Can I use a CMOS level RS232 port w/o using C-100?
No. It needs to be at TTL levels. I would try a C-100 to compare. Download and used the Direct Talk application. It has good samples of data being sent and received. Two major things you need to consider. 1- All data being sent and received in full byte spectrum (0-255). 2- Most commands need CRC16 checksum.