Re: Plus/4 CPU

From: Nicolas Welte (
Date: 2001-02-08 10:00:37

Nick Coplin wrote:
> I previously check with Ruud, but unfortunately his group didn't have any 7501/8501 CPUs.
> My sample of 6 machines (3 Plus/4 and 3 C16) gives 5.5 dead CPUs (the 0.5 means it works for about 5-10mins and then stops....)
> I'd also be interested in one of these, but I think its pretty rare to find a working sample....

I've heard the failure rate of the original 7501 is much higher than that of
the 8501, but still it's not easy to find replacement CPUs. If there's really
no spare 8501 on this planet left, the following project might be an option:
Use a 6502, a 6522 (for the processor port), a set of two LS244 and one LS245
for the bus tri-state and direction control, a 15 input (and/or/nor/nand, I
forgot) gate to select adresses 0/1 and some other minor control logic. This
applies for all of the 6510/7501/8500/8501/8502 so far, and the x501 might
need an additional gate on the R/W line that is controlled by the "gate in"
pin. Though my machines were working fine even with the gate in disconnected,
which affected the R/W timing only a little bit.

If you add up the costs for a new 6502 and 6522 you'll see immediately why
this project isn't an option yet. BTW, do they make new 6502s in PLCC44
casings? I think I know that you can get the 65C02 and 65C22 in this package,
but while the 65C22 would be fine, the 65C02 isn't an option due to its
different command set. If everything is SMD mounted, it might be possible to
make a small adaptor board that is not much larger than the original CPU. 


PS: I have headaches when I think about the recently discovered behaviour of
writes/reads to unconnected bits of adresses 0/1. IIRC, the data direction
bit is connected, but the data port bit itself is not. Hard to emulate with a
6522 that has both registers in a single chip.
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