection is that bad, it's better to 
 +redial for a better connection, or mail a floppy disk. 
 + 
 + 
 +==== Graceful Abort ==== 
 + 
 +The YAM and Professional-YAM X/YMODEM routines recognize a sequence of two 
 +consecutive CAN (Hex 18) characters without modem errors (overrun, 
 +framing, etc.) as a transfer abort command. ​ This sequence is recognized 
 +when is waiting for the beginning of a block or for an acknowledgement to 
 +a block that has been sent.  The check for two consecutive CAN characters 
 +reduces the number of transfers aborted by line hits.  YAM sends eight CAN 
 +characters when it aborts an XMODEM, YMODEM, or ZMODEM protocol file 
 +transfer. ​ Pro-YAM then sends eight backspaces to delete the CAN 
 +characters from the remote'​s keyboard input buffer, in case the remote had 
 +already aborted the transfer and was awaiting a keyboarded command. 
 + 
 + 
 +==== CRC-16 Option ==== 
 + 
 +The XMODEM protocol uses an optional two character CRC-16 instead of the 
 +one character arithmetic checksum used by the original protocol and by 
 +most commercial implementations. ​ CRC-16 guarantees detection of all 
 +single and double bit errors, ​ all errors with an odd number of error 
 +bits, all burst errors of length 16 or less, 99.9969% of all 17-bit error 
 +bursts, and 99.9984 per cent of all possible longer error bursts. ​ By 
 +contrast, a double bit error, or a burst error of 9 bits or more can sneak 
 +past the XMODEM protocol arithmetic checksum. 
 + 
 +The XMODEM/CRC protocol is similar to the XMODEM protocol, except that the 
 +receiver specifies CRC-16 by sending C (Hex 43) instead of NAK when 
 +requesting the FIRST block. ​ A two byte CRC is sent in place of the one 
 +byte arithmetic checksum. 
 + 
 +YAM's c option to the r command enables CRC-16 in single file reception,​ 
 +corresponding to the original implementation in the MODEM7 series 
 +programs. ​ This remains the default because many commercial communications 
 +programs and bulletin board systems still do not support CRC-16, 
 +especially those written in Basic or Pascal. 
 + 
 +XMODEM protocol with CRC is accurate provided both sender and receiver 
 +both report a successful transmission. ​ The protocol is robust in the 
 +presence of characters lost by buffer overloading on timesharing systems. 
 + 
 +The single character ACK/NAK responses generated by the receiving program 
 +adapt well to split speed modems, where the reverse channel is limited to 
 +ten per cent or less of the main channel'​s speed. 
 + 
 +XMODEM and YMODEM are half duplex protocols which do not attempt to 
 +transmit information and control signals in both directions at the same 
 +time.  This avoids buffer overrun problems that have been reported by 
 +users attempting to exploit full duplex asynchronous file transfer 
 +protocols such as Blast. 
 + 
 +Professional-YAM adds several proprietary logic enhancements to XMODEM'​s 
 +error detection and recovery. ​ These compatible enhancements eliminate 
 +most of the bad file transfers other programs make when using the XMODEM 
 +protocol under less than ideal conditions. 
 + 
 + 
 +==== XMODEM-1k 1024 Byte Block ==== 
 + 
 +Disappointing throughput downloading from Unix with YMODEM((The name hadn't been coined yet, but the protocol was the same.)) lead to the 
 +development of 1024 byte blocks in 1982.  1024 byte blocks reduce the 
 +effect of delays from timesharing systems, modems, and packet switched 
 +networks on throughput by 87.5 per cent in addition to decreasing XMODEM'​s 
 +per byte overhead 3 per cent on long files. 
 + 
 +The choice to use 1024 byte blocks is expressed to the sending program on 
 +its command line or selection menu.((See "​KMD/​IMP Exceptions to YMODEM"​ below.)) 1024 byte blocks improve throughput 
 +in many applications,​ but some environments cannot accept 1024 byte 
 +bursts, especially minicomputers running 19.2kb ports. 
 + 
 +An STX (02) replaces the SOH (01) at the beginning of the transmitted 
 +block to notify the receiver of the longer block length. ​ The transmitted 
 +block contains 1024 bytes of data.  The receiver should be able to accept 
 +any mixture of 128 and 1024 byte blocks. ​ The block number (in the second 
 +and third bytes of the block) is incremented by one for each block 
 +regardless of the block length. 
