i found this on myoldmac.net:
Hello to all pre-ADB Mac Users,
I like to announce an open source, open hardware keyboard protocol
adaptor project to connect the MacPlus keyboard protocol and PS2.
With this adaptor, you can connect your original MacPlus-keyboard
to your PeeCee, or you can take a mass-production-PS2-keyboard
to run your MacPlus, if the original keyboard is lost or broken.
The Hardware is based on an low-cost AT89C2051 8051 type
microcontroller and will fit for both, translation from
a PS2-Keyboard to a MacPlus Computer or vis versa from
the MacPlus Keyboard to a PS2-Host.
Note: do not use telephone-headset-cables as keyboard cable!
telephone cables are connected in roll-over mode
while the mac-keyboard cable is straigt-through.
There are two firmware versions available:
kbdbabel_ps2_macplus_8051.asm from PS2-keyboard to the Mac
kbdbabel_mac_ps2_8051.asm from the MacPlus keyboard to PS2.
Note: if you want to switch the direction on the adaptor,
you need to change or re-program the micronontroller.
Note2: This adapter may be used as well for Mac 512k / Mac 128k,
when some adaptions on the scancode translation table are made.
Sorry, I did not get my hands on a Mac512k/Mac128k keyboard
to analyze the protocol.
Some documentation about the Mac Plus Keyboard protocol
is available as well (draft, germand keyboard)
http://kbdbabel.sourceforge.net/doc/kbd ... 070606.pdf
Schematic, PCB mask and firmware source code is available here
http://kbdbabel.cvs.sourceforge.net/*ch ... s2_sch.eps
http://kbdbabel.cvs.sourceforge.net/*ch ... s2_pcb.eps
http://kbdbabel.cvs.sourceforge.net/*ch ... 2_8051.asm
http://kbdbabel.cvs.sourceforge.net/*ch ... s_8051.asm
The open software is licensed under the terms of the GNU GPL,
schematics and designs under Creative Commons CC-BY-SA 2.5.
More information, and other keyboard adaptors can be found on
cu from Cologne, Alexander / kbdbabel