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Thread: Differences (apart from clock speed) between Z80 versions?

  1. #1
    Join Date
    Apr 2015
    Morisset, NSW Australia

    Default Differences (apart from clock speed) between Z80 versions?

    There's several forums I could ask this in but, given it occurred putting together a CP/M machine, I thought I'd ask here.

    I recently built one of the variants out there of Grant Searle's CPM machine - this one in particular (why that one? Someone not too far from me had spare boards they'd had made up, no better reason). As per this specific variant, I went with a 3.6864MHz clock. But given a plan to later try the faster speed I got a Z80BSIO-2 and (because it was cheaper than any of the other options) a Z80H.

    Fast forward, and when I tried to connect via serial I got a) nothing, then b) really noisy comms at the expected 57600. After some scope work confirmed the clock was good, the reset signal was passing through (without bounce) everywhere expected, and inspection & scope convinced me that there were no obvious soldering probs, I went back to the problems. Along the way I'd noticed that sometimes after a reset the address lines went active on the Z80 at the rate you'd expect, other times nothing at all. A bit of searching on the serial problem turned up someone else who had seen the same thing on the same board, which was fixed by replacing the Z80. So I tried the same thing - got a Z80A and a Z80ASIO-2. And, lo and behold, all is working fine. System comes up consistently and I'm not seeing any serial noise any more.

    So, to my questions:
    • Is there any electrical differences between the various Z80 chips original through H? I.e. is the H not a straight drop-in replacement?
    • Is it a really bad idea to mix revisions between the support chips (e.g. SIO) and the CPU?
    • Or did I just luck out with a bad Z80H? (I haven't tried the Z80B-SIO2 with Z80A but I probably will)


  2. #2
    Join Date
    Dec 2012
    Blog Entries


    It seems that it is nearly impossible to find any sort of errata for Zilog chips. I tried for quite a while to find Z180 errata, and failed, but just had to assume that some behavior that another person and I found were the result of the revisions we had just didn't work for certain operations involving some of the internal peripheral registers.

    Don't know if this applies somehow to your situation, but when I built my P112 board, I had an issue where it didn't always reset correctly. Turns out I had entirely missed soldering one leg of a resistor in the reset circuit, and it made intermittent contact with the plating in the pad's hole and would work sometimes.


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