I have had success using a Megahertz 2496 XJack modem on the 100LX with DataComm, cc:Mail, and fastCOMM!. All that is needed is to put the CIC100 /GEN 1 command in your AUTOEXEC.BAT. Then the 100LX beeps every time it finds a different card type in the slot. It will recognize the modem card and configure the slot as COM 2. [See the HP 100LX User's Guide page 7-21]
It appears very well made and comes with MTEZ with Express FAX for DOS, and Eclipse FAX software for WINDOWS.
The modem manual mentions nothing about the 100LX, but I did discovered one problem. Inserting the modem with the computer on will lock up the 100LX.
Mark Scardina
CompuServe ID: [76711,732]