Fixes: VDC R/W

Continuing with the series on fixes compared to the original Commodore PCB, we now come to these two wires:

(Ignore the capacitor, that’s a different story)

If we look closely, we can see that one pin on the VDC has been isolated from the PCB traces by drilling them out in two places. Then new wires have been pulled to connect the now isolated pin on the VDC somewhere and another wire was used to bypass that segment for the original trace. Checking what the pin is it shows up as pin 9, which is documented as R/W.

Schematics

Just like with R102, there is a clue available if one compares the rev 6 schematic with rev 7:

So, somehow in the revision change they changed R/W to F_R/W. So what is this F_RW and where does it come from? We can find it in the rev 7 schematic in the section that contains the glue logic that handles the Z80 and 8502 CPUs.

So this was for sure an intentional change which just like the previous case with the R102 was not integrated into a new PCB revision, instead it was retro-fitted onto the PCB and manually patched by the assembly workers. If we look at the signals using an oscilloscope, we can clearly see that the yellow R/W signal is about a volt lower than F_R/W on the highs.

My guess is that in some cases the VDC simply didn’t trigger correctly on this signal, it’s my understanding that it was a bit of a troublesome chip in many aspects.

The fix

Finding space on the C128 PCB is not easy, it’s pretty well packed. We have to find a path between the two pins marked in white:

After a lot of thinking, looking and shoving things around I landed at the following path:

This seems to work just fine so I’m happy with this. No more traces to cut, no more bodges to solder.

Stay tuned for the next tale of mischief on the boards.

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