> We have provided an external isolated RS-232 to RS-485 convertor plugged
> into the COM port on the PC side.

Was it you or someone else who posted a link to a RS-232 to RS-485
convertor?
I downloaded the zip file, saw it required Eagle software to view,
downloaded the freeware Eagle package, then went looking for the
converter...

And could I find it...? Looking very hard on my HDD didn't work!
Couldn't even find the post in the archives.

Please, I know I didn't imagine it, I remember the person saying that they
found that they needed one and designed one, and wished one had been
available at the time... ring a bell?