 + 
 +The sender must not change between 128 and 1024 byte block lengths if it 
 +has not received a valid ACK for the current block. ​ Failure to observe 
 +this restriction allows transmission errors to pass undetected. 
 + 
 + 
 +If 1024 byte blocks are being used, it is possible for a file to "​grow"​ up 
 +to the next multiple of 1024 bytes. ​ This does not waste disk space if the 
 +allocation granularity is 1k or greater. ​ With YMODEM batch transmission,​ 
 +the optional file length transmitted in the file name block allows the 
 +receiver to discard the padding, preserving the exact file length and 
 +contents. 
 + 
 +1024 byte blocks may be used with batch file transmission or with single 
 +file transmission. ​ CRC-16 should be used with the k option to preserve 
 +data integrity over phone lines. ​ If a program wishes to enforce this 
 +recommendation,​ it should cancel the transfer, then issue an informative 
 +diagnostic message if the receiver requests checksum instead of CRC-16. 
 + 
 +Under no circumstances may a sending program use CRC-16 unless the 
 +receiver commands CRC-16. 
 + 
 +=== Figure 1: XMODEM-1k Blocks === 
 + 
 +      SENDER ​                                 RECEIVER 
 +                                              "s -k foo.bar"​ 
 +      "​foo.bar open x.x minutes"​ 
 +                                              C 
 +      STX 01 FE Data[1024] CRC CRC 
 +                                              ACK 
 +      STX 02 FD Data[1024] CRC CRC 
 +                                              ACK 
 +      STX 03 FC Data[1000] CPMEOF[24] CRC CRC 
 +                                              ACK 
 +      EOT 
 +                                              ACK 
 + 
 +=== Figure 3: Mixed 1024 and 128 byte Blocks === 
 + 
 +      SENDER ​                                 RECEIVER 
 +                                              "s -k foo.bar"​ 
 +      "​foo.bar open x.x minutes"​ 
 +                                              C 
 +      STX 01 FE Data[1024] CRC CRC 
 +                                              ACK 
 +      STX 02 FD Data[1024] CRC CRC 
 +                                              ACK 
 +      SOH 03 FC Data[128] CRC CRC 
 +                                              ACK 
 +      SOH 04 FB Data[100] CPMEOF[28] CRC CRC 
 +                                              ACK 
 +      EOT 
 +                                              ACK 
 + 
 +===== YMODEM Batch File Transmission ===== 
 + 
 +The YMODEM Batch protocol is an extension to the XMODEM/CRC protocol that 
 +allows 0 or more files to be transmitted with a single command. ​ (Zero 
 +files may be sent if none of the requested files is accessible.) The 
 +design approach of the YMODEM Batch protocol is to use the normal routines 
 +for sending and receiving XMODEM blocks in a layered fashion similar to 
 +packet switching methods. 
 + 
 +Why was it necessary to design a new batch protocol when one already 
 +existed in MODEM7?​((The MODEM7 batch protocol transmitted CP/M FCB bytes f1...f8 and t1...t3 one character at a time.  The receiver echoed these bytes as received, one at a time.)) The batch file mode used by MODEM7 is unsuitable 
 +because it does not permit full pathnames, file length, file date, or 
 +other attribute information to be transmitted. ​ Such a restrictive design, 
 +hastily implemented with only CP/M in mind, would not have permitted 
 +extensions to current areas of personal computing such as Unix, DOS, and 
 +object oriented systems. ​ In addition, the MODEM7 batch file mode is 
 +somewhat susceptible to transmission impairments. 
 + 
 +As in the case of single a file transfer, the receiver initiates batch 
 +file transmission by sending a "​C"​ character (for CRC-16). 
 + 
 +The sender opens the first file and sends block number 0 with the 
 +following information.((Only the data part of the block is described here.)) 
 +     
 +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 null. 
 + 
 +=== Pathname === 
 +The pathname (conventionally,​ the file name) is sent as a null 
 +terminated ASCII string. ​ This is the filename format used by the 
 +handle oriented MSDOS(TM) functions and C library fopen functions. 
 +An assembly language example follows: 
 +    DB      '​foo.bar',​0 
 +No spaces are included in the pathname. ​ Normally only the file name 
 +stem (no directory prefix) is transmitted unless the sender has 
 +selected YAM's f option to send the full pathname. ​ The source drive 
 +(A:, B:, etc.) is not sent. 
