One of the first things I found out, is that the Psion
serial port is cross wired.
That is, to connect to another device via the serial port you have to make a cross over cable.
I didn't draw this, and I can't remember where I got it from.
This explains why I couldn't get a modem
to work with the Psion Message Suite. All the wires were crossed.
Below is OPL code for sending and receiving bytes or strings via the serial port of your Psion 5/Revo, probably works on most Psions. It has to be noted that at the end of each byte or string received, there are two more bytes that may have to be read and ignored. On the Revo, if you change the TTY:A to TTY:B, you can use the infrared port directly. I've not had much success with reading the port, but it transmitts OK.
A modified version of this code is used for communicating with my I/O board.
Print "Press, (s) to send. (r) to receive. (q) to quit"
if v$ = "s"
print "send what ? ";
if v$ = "r"
IF stop%=2 :frame%=frame% OR 16 :ENDIF
IF parity% :frame%=frame% OR 32 :ENDIF
srchar%(1)=baud% OR (baud%*256)
srchar%(2)=frame% OR (parity%*256)
srchar%(3)=(hand% AND 255) OR $1100
IF err% :RAISE err% :ENDIF
More serial port info can be found here. But it's technical, not safe and not finished.