RS232 problems Fanuc 18M

Hi all,
I have an 18MC with the following IO problems:
IO ports 0 & 1 (JD5A) will receive files from the PC no problem but will not upload to the PC. The PC times out and on the machine 'Output' flashes indefinitely and I have to reset to stop it. Cable works on other machines.
IO port 2 (JD5B) does not work at all. Parameters for baud rate etc. are present and set correctly, but if I try to read or punch with IO set to 2 nothing happens when I press EXEC. No flashing 'Input' of 'Output', machine doesn't lock up, just nothing at all happens. Also, the PC won't establish an initial connection.
Some searching suggests there may be a blown chip on JD5A, and that JD5B was an option that may not have been activated.
I have no problem replacing a blown chip, but am reluctant as I currently have no way to back up parameters, memory etc.
Does anybody know how to activate JD5B if it is currently not activated?
Also, any other suggestions greatly appreciated.
Thanks in advance
Karl

What about Flow control settings on PC? Try to change them

RS232 Communications | Fanuc 0i, 16i, 18i, 21i Series and 16/18 Models A, B and C | SharpCam Ltd

Check the baud rates on the CNC and on the PC, are they the same?

Thanks for the replies and the info.
I have checked that the settings are the same at the machine and the PC, and also tried different baud rates and other settings.
Settings that I usually run are Baud 9600, data bits 7, stop bits 2, parity even, software handshaking. Thats always worked in the past.
Cable is 25 pin (machine) to 9 pin (pc) with pinout:
2 - 2
3 - 3
7 - 5
4 & 5 bridged and 6,8,20 bridged at the machine end,
1,4,6 bridged and 7 & 8 bridged at the PC end.
Next I will put an oscilloscope on the cable to see what signals are actually being sent.
If it is a hardware issue, I am reluctant to start poking around unless I can get JD5B activated and can do a backup.