On Sun, Aug 16, 2015 at 8:48 PM, drlegendre . <[email protected]> wrote: > The Zilog P/N of the chip is "Z84C0008PEC", and it's further marked "Z80 > CPU" with a datecode of "8904" - April of 1989?
Assuming that your Z84C00 is not damaged, I think there's one difference that might make it not work in a circuit that works with the NMOS Z80. The clock input signal of the Z80 CPU, either NMOS or CMOS, is *not* TTL-compatible, but rather has a Vihc min spec of Vcc-0.6V, which for a 5V supply is 4.4V. Back in the day, many Z80 system designers ignored that problem, and drove the Z80 clock input from a normal TTL gate, which was marginal at best since the TTL Voh min spec is only 2.4V. In practice, you could usually get away with that in a 2.5 MHz design (original Z80), but at 4 MHz or higher it tended to be noticeably unreliable. The Mostek Z80 databook gives suggested drive circuits; I imagine that the Zilog documentation must have also. Anyhow, even though the Vihc spec is the same for NMOS and CMOS, the NMOS part might better tolerate a clock input that didn't reach as high as the spec. The other thing that could do it is if the circuit might have been designed such that a Z80 with very short clock-to-signal delays wouldn't work. That would be bad design since the Z80 has never had a minimum spec for those delays, so in such a case even a 6 or 8 MHz NMOS part might not work.
