Null Modem Requirements

A synchronous null modem is a specially configured cable that simulates two back-to-back modems. When the distance between two hosts is not great, you might be able to use a null modem cable instead of more expensive synchronous modems or synchronous modem eliminators. The use of the null modem cable described here allows you to connect your Sun machine to another Sun or non-Sun machine, for synchronous communication over a short distance.

The maximum distance at which a null modem cable can work is determined by the specification for your serial port interface and depends greatly on the quality of the cable you use.

You must perform the following steps to use a null modem cable for machine-supplied clocking:

• Build the null modem cable, according to the pin-outs provided here.

• In the x25tool Link Editor, set the Interface parameter so that one side is DTE and the other side is DCE.

• In the x25tool Link Editor, set the Tx Clock parameter to the bit rate you want.

You can use a null modem cable with any of the Sun RS-232-C or RS-449 serial port options, except the EXPX device, which cannot supply a clock.