Kev pointed me at the right page on the schematics (page 163 in the 9100_svc_part2.pdf). Looking at the schems I think this project just got easier.

I was expecting to have to use a MAX232 (or similar) TTL (0 - 5v) to RS232 driver chip (+/- 3v). But looking at the 9100 schematic the keyboard signal is just fed to a 74HCT14 / 74LS04 pair which raise the levels back to TTL. So I think I can just drive this at TTL levels directly from the microcontroller. Currently looking at one of the small Atmel parts.

So this becomes a single chip solution. I like it.

Anyone want to take a look at the schematic and vet my answer? Also it looks like though !KBDATA is at pin 1 it just gets fed to ground so I only need to feed KBDATA.

Now all I need to do is figure out what BAUD rate the 9100 is expecting its data at.

David