TTY-Redirector/Pseudo-TTYs under Linux

1. Download and installation

• In many Linux distributions socat can be installed directly using the respective software management. Simply search for socat.

• The homepage of the socat project with download links, information about supported Linux distributions and comprehensive documentation can be found at http://www.dest-unreach.org/socat/. Suitable Com-Servers can be found here.

2. Create pseudo-TTYs

• The following command creates the pseudo-TTY csTTY1 and redirects it to the Com-Server 10.40.21.18 located in the network:

  socat PTY,link=/dev/csTTY1,echo=0,wait-slave  TCP:10.40.21.18:8000,retry=5

• The keyword PTY first creates the pseudo-TTY with the name csTTY1 in the directory /dev. The associated option echo=0 deactivates the local echo of the TTY.

• By using wait-slave the TCP connection to the 2nd data channel is not established until the created pseudo-TTY is opened by an application, so that if a TTY is closed, other stations in the network can access the Com-Server and the connected serial device.

• The keyword TCP specifies the 2nd data channel, which is connected to the previously created pseudo-TTY. The IP address and the TCP port number of the Com-Server are specified as address parameters. retry=5 specifies the number of connection attempts in case the Com-Server is currently being used by another station.

3. Notes and special features

• The required serial transmission parameters (baud rate, parity etc.) must be first configured manually in the Com-Server setup. Automatic configuration coupled to the parameters of the opening application is not possible.

• Via socat created pseudo-TTYs are useful for all applications which communicate with the terminal device only on the level of the serial data lines Rx and Tx. Transmission on the RS232 control lines (RTS, CTS, etc.) is not possible.

• The wait-slave option used in the example above provides for a need-based network connection opening. At the same time, closing the TTY and the network connection to the Com-Server also ends the socat process, so that a script-based restart must follow. When the socat command is executed as part of the system start, the procedure differs depending on the Init system of whichever Linux distribution is used.

• You can also perform a character-oriented test of the virtual TTY using socat. The following command creates a transparent connection between STDIN/STDOUT of the Linux system and the Com-Server through the pseudo-TTY csTTY1:

  socat STDIO,raw,echo=0,escape=0x03 /dev/csTTY1,raw,echo=0