Dennis Hess (KA0ZIS) provided this information on using the BayPac modem and BayCom software with the HP-100LX palmtop.
To use the BayPac with the HP-100, you will need
Hewlett Packard's serial cable #F1015-80002
and 9-pin to 25-pin adapter #5181-6642.
To attach the BayPac to the HP-100, just plug
one end of the serial cable into the HP-100 and plug
the other end of the cable into the 9-pin to 25-pin adapter. Now, plug your BayPac modem into the
9-pin to 25-pin adapter.
Load the BayCom software onto the HP-100 and configure it like you would on any PC. The only setting that you may have to change is the serial port selection.