The PIM documentation shows 4 lines Tx Rx DTR and Gnd. The ICD PR: manual says R1 has full handshaking and R2 has DTR and RTS set to "ready". All the ports on the PR: and the first 3 on the 850 should all work. However, I tried both and it made no difference. I also tried R1 and R2 on the 850 and both worked.
I'm not using a terminal program, just typing in commands from BASIC XL
Open #5,13,0,"R1"
XIO 36,#5,13,0,"R1" (4800 baud, 8 bit word, 1 stop bit, don't monitor the handshake lines)
XIO 38,#5,0,0,"R1" (light ATASCII translation, ignore parity, no LF after CR)
XIO 34,#5,240,0,"R1" (set DTR and RTS)
XIO 40,#5,0,0,"R1" (start concurrent mode)
once in concurrent mode, I can print to the serial port:
Print #5;"●070101FF20DA" (not a real command, just an example)
This works fine on the 850. I've tried most of the reasonable possibilities for configuring the ports, monitoring the handshake lines, turning RTS and DTR on and off, heavy and no ATASCII translation. Nothing will make the PR: talk to the PIM. I'm fairly certain it's not a configuration issue.
I think the culprit might be the lack of a dedicated power supply. The voltage drop from the SIO might be just enough to keep it under the threshold for the PIM. Unfortunately the PIM documentation doesn't have the voltage requirements. I'm using a relatively long serial cable too so that might make a difference.