 + 
 +== 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. 
 + 
 +  * The receiver should accommodate file names in lower and upper case. 
 + 
 +  * When transmitting files between different operating systems, file names must be acceptable to both the sender and receiving ​          ​operating systems. 
 +  *  If directories are included, they are delimited by /; i.e., "​subdir/​foo"​ is acceptable, "​subdir\foo"​ is not. 
 + 
 +=== Length === 
 +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 number of data bytes in the file.  The file length does not 
 +include any CPMEOF (^Z) or other garbage characters used to pad the 
 +last block. 
 + 
 +If the file being transmitted is growing during transmission,​ the 
 +length field should be set to at least the final expected file 
 +length, or not sent. 
 + 
 +The receiver stores the specified number of characters, discarding 
 +any padding added by the sender to fill up the last block. 
 + 
 +=== Modification Date === 
 +The mod date is optional, and the filename and length 
 +may be sent without requiring the mod date to be sent. 
 + 
 +If the modification date is sent, a single space separates the 
 +modification date from the file length. 
 + 
 +The mod date is sent as an octal number giving the time the contents 
 +of the file were last changed, measured in seconds from Jan 1 1970 
 +Universal Coordinated Time (GMT). ​ A date of 0 implies the 
 +modification date is unknown and should be left as the date the file 
 +is received. 
 + 
 +This standard format was chosen to eliminate ambiguities arising from 
 +transfers between different time zones. 
 + 
 + 
 +=== 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 
 +string. ​ Unless the file originated from a Unix system, the file mode 
 +is set to 0.  rb(1) checks the file mode for the 0x8000 bit which 
 +indicates a Unix type regular file.  Files with the 0x8000 bit set 
 +are assumed to have been sent from another Unix (or similar) system 
 +which uses the same file conventions. ​ Such files are not translated 
 +in any way. 
 + 
 + 
 +=== Serial Number === 
 +If the serial number is sent, a single space separates the 
 +serial number from the file mode.  The serial number of the 
 +transmitting program is stored as an octal string. ​ Programs which do 
 +not have a serial number should omit this field, or set it to 0.  The 
 +receiver'​s use of this field is optional. 
 + 
 + 
 +=== Other Fields === 
 +YMODEM was designed to allow additional header fields to be 
 +added as above without creating compatibility problems with older 
 +YMODEM programs. ​ Please contact Omen Technology if other fields are 
 +needed for special application requirements. 
 + 
 +The rest of the block is set to nulls. ​ This is essential to preserve upward compatibility. ((If, perchance, this information extends beyond 128 bytes (possible with Unix 4.2 BSD extended file names), the block should be sent as a 1k block as described above.)) 
 +If the filename block is received with a CRC or other error, a 
 +retransmission is requested. ​ After the filename block has been received, 
 +it is ACK'ed if the write open is successful. ​ If the file cannot be 
 +opened for writing, the receiver cancels the transfer with CAN characters 
 +as described above. 
 + 
 +The receiver then initiates transfer of the file contents according to the 
 +standard XMODEM/CRC protocol. 
 + 
 +After the file contents have been transmitted,​ the receiver again asks for 
 +the next pathname. 
 + 
 +Transmission of a null pathname terminates batch file transmission. 
 + 
 +Note that transmission of no files is not necessarily an error. ​ This is 
 +possible if none of the files requested of the sender could be opened for 
 +reading. 
 + 
 + 
 +The YMODEM receiver requests CRC-16 by default. 
 + 
 +The Unix programs sz(1) and rz(1) included in the source code file 
 +RZSZ.ZOO should answer other questions about YMODEM batch protocol. 
