Somewhere I have the original Intel 8042 datasheet and the Holtek HT6542B replacement for i8042. Think they have sample circuit diagrams which could be useful...
will attach when I find them in my archives... ohh the Holtek was easy, think the Intel is in a Data book...

Hi @Horun, many thanks for the info.

Just FYI, I am working on a separate project where I have interfaced a 8042 (marked AMIBIOS) to an Arduino Nano. Consequently I have downloaded all the information on the 8042 and clones that I can find, so I recognise the PDF which you linked. I have already posted in an 8042 thread and will post again later, so please read and add any knowledge if you have it!

Schematics of a full 486 motherboard, any would be helpful as an example, would confirm the wiring of the 8042 to the keyboard socket and also the battery backup circuit. The seller hacked three wires to fix the broken wires associated with the keyboard, but the battery backup circuitry has damage which I am guessing (will try later today) will prevent the CMOS (which I think is in the Ali chipset) being powered. If so this will be a PITA (until repaired) as the CMOS will not remember settings after power-off. I meant to buy a different 486 board from the same seller at the same price, but had multiple tabs open and selected the wrong one 🙁

Ok have you seen this: http://web.archive.org/web/20000610020233/htt … chems/index.htm
http://web.archive.org/web/19991110232114/htt … hems/ulpsch.htm
the files are .PS but someone at retrotronics.org converted them to PDF. There is a keyboard section schematic using s82c42pe that may help...
also there is a sort of mirror here but less schematics: http://intel-vintage-developer.eu5.org/DESIGN … WEB/INTAR_2.HTM
attached the 486eval pdf