Updates to tcpser based on https://github.com/FozzTexx/tcpser
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Chris Osborn 95f4b515ef Don't use high speed Linux baud rates when they haven't been defined. 2 years ago
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bridge Added parity generation. 7 years ago


This is yet another fork of Jim Brain's tcpser serial to IP modem emulation program.

The original source code can be found here: http://www.jbrain.com/pub/linux/serial/

My changes are based upon the rc12 archive dated 11Mar09.

I fixed the bug with being unable to connect to real telnet servers.

I also made the modem routines automatically detect parity. If even, odd, or mark parity is detected then the telnet connection will be limited to 7 bit. Space parity will be treated as 8N1 and will allow telnet binary mode.

I also incorporated geneb's changes found at https://github.com/geneb/tcpser

Chris Osborn fozztexx@fozztexx.com


TCPSER turns a PC serial port into an emulated Hayes compatible modem that uses TCP/IP for incoming and outgoing connections. It can be used to allow older applications and systems designed for modem use to operate on the Internet. TCPSER supports all standard Hayes commands, and understands extended and vendor proprietary commands (though it does not implement many of them). TCPSER can be used for both inbound and outbound connections.


TCPSER is distributed under the GPL 2.0 or later



Simply clone the repository, and use the appropriate make command generate the exectutable. If unsure, try the default make command first.

OS Command
Default/Linux/macOS make
Solaris make -f Makefile.solaris
*BSD gmake

Windows 95/OSR2/98/SE/ME/NT/2000/XP/2003

Win32: make -f Makefile.win32

Note that at least the cygwin1.dll library is required to operate tcpser under Windows. I recommend downloading a recent version from www.cygwin.com

This version of tcpser supports setting up an ip232 port instead of using a real serial port. This is for use with the version of WinVICE 1.19 that has the ACIA fix and ip232 support, allowing WinVICE to use tcpser as if it was connected via a serial cable.


tcpser -d <dev> -s <speed> -l <log_level> -t <tracing options> ...


tcpser -v <port> -s <speed> -l <log_level> -t <tracing options> ...


tcpser -d /dev/ttyS0 -s 38400 -l 7 -tsSiI -i "s0=1" -p 6400

Will start tcpser on ttyS0 at 38400 bps, level 7 logging, tracing of inbound serial, outbound serial, inbound IP, outbound IP, init modem to answer after 1 ring, and listen for incoming connections on port 6400

tcpser -v 25232 -s 38400 -l 4 -p 23

Will set up an ip232 port at 25232, report 38400 bps connections, level 4 logging, and listen for incoming connections on port 23.

tcpser -h will provide additional information

tcpser can be configured to send the contents of a file upon:

Event Flags
connect -c -C
answer -a -A
no-answer -I
busy -B
inactivity-timeout -T

For connect and answer, there are separate options for sending a file to the local serial connection (-c, -a) and the remote IP connection (-C, -A).

If tcpser connects to a telnet service, tcpser will negotiate the connection using the telnet protocol. If telnet is detected, then tcpser will support RFC 856 (Telnet Binary Transmission), so that 8-bit file transfers will work correctly.

tcpser can be configured to support multiple serial/ip232 ports on one TCP/IP port. Simply repeat the -s and -d/-v parameters on the command line for each serial/ip232 port to be configured. Options s,S,a,A,c,C,I, and T will "propagate" to subsequent connections, unless they are redefined. Defaults for s and S are 38400. This configuration enables the operation of a multi-line BBS on one TCP/IP port.

Frequently used addresses can be configured in the "phonebook", like so:

tcpser .... -nhome=jbrain.com:6400

This is also useful for systems that do not accept non-numeric phone numbers:

tcpser .... -n9169651701=bbs.fozztexx.com

One can even "hide" a regular IP address or DNS entry by aliasing it to something else:

tcpser .... -njbrain.com=bestbbs.com

At this point, phonebook support is very alpha, so use with care.


All of the standard Hayes commands should behave as expected. Some of of the proprietary commands are not implemented, but should not cause errors.


Command Effect
ats0=1 set number of rings to answer
ata answer the line
ath0 hang up
ats12? query S register 12
ate0 turn off echo
at&k3 set flow control to RTS/CTS
atdtjbrain.com:6400 "dial" jbrain.com, port 6400 (defaults to port 23)
atdl "dial" last number
a/ repeat last command

Commands can be chained, as on a regular modem:


tcpser supports the Hayes break sequence semantics, so +++ should operate correctly, even if the sequence of characters is used in normal data transmissions.


tcpser can be used with a regular null modem cable, but it utilizes the DTR line on the PC serial port to reflect the state of the DCD line as seen by the target system. On a normal null-modem cable, DTR is mapped to DCD/DSR, which implies DSR will also reflect the state of DCD on the target machine. However, some systems (notably those utilizing the 6551 ACIA communication IC) will not transmit unless DSR is held high. In this case, a quick qorkaround is to force DCD to be held high by adding -i"&c0" to the tcpser parameter list. However, this also prevents normal operation of the DCD line, which is needed by some BBS systems. A more permanent solution is to construct a modified null-modem cable or modify an existing cable to the following specifications:

PC      Target



    | |


This differs from a regular null-modem cable in that the target machine has DSR looped to DTR, not to DCD. Note that this cable is directional.

Normally, the target machine will configure DSR to float to a high state if unconnected. As well, PCs do not require a valid DSR line for operation. Thus, a simpler cable can be constructed that is bi-directional:


Unless there are issues, we recommend this simplified version, as it can be installed in either direction.

As an even simpler solution, many have simply taken a normal rs232 DE-9F to DE-9M cable and removed pin 6 from the male end (DSR). This is fine, but the cable must be installed between the null modem adapter and the target machine:

PC ----- null-modem adapter ----- cable with pin 6 removed ------ target machine

Any other configuration will not work correctly.

Platform notes

Win32 users should use /dev/ttyS0-3 for COM1-4. At present, using "com1:" does not operate correctly.

Raymond Day sends the following Ubuntu 7.10 autorun scripts:



Make a file named something like tcpser with this code in it:

# Start tcpser at 1200 boud on both RS232 ports for Q-Link Reloaded.
case "$1" in
 tcpser -d /dev/ttyS0 -s 1200 -i"e0&k0&c0" -n"5551212"=
 tcpser -d /dev/ttyS1 -p 6401 -s 1200 -i"e0&k0&c0" -n"5551212"=
 echo "Usage: $0 { start | stop }"
exit 0

This has been tested on the following platforms:

  • Linux 2.4.20-8
  • Windows XP
  • Windows XP SP1
  • Slackware 10.0


tcpser has a small but active user community. Help can be found by asking a question in comp.sys.cbm, on the NEWNet #c64friends IRC channel, or by emailing the author.

Jim Brain brain@jbrain.com www.jbrain.com

The ip232 support was added by Anthony Tolle. For questions regarding that, e-mail atolle@gcns.com