Hi Tom, did you try your MFM in one of your 4051 computers?
I suggest running my Checksum program on the 4051 or 4054 with MFM in slot 2. The checksum program will indicate which Expansion slot it finds each ROM - including the RS-232 Printer Interface.
In my photo attached - I think the RS-232 Printer Interface ROM is the "Unknown ROM Pack" with 2350 for the first 4KB checksum.
RLSD is now called DCD - which is connected to ground in the MFM schematic Rev 0.1 that I'm looking at from Jos' MFM pdf.
That schematic shows the RS232 pin 1 is connected to RTS on the 6850 although the signal name on the schematic is dcd.
You can probably rewire the 75155 buffer input to DCD on the 6850 - but you would need to cut the DCD trace to GND - or lift the UART DCD pin out of the socket and wire to the lifted 75155 pin if that IC is also socketed.
@jdreesen would know if the schematic I'm looking at is the current MFM board schematic.
If you don't care about the 4050 speaker - it is a GPIO that you could use - but it would require an assembly language routine to set or clear it.
Alternatively - you might try using the COM Option 1 interface.
Appendix A-1 has an interesting paragraph on ERROR message 141:
141-I/O Function: Programmers error. Attempt at BASIC I/O with device 40 with incorrect
secondary address. INPUT from 40 is only correct when secondary address=4(OLD),
13(INPUT) and 0(STATUS). Output correct when secondary address=1(SAVE), 12(PRINT),
19(LIST),
30(ON) and 31(OFF).
I tried
PRINT@40,30:0, PRINT@40,30:1, PRINT @40,31:0 and PRINT @40,30:1 and got no error!
I wonder if 30 is the secondary address for controlling the COM RLSD signal on pin 8 of the 25 pin connector on the COM Backpack. I searched that manual for mention of 30 or 31 and didn't find any other mention
I also tried PRINT @40,11:0 and got error 138 - which is not listed in Appendix A-1!
The RS-232 Printer Interface manual page 3-2 shows how to control the RLSD signal:
PRINT @41,11:0 to set the RLSD OFF
PRINT @41,11:1 to set the RLSD ON.
I think it might be worth your experimenting with the COM backpack RLSD signal!