When using COM 3 and COM 4 with CPACK 100, it is IMPERATIVE (as documented early in the manual) that you run CPACK 100, go into SETUP, and configure the COM ports properly. The default IRQ settings for COM 3 and COM 4 are, I believe, 0, which is of course, wrong no matter what your hardware set up is. This MUST be done or COM3/COM4 won't operate correctly.
Everett Kaser
CompuServe ID: [70662,2540]