We spent a long time trying to get our XServe Macs to talk serial to serial. Putting together many of the hints and pieces, we finally came up with a relatively simple Terminal solution. Just connect the serial cables together of two machines and run this on one of them:
#!/bin/bash
sudo /System/Library/StartupItems/SerialTerminalSupport/SerialTerminalSupport stop
screen /dev/cu.serial 57600
sudo /System/Library/StartupItems/SerialTerminalSupport/SerialTerminalSupport startMac OS X Hints
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