JMRI Install Guide: MacOS Serial Port conections
Macintosh serial ports use a "mini DIN-8" connector, which does not contain all of the signals of a typical RS232 connector. This page discusses how to wire an adapter so that a Macintosh can work with JMRI and a LocoBuffer or LocoBuffer II. The same adapter may also work with other protocols, particularly those that use "hardware flow control".
A good survey of Macintosh serial connections, including good debugging techniques, is available at http://francis.courtois.free.fr/jc1/serial/main.html.
Note that the Macintosh has only one status output and input.
If the serial device uses "hardware flow control", it will assert the CTS lead to indicate when it's OK to send data. That has to be connected to the Macintosh input lead, or the program won't be able to send.
The status output will be set active by JMRI at startup. It should be connected to whatever status leads (e.g. RTS) the serial device looks at.
The information on this page was provided by Dick Bronson and Walter Thompson.