HP 400 - Manual (Page 41)

2-6 Installation and Configuration
3. Connect the computer end of the parallel cable firmly into
the parallel port on your computer and tighten the screws
on the connector. 4. Turn on the computer and the printer. The printer Power
light should come on.
5. Establish communication between the computer and the
printer by sending the following DOS command:
MODE LPT1:,,P
For DOS 5.0 and DOS 6, send
MODE LPT1: RETRY=R
This command causes infinite retries when sending data
so that the computer does not timeout while waiting for
the printer to print. All data is directed to parallel port 1. If
the printer is connected to a different interface port on
the computer (such as parallel port 2), modify the Mode
command.
Note: This DOS command can be added to the
AUTOEXEC.BAT file so that it will not be necessary to
type the command whenever the computer is turned
on. Refer to your DOS manual for instructions on
adding this command to the AUTOEXEC.BAT file.