SX680 comms error 81 when updating plu(s)

Hi, I wonder if anyone can throw any light on this:

Using an SX-680 till, over CAT-5 to D9pin COM port on PC

When we try to update the PLUs, it says
"Error (81) Write failure (write till) During (6) Send Updates, Retry 1, Ptr 0"
That's not on the David Error codes, ha ha

We do have two other tills which update perfectly. Swapping the cables or the ports doesn't help!!
The comms settings on all 3 tills are the same (9600/8/none/1 etc). I have 4 COM ports on a PCI card

[Can the COM port at the till end be reset via the P2 key setting? and can a till use COM 3 and 4 as well as COM 1 and 2 ?]

Any suggestions, thoughts or info you can think of would be a great help, as updating 1000s of PLUs by hand is not really an option.
Thanks, Baz