help testing serial port

[Doug Poland]

Hi,

I'm trying to hook up an APC SmartUPS to a 16450 serial port on an
old 80486.  The box is running 4.8-STABLE and is otherwise
up-to-date.

apcupsd is telling me that it cannot communicate with the UPS via
the serial port.  Both the cable and UPS are fine, I can commuicate
with the UPS with the same cable on another 80486 with a 16550A
serial port.  Also, the apcupsd.conf has been successfully tested on
another machine.

How can I test the serial port itself to see if it's working?  I tried
cu -s 2400 -e -o -h -l /dev/cuaa0 and said it was connected.  I
exited cu but now cannot attach ( cu: /dev/cuaa0: Line in use ). 
Obviously I don't understand the fundamentals and tools necessary to
communicate with a serial port.  The handbook and list archives
haven't been much help.

Could someone point me to a FAQ/archive/tutorial on how to debug a
serial port?

-- 
Regards,
Doug