 + 
 +=== Figure 3: YMODEM Batch Transmission Session === 
 + 
 +      SENDER ​                                 RECEIVER 
 +                                              "sb foo.*<​CR>"​ 
 +      "​sending in batch mode etc."​ 
 +                                              C (command:​rb) 
 +      SOH 00 FF foo.c NUL[123] CRC CRC 
 +                                              ACK 
 +                                              C 
 +      SOH 01 FE Data[128] CRC CRC 
 +                                              ACK 
 +      SOH 03 FC Data[128] CRC CRC 
 +                                              ACK 
 +      SOH 04 FB Data[100] CPMEOF[28] CRC CRC 
 +                                              ACK 
 +      EOT 
 +                                              NAK 
 +      EOT 
 +                                              ACK 
 +                                              C 
 +      SOH 00 FF NUL[128] CRC CRC 
 +                                              ACK 
 + 
 +=== Figure 4: YMODEM Batch Transmission Session-1k Blocks === 
 + 
 +      SENDER ​                                 RECEIVER 
 +                                              "sb -k foo.*<​CR>"​ 
 +      "​sending in batch mode etc."​ 
 +                                              C (command:​rb) 
 +      SOH 00 FF foo.c NUL[123] CRC CRC 
 +                                              ACK 
 +                                              C 
 +      STX 02 FD Data[1024] CRC CRC 
 +                                              ACK 
 +      SOH 03 FC Data[128] CRC CRC 
 +                                              ACK 
 +      SOH 04 FB Data[100] CPMEOF[28] CRC CRC 
 +                                              ACK 
 +      EOT 
 +                                              NAK 
 +      EOT 
 +                                              ACK 
 +                                              C 
 +      SOH 00 FF NUL[128] CRC CRC 
 +                                              ACK 
 + 
 + 
 + 
 +=== Figure 5: YMODEM Filename block transmitted by sz === 
 +<​code>​ 
 +      -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 | 
 +      20 31303036 34340000 00000000 00000000 ​  ​|100644..........| 
 +      30 00000000 00000000 00000000 00000000 
 +      40 00000000 00000000 00000000 00000000 
 +      50 00000000 00000000 00000000 00000000 
 +      60 00000000 00000000 00000000 00000000 
 +      70 00000000 00000000 00000000 00000000 
 +      80 000000CA 56 
 +</​code>​ 
 +=== Figure 6: YMODEM Header Information and Features ==== 
 + 
 +    _____________________________________________________________ 
 +    | Program ​  | Length | Date | Mode | S/N | 1k-Blk | YMODEM-g | 
 +    |___________|________|______|______|_____|________|__________| 
 +    |Unix rz/sz | yes    | yes  | yes  | no  | yes    | sb only  | 
 +    |___________|________|______|______|_____|________|__________| 
 +    |VMS rb/sb  | yes    | no   | no   | no  | yes    | no       | 
 +    |___________|________|______|______|_____|________|__________| 
 +    |Pro-YAM ​   | yes    | yes  | no   | yes | yes    | yes      | 
 +    |___________|________|______|______|_____|________|__________| 
 +    |CP/M YAM   | no     | no   | no   | no  | yes    | no       | 
 +    |___________|________|______|______|_____|________|__________| 
 +    |KMD/​IMP ​   | ?      | no   | no   | no  | yes    | no       | 
 +    |___________|________|______|______|_____|________|__________| 
 + 
 +==== KMD/IMP Exceptions to YMODEM ==== 
 + 
 +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 
 +option to the sending program. ​ Although this two character sequence works 
 +well on single process micros in direct communication,​ timesharing systems 
 +and packet switched networks can separate the successive characters by 
 +several seconds, rendering this method unreliable. 
 + 
 +Sending programs may detect the CK sequence if the operating enviornment 
 +does not preclude reliable implementation. 
 + 
 +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. 
 + 
 +===== 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 
 + 
 + 
 +===== 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 ==== 
 + 
 +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. 
 +  * 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: 
 +          <​SOH><​blk #><​255-blk #><​--128 data bytes--><​cksum>​ 
 +    in which: 
 +    <​SOH> ​        = 01 hex 
 +    <blk #>       = binary number, starts at 01 increments by 1, and 
 +                    wraps 0FFH to 00H (not to 01) 
 +    <255-blk #>   = blk # after going thru 8080 "​CMA"​ instr, i.e. 
 +                    each bit complemented in the 8-bit block number. 
 +                    Formally, this is the "ones complement"​. 
 +    <​cksum> ​      = the sum of the data bytes only.  Toss any carry. 
 + 
 +==== File Level Protocol==== 
 + 
 +=== 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. 
 + 
 + 
 +=== 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 
 +that looked like an <​ack>​. ​ Abort the transmission,​ sending a <​can>​ 
 + 
 + 
 +=== Sending_program_considerations === 
 +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 
 +10 second timeout before retrying. ​ I suggest NOT doing this, and letting 
 +the protocol be completely receiver-driven. ​ This will be compatible with 
 +existing programs. 
