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Thread: 8008 and Z80 ICEs

  1. #11


    @BloodyCactus: It just works.. The official level of a TTL "High" signal (according to Wikipedia) is from 2.0v - Vcc (5v), so a 3.3v high level would count as a 1 when read by a TTL IC. The CPLD only has to drive the first input signal of a TTL device; all ongoing signals would be at 5v. What is more important is the signals coming into the CPLD. If its (input) pins are not 5v tolerant there will be damage, but the part I am using (XC9536XL) is 5v tolerant.

  2. #12
    Join Date
    Jun 2014
    Salt Lake City, Utah
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    Quote Originally Posted by glitch View Post
    I'd be interested in one of each. Thanks for reviving this project!

    I was over-excited to see "8008" in the title -- perhaps a moderator can correct that?
    Yep, ditto on that
    Spread the joy of Vintage Addiction


  3. Default

    Do you have any ice boards fore sale, and if so how much are they?

    Thank you,


  4. #14


    Quote Originally Posted by BloodyCactus View Post
    Im curious how well the 3.3v CPLD drives 5v TTL CPU lines and downstream TTL logic.
    Also keep in mind that old NMOS TTL only outputs ~3V anyway. Confused me quite a lot when doing my Z80 board.

  5. #15
    Join Date
    Jan 2014
    Western North Carolina, USA


    Quote Originally Posted by Svenska View Post
    Also keep in mind that old NMOS TTL only outputs ~3V anyway. Confused me quite a lot when doing my Z80 board.
    Indeed. TTL "outputs" don't actually output voltage directly.... they sink current that is sourced by the "inputs." NMOS wa designed to be somewhat compatible with TTL, but MOS inputs are not current sources, and that can complicate things sometimes. I still remember the first few weeks of the first semester of digital systems in college; long ago enough that we still breadboarded 7400-series TTL directly, rather than using one of the many University Program boards from Altera and others to do digital logic in FPGAs. Everyone in the class was pretty confused about what the professor was saying about inputs being the current sources and outputs being sinks, and how TTL outputs have so much more drive capability outputting a low than a high. It was actually several weeks into the semester before the analog bipolar transistor theory class caught up with the digital systems class, and then all of a sudden it clicked why TTL inputs and outputs are the way they are...... and why fan-out and fan-in are 'things' and need to be paid attention to.

    The analysis of the logic high output is a little different from the logic low output, but it still goes to a collector-emitter drop plus a diode drop, but that diode drop is current-dependent.... been a long time since I did the analog analysis of the totem-pole output and how it 'fast floats' the input.
    Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"


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