I've only done it under the Sharp ROM on a 5500 and 6000, and I haven't used it in a while but IIRC the serial port is ttyS0. Of course just like any other console the settings need to be 8-none-1 at 9600, unless the device defaults have changed.
I found minicom works okay although the fonts can be a little squirrely. There is/was another QPE based terminal program that was much better suited to the task but I can't remember what it's called and I lost the bugger during one of many (re)flash sessions.
Either way, this setup has really pulled my buns out of the fire on more than one occasion when I locked myself out of a switch or router.
Hope this helps,
DirkP