I'm having the same issue using the RXD instruction. My problem is I don't see anything come through to the PLC when I try to send a byte to the PLC from the PC. I am able to receive on the PC side when a byte is sent from the PLC using the TXD instruction. One other problem I'm having is the serial communication error (A392.12) is always on and I cannot figure out why. I'm attaching captures of CP1L settings, RS-232 communication cable that was wired up, and the RXD and TXD code I'm using if someone could help me out.