This page contains some links to archived programming cable information for GRE-made scanners from the PRO-92 through the PSR-500. Use any of the schematics at your own risk.
First, a description of how a computer with an EIA-232 port is connected to one of these scanners: Cables.pdf
Next, a description of how the original GRE cable (included with their "Scanner Data Manager") worked: GRE Cables
"pfranc" used to make a cable that, while originally designed for Casio calculators, did work with the PRO-92 through PRO-96. I used the one that came with my Casio QV-3000 digital camera for several years (with a stereo-mono adapter and a 3.5mm-2.5mm adapter): pfranc PC-Link cable. I don't know that this cable is still available, but there are some links on that page that may help.
Finally, a couple of schematics for building your own cables (these are pretty old - some parts may not be readily available): cable #1 and cable #2
