An initial readme.txt, most of this text comes from the wiki.synchro.net page

src/vdmodem/readme.txt

+Synchronet Virtual DOS Modem (SVDM) for Windows - Copyright 2022 Rob Swindell
+
+For the current/complete documentation on the latest build/version, see
+https://wiki.synchro.net/util:svdm
+
+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. 
+
+SVDM should run on 32-bit and 64-bit editions of Windows XP and later.
+
+For operation on 64-bit editions of Windows, you will need to also install
+NTVDMx64: http://www.columbia.edu/~em36/ntvdmx64.html
+
+On 32-bit editions of Windows Vista and later, the SBBSEXEC.DLL must also
+be copied to your Windows/System32 directory (by an administrator).
+
+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.
+ 
+===== Uses =====
+  * Use 16-bit DOS terminal programs (e.g. Telix, ProComm, Qmodem, Telemate)
+    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)
+
+===== Features ===== 
+  * Open source (Microsoft Visual C)
+  * Emulates an NS16550 UART
+  * Provides a FOSSIL/PC-BIOS (int14h) software interface
+  * IPv4 and IPv6 support (for inbound and outbound connections)
+  * Inbound connection filtering based on IP address/range
+  * 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
+     * Optional auto-answer (ATS0=1)
+     * RING result upon incoming connections
+     * Verbal and numeric result modes
+     * Caller ID support (enabled with AT+VCID=1 or AT#CID=1)
+     * "Dial" IP addresses or hostnames
+       with optionally-specified TCP port and/or protocol
+
+===== Install =====
+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).
+
+===== Usage =====
+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)
+        -n<node>  Specify node number
+        -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
+
+==== Flow Control ====
+When utilizing UART (COM Port) virtualization (e.g. with most DOS terminal
+programs and some DOS door programs), be sure to enable RTS/CTS hardware flow
+control in the DOS program and do **not** enable DTR/DSR or XON/XOFF
+(software) flow control.
+
+==== Ports ====
+
+For DOS programs using UART/COM Port I/O, the COM port number (or I/O port and
+IRQ) must match what SVDM is virtualizing. By default, SVDM virtualizes COM 1
+(I/O Port 3F8, IRQ 4), but this can be changed globally or per program via the
+svdm.ini file.
+
+For DOS programs using the PC-BIOS/FOSSIL int14h interface, all COM ports are
+supported and treated identically (i.e. the DX register value is ignored).
+
+==== Baud Rates ====
+
+The DTE baud rate set by the DOS program is not used by SVDM (i.e. the
+transmit and receive data rates are only limited by the TCP/IP network). If
+you wish to artificially limit the receive data rate for some reason (not
+normally necessary), you can use the -r command-line option or the "Rate"
+key in the root section of svdm.ini to set the maximum receive rate, in
+characters per second.
+
+A DTE rate of 38400 bps is reflected by virtual device driver and if the DOS
+program attempts to change this value, there is no effect.
+
+==== Connect Rates ====
+
+For incoming TCP connections, SVDM will report either "CONNECT 9600" or just
+"CONNECT" (assumed 300 bps) depending on the modem's extended response
+(ATX) setting.
+
+==== Bits o' Parity ====
+
+The communication data (word) bit width, stop bits, and parity settings used
+by the DOS program are not used by SVDM. The equivalent of 8 data bits, no
+parity, and one stop bit, is assumed.
+
+==== Disconnect ==== 
+If the DOS program de-asserts ("drops") DTR, the TCP connection will be
+terminated (by default). Alternatively, the DOS program can send the escape
+sequence (by default, "+++", surrounded by a one second guard time of
+inactivity) to enter command mode and then send "ATH" or "ATH0" to disconnect.
+
+===== Examples =====
+
+Run Telix (popular comm program for MS-DOS from the 1980s):
+  svdm telix
+
+Run Renegade BBS Software (in WFC mode), listening on all network interfaces
+for incoming TCP connections:
+  svdm -l renegade
+  
+===== Dialing =====
+
+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 (in brackets):
+  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
+
+===== Configure =====
+
+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
+
+===== AT Command Set =====
+
+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.
+ 
+For the current/complete documentation on the latest build/version, see
+https://wiki.synchro.net/util:svdm