11.12. Serial Printers

If your printer is attached by a serial line, then you may need to set the serial line characteristics before sending the job to the printer. Here are a set of guidelines to following when attaching a serial port printer to a serial line.

1. Check to make sure that the line is not enabled for login. Logins are usually managed by the getty (BSD) or ttymon (Solaris, SystemV). Check your system documentation and make sure that these daemons are not managing the serial line.

2. Check the permissions and ownership of the serial line. For the most easy testing, set the permissions to 0666 (everybody can open for reading and writing). After you have made sure that you can send jobs to the printer, you might want to change the ownership of the serial line to the lpd server and change the permissions to 0600.

3. Make sure that you can print a test file on the printer via the serial port. This may require setting the line characteristics and then sending a file to the printer. You should try to use 8 bit, no parity, with hardware flow control and no special character interpretation, and definitely no LF to CR/LF translation. The problem is that different versions of UNIX systems have different sets of stty(1) commands to do this. The following simple test script can help in this.

    #!/bin/sh
    # 9600, no echo, no CR
    FLAGS= 9600 -raw -parenb cs8 crtscts
    DEV= /dev/tty01
    (stty $FLAGS; stty 1>&2; cat $1 ) <$DEV >$DEV

This shows using stty to set the flags, then to print the current settings, and then using cat a file to the output. If you attach a dumb terminal to the serial port, you can even use this script to ensure that input from the device is echoed to the output with the correct speed, parity, etc.

Experience has shown that serially connected printers are the least reliable and lowest speed. Where possible, it is strongly recommended that they be attached to a network print box which will provide a Socket API interface and handle the low level network to serial port protocol conversions.