Reverse engineering vintage PCBs

Al Kossow aek at
Fri Jan 9 08:53:45 CST 2015

On 1/8/15 6:35 PM, Alan Hightower wrote:
> I appreciate all the input from folks. With 3600 though holes and at
> least 4 layers, I'm afraid I will make mistakes using any manual method
> like ohm tracing without a check or balance. With so many permutations
> and little personal knowledge about the WE chipset, I'm sure I will miss
> an occasional trace that shoots to the other side of the board.

Looking at the picture, it isn't that hard to decompose what's there into
functional blocks. At that density, the PC layout guy will have kept the
parts that are closely related together. You have an expansion bus interface
on the left with bus buffering, HD controller, UART, floppy interfaces and a
DMA controller for it on the top, processor upper right, four eproms, so there
is a 32 bit data bus, and some sort of memory interface coming out the bottom.

Get the SYS V driver sources and headers from Seth, that will tell you what the
interrupts and DMA channels are hooked to and where the peripherals are mapped.

Start with the CPU and trace the address and data bus paths. You don't need to
buzz out every pin, just identify what buffer is connected to what and start
ticking off the ICs from the top board layout out through the peripherals,
expansion bus, and memory bus. Once that's done, you'll have decoders, state
machines and glue logic which should be functionally associated with one of
the big blocks. It doesn't look like there are many bipolar proms or PLA/PALs
I see one 82S153 sequencer near the CPU, but Signetics parts (82S100, 105, 153)
don't have protection fuses.

I have one document that I haven't put on line yet that I think describes the
expansion bus, I'll try to get that uploaded today.

Is there a description somewhere of what the various 3B2 CPU boards had on them
or what the memory board looks like?

It's a drag the guy on eBay wants so much for the docs they've listed.

More information about the cctalk mailing list