 + 
 +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 
 +receiver-driven,​ with the sender only having the high-level 1-minute 
 +timeout to abort. 
 + 
 + 
 +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 
 +getting garbaged. ​ <xx> represents the checksum byte. 
 + 
 +=== Figure 9: Data flow including Error Recovery === 
 + 
 +    SENDER ​                                 RECEIVER 
 +                                  times out after 10 seconds, 
 +                                  <​--- ​             <​nak>​ 
 +    <soh> 01 FE -data- <​xx> ​      ​--->​ 
 +                                  <​--- ​             <​ack>​ 
 +    <soh> 02 FD -data- xx         ​---> ​      (data gets line hit) 
 +                                  <​--- ​             <​nak>​ 
 +    <soh> 02 FD -data- xx         ​--->​ 
 +                                  <​--- ​             <​ack>​ 
 +    <soh> 03 FC -data- xx         ​--->​ 
 +    (ack gets garbaged) ​          <​--- ​             <​ack>​ 
 +    <soh> 03 FC -data- xx         ​---> ​             <​ack>​ 
 +    <​eot> ​                        ​--->​ 
 +                                  <​--- ​      <​anything except ack> 
 +    <​eot> ​                        ​--->​ 
 +                                  <​--- ​             <​ack>​ 
 +    (finished) 
 + 
 +==== Programming Tips ==== 
 + 
 +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. 
 + 
 +After receiving the <​soh>,​ the receiver should call the character 
 +receive subroutine with a 1-second timeout, for the remainder of the 
 +message and the <​cksum>​. ​ Since they are sent as a continuous stream, 
 +timing out of this implies a serious like glitch that caused, say, 
 +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. 
 + 
 +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. 
 + 
 +=== Figure 10: Message Block Level Protocol, CRC mode === 
 + 
 +    Each block of the transfer in CRC mode looks like: 
 +         <​SOH><​blk #><​255-blk #><​--128 data bytes--><​CRC hi><​CRC lo> 
 +    in which: 
 +    <​SOH> ​       = 01 hex 
 +    <blk #>      = binary number, starts at 01 increments by 1, and 
 +                   wraps 0FFH to 00H (not to 01) 
 +    <255-blk #>  = ones complement of blk #. 
 +    <CRC hi> ​    = byte containing the 8 hi order coefficients of the CRC. 
 +    <CRC lo> ​    = byte containing the 8 lo order coefficients of the CRC. 
 + 
 +==== CRC Calculation ==== 
 + 
 +=== Formal_Definition === 
 +To calculate the 16 bit CRC the message bits are considered to be the 
 +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 + 
 +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 */ 
 +    unsigned short 
 +    updcrc(c, crc) 
 +    register c; 
 +    register unsigned crc; 
 +    { 
 +            register count; 
 + 
 +            for (count=8; --count>​=0;​) { 
 +                    if (crc & 0x8000) { 
 +                            crc <<= 1; 
 +                            crc += (((c<<​=1) & 0400)  !=  0); 
 +                            crc ^= 0x1021; 
 +                    } 
 +                    else { 
 +                            crc <<= 1; 
 +                            crc += (((c<<​=1) & 0400)  !=  0); 
 +                    } 
 +            } 
 +            return crc; 
 +    } 
 +</​code>​ 
 +==== 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 
 +initial handshake which is used to determine if both the sending and the 
 +receiving programs support the CRC mode. All Modem Programs should support 
 +the checksum mode for compatibility with older versions. ​ A receiving 
 +program that wishes to receive in CRC mode implements the mode setting 
 +handshake by sending a <C> in place of the initial <​nak>​. ​ If the sending 
 +program supports CRC mode it will recognize the <C> and will set itself 
 +into CRC mode, and respond by sending the first block as if a <nak> had 
 +been received. If the sending program does not support CRC mode it will 
 +not respond to the <C> at all. After the receiver has sent the <C> it will 
 +wait up to 3 seconds for the <soh> that starts the first block. If it 
 +receives a <soh> within 3 seconds it will assume the sender supports CRC 
 +mode and will proceed with the file exchange in CRC mode. If no <soh> is 
 +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. 
 + 
 + 
 +=== 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>​. 
 + 
 +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. 
