Synchronet Virtual DOS Modem

SVDM is a modem emulator for Windows which utilizes the Synchronet Virtual UART/FOSSIL Driver (DOSXTRN.EXE/SBBSEXEC.DLL) to enable support for DOS communications programs on Windows NT-based operating systems. 64-bit Windows support is enabled by also installing NTVDMx64.

SVDM should run on 32-bit and 64-bit editions of Windows XP and later.

Although SVDM reuses components of the Synchronet BBS Software and shares some of its source code and libraries, it is is not technically “part of” Synchronet BBS nor is it required for the normal use or operation of a Synchronet BBS.

• Use 16-bit DOS terminal programs (e.g. Telix, ProComm, Qmodem, Telemate, etc.) to connect to Internet-connected BBSes
• Use 16-bit DOS terminal program's “host mode” to accept incoming Telnet or “Raw TCP” connections from Internet clients
• Use 16-bit DOS BBS program (e.g. Synchronet v1/2, WWIVv4, TriBBS, etc.) to accept incoming Telnet or “Raw TCP” connections from Internet users
• Run 16-bit DOS door programs from Internet-connected BBSes (with the SVDM -h command-line option)

• Open source (Microsoft Visual C)
• Emulates an NS16550 UART
• Provides a FOSSIL/PC-BIOS (int14h) software interface
• IPv4 and IPv6 support
• Telnet client and server support
• Raw TCP client and server support
• Accurate Hayes Smartmodem / USRobotics Sportster Modem “AT Command Set” emulation
  • Complete with virtual NVRAM settings/number storage (i.e. AT&W, AT&Zn=S)
  • Optional auto-answer (ATS0=1)
  • RING result upon incoming connection attempts
  • Verbal and numeric result modes
  • “Dial” IP addresses or hostnames with optionally-specified TCP port and/or protocol

SVDM currently does not include an “installer”, so you will need to copy the following files to a directory ideally in your system's search path:

• svdm.exe - the main program (modem emulator)
• svdm.ini - configuration settings, optional
• dosxtrn.exe - 16-bit FOSSIL driver and VDD loader
• sbbsexec.dll - 32-bit Virtual UART/Device Driver

On 32-bit editions of Windows Vista and later, sbbsexec.dll must also be copied to your Windows/System32 directory (by an administrator).

The general usage is to use svdm to execute the 16-bit DOS communications program of interest. Any command-line options you want to pass to the program may be specified on the command-line after the program path/filename.

usage: svdm [-opts] <program> [options]
opts:
        -telnet   Use Telnet protocol by default
        -raw      Use Raw TCP by default
        -4        Use IPv4 address family by default
        -6        Use IPv6 address family by default
        -l[addr]  Listen for incoming TCP connections
                  [on optionally-specified network interface]
        -p<port>  Specify default TCP port number (decimal)
        -d        Enable debug output
        -h<sock>  Specify socket descriptor/handle to use (decimal)
        -r<cps>   Specify maximum receive data rate (chars/second)
        -c<fname> Specify alternate configuration (.ini) path/filename
        -V        Display detailed version information and exit

Run Telix:

svdm telix

Run Renegade BBS Software (in WFC mode), listening on all IPv4 network interfaces for incoming TCP connections:

svdm -l -4 renegade

SVDM responds to the ATD dial command to perform an outbound TCP connection. If the first character of the dial-string (following the 'D') is an uppercase 'T' or 'P', this character is ignored as it was used to specify “tone” or “pulse” dialing on the old telephone system and not relevant to TCP/IP connections. For example, any of the following commands may be used to “dial” test.synchro.net:

ATDTtest.synchro.net
ATDPtest.synchro.net  
ATDtest.synchro.net
atdtest.synchro.net

The address to “dial” may be specified as a DNS hostname, an IPv4 address, or an IPv6 address:

ATDvert.synchro.net
ATD192.168.1.2
ATD[::1]

The TCP port to connect to may be specified following the address and a colon:

ATDvert.synchro.net:23
ATD192.168.1.2:23
ATD[::1]:23

The TCP protocol to connect with may be specified (followed by a colon) before the address:

ATDtelnet:vert.synchro.net
ATDtelnet:192.168.1.2
ATDraw:[::1]

The last “number dialed” can be dialed again by dialing 'L' or 'l':

ATDL

Up to 20 dial strings (numbers) can be stored using the AT&Z command:

AT&Z0=vert.synchro.net
AT&Z1=L

Saved dial strings (numbers) can be queried/displayed using the AT&Z command:

AT&Z0?
AT&ZL?

Save dial strings (numbers) can be dialed by using the ATDSn command:

ATDS0

If your DOS terminal program of choice has trouble dialing long dial strings (e.g. accommodating long DNS hostnames or IPv6 addresses), try using the saved number storage feature to resolve that limitation. After enter the following command, dialing “S0” as a “phone number” would actually connect to “telnet:vert.synchro.net:23”.

AT&Z0=telnet:vert.synchro.net:23

SVDM's behavior can be customized or modified by specifying command-line parameters, invoking modem “AT commands”, or creating/modifying a .ini file.

The default configuration file svdm.ini will be loaded from the same directory where the executed svdm.exe is located.

Settings changed via modem AT command take precedence over command-line options which take precedence over .ini file settings.

Settings in the Root section may also be specified in a program-specific section (named after the program itself), to create program-specific settings.

Settings in the UART section may also be over-ridden in a program-specific section but naming the section [<program>.UART]. For example, by default SVDM emulates a UART for COM Port 1 (I/O address 0x3f8, IRQ 4), but you could change the emulated COM Port for one particular program by creating a program-specific UART section in the .ini file:

[telix.UART]
ComPort=2

SVDM will recognize and respond to modem commands only when in “command mode”.

When there is no active connection, “command mode” is the current mode. When there is an active connection, the terminal can force the modem into command mode by sending the Hayes-302 escape sequence (by default, “+++”) with the minimum guard time of inactivity before and after the escape sequence. The duration of the guard time (default of one second) and the character repeated 3 times for the escape sequence are configurable via modem S-registers.

Modem commands are prefixed by “AT” (case-insensitively), which stands for “attention”, hence the name “AT Command”.

After the initial “AT” is sent to the modem, zero or more commands may be specified, followed by a final carriage-return (CR) character. The ASCII values of the carriage-return and line-feed characters are configurable via modem S-registers. The backspace character (configurable via S-register) is recognized as a destructive edit command to modify a command sequence before the command is submitted (usually with the ENTER key).

If all of the commands are valid, then the modem will respond with the “OK” response (or 0 in numeric response mode). There are exceptions (e.g. D command).

If any of the commands are invalid, then the modem will respond with the “ERROR” response (or 4 in numeric response mode).

Any white-space characters within the AT command sequence are ignored.

Some commands (e.g. D, &Zn=), must be the last command of the command sequence since they accept an arbitrary string argument that may contain otherwise-valid AT command characters and sequences.

S-Registers store additional/advanced modem settings as decimal-numeric values:

• Utility index
• sbbsexec
• sbbsexec.ini
• Initial demonstration/announcement video