The pinouts of the A500 bus connector are pin 1 closest to Denise to pin 85 at the other end, usually marked on the motherboard. The pin numbers alternate from top to bottom from pin 1 to pin 86, with all uneven numbered pins on top and uneven numbered pins at the bottom.
Pins 1 - 4 are all earthed, that is the first two upper and lower pins on the connector, closest to Denise.
If you have the A500 schematics then you should have each pin assignment, if not either download them from my old site
http://www.whirljack.net/jeremy/clockmeister/ or email me at clockmeister at internode dot on dot net and I'll help you with your project.
My interface that can be seen in the images section on amiga.org borrows heavily from the A500IDE project, in particular the first prototype.
Hope this helps :-)
