Casio fx-8500G weird behavior with FA-80 interface

[Dave Britten]

This is a long-shot, but I'm curious if anyone here has ever seen this. I've got an fx-8500G and a pair of FA-80 cassette/printer interfaces. When I connect the calculator to the interface and power it up, it suddenly acts like it's an fx-7000G with slightly more RAM - none of the fx-8000/8500 features seem to be present. Pressing MODE 0 to enter the text editor just types a 0 on the "home" screen, and MODE Ans to recall the input buffer just types Ans. Also, most of the RAM is reportedly missing, with only around 1500 bytes available according to the program write mode screen. And of course, none of the cassette I/O commands are recognized.

When I take the calculator off the interface and switch it on, everything is back to normal. The editor, all the RAM, and the fx-8000/8500 commands and features are back.

I also have an fx-8000G which works fine with these interfaces (as far as cassette I/O is concerned - I don't have a parallel printer to test with). Anybody ever run into this before? I opened up the 8500 and reflowed the solder on the I/O connector, and replaced the memory-backup super capacitor, on the off chance that it had enough current leakage that some part of the calculator wasn't getting enough power when the I/O port was connected.

Note that I do not have an oscilloscope or logic analyzer to get really fancy with troubleshooting.