 + 
 + 
 +==== Data Flow Examples with CRC Option ==== 
 +Here is a data flow example for the case where the receiver requests 
 +transmission in the CRC mode but the sender does not support the CRC 
 +option. This example also includes various transmission errors. ​ <​xx>​ 
 +represents the checksum byte. 
 + 
 +=== Figure 12: Data Flow: Receiver has CRC Option, Sender Doesn'​t === 
 + 
 +    SENDER ​                                       RECEIVER 
 +                            <​--- ​               <C> 
 +                                    times out after 3 seconds, 
 +                            <​--- ​               <C> 
 +                                    times out after 3 seconds, 
 +                            <​--- ​               <C> 
 +                                    times out after 3 seconds, 
 +                            <​--- ​               <C> 
 +                                    times out after 3 seconds, 
 +                            <​--- ​               <​nak>​ 
 +    <soh> 01 FE -data- <xx> ---> 
 +                            <​--- ​               <​ack>​ 
 +    <soh> 02 FD -data- <xx> ---> ​       (data gets line hit) 
 +                            <​--- ​               <​nak>​ 
 +    <soh> 02 FD -data- <xx> ---> 
 +                            <​--- ​               <​ack>​ 
 +    <soh> 03 FC -data- <xx> ---> 
 +       (ack gets garbaged) ​ <​--- ​               <​ack>​ 
 +                                    times out after 10 seconds, 
 +                            <​--- ​               <​nak>​ 
 +    <soh> 03 FC -data- <xx> ---> 
 +                            <​--- ​               <​ack>​ 
 +    <​eot> ​                  ​--->​ 
 +                            <​--- ​               <​ack>​ 
 + 
 +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 
 +example also includes various transmission errors. ​ <​xxxx>​ represents the 
 +2 CRC bytes. 
 + 
 +=== Figure 13: Receiver and Sender Both have CRC Option === 
 + 
 +    SENDER ​                                      ​RECEIVER 
 +                              <​--- ​                <​C>​ 
 +    <soh> 01 FE -data- <​xxxx>​ ---> 
 +                              <​--- ​                <​ack>​ 
 +    <soh> 02 FD -data- <​xxxx>​ ---> ​        (data gets line hit) 
 +                              <​--- ​                <​nak>​ 
 +    <soh> 02 FD -data- <​xxxx>​ ---> 
 +                              <​--- ​                <​ack>​ 
 +    <soh> 03 FC -data- <​xxxx>​ ---> 
 +    (ack gets garbaged) ​      <​--- ​                <​ack>​ 
 +                                         times out after 10 seconds, 
 +                              <​--- ​                <​nak>​ 
 +    <soh> 03 FC -data- <​xxxx>​ ---> 
 +                              <​--- ​                <​ack>​ 
 +    <​eot> ​                    ​--->​ 
 +                              <​--- ​                <​ack>​ 
 + 
 + 
 +===== 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. 
 +    omen Any ACU 2400 1-503-621-3746 se:--se: link ord: Giznoid in:--in: uucp 
 + 
 + 
 + 
 +===== Revisions ===== 
 + 
 +=== 10-27-87 === 
 +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,​ minor revisions for clarity per user comments. 
 +=== 8-03-87 === 
 +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 ===== 
 + 
 + 
 +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 
 +         Omen Technology Inc 
 +         ​17505-V Sauvie Island Road 
 +         ​Portland Oregon 97231 
 +         ​VOICE:​ 503-621-3406 :VOICE 
 +         ​Modem:​ 503-621-3746 Speed: 19200(Telebit PEP),​2400,​1200,​300 
 +         ​Usenet:​ ...!tektronix!reed!omen!caf 
 +         ​CompuServe:​ 70007,​2304 
 +         ​GEnie:​ CAF 
 + 
 +Unlike ZMODEM and Kermit, XMODEM and YMODEM place obstacles in the path of 
 +a reliable high performance implementation,​ evidenced by poor reliability 
 +under stress of the industry leaders'​ XMODEM and YMODEM programs. ​ Omen 
 +Technology provides consulting and other services to those wishing to 
 +implement XMODEM, YMODEM, and ZMODEM with state of the art features and 
 +reliability. 
 ===== See Also ===== ===== See Also =====
   * [[:​ref:​|Reference Library]]   * [[:​ref:​|Reference Library]]
  
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