Partial Z-80 computer - Something seems wrong.

Discussion in 'The Projects Forum' started by dag10, Oct 24, 2009.

  1. dag10

    Thread Starter New Member

    Oct 10, 2009

    I'm building a Z80 minicomputer on wire-wrapped perforated board. I just finished wiring the address bus and data bus pins to input pins on buffers (A 2-way buffer for the data bus). I connected all unused inputs to +5v, and created a single-step clock. My voltmeter shows that the power supply is +5.01v.

    At this point I powered on the computer, took it out of reset, and stepped a couple times, and checked the output of the pins with the voltmeter. They seem odd.

    I would normally assume that the address bus value would be incrementing, as the input from the data bus is 0x00000000 (NOP). However, the output of the pins are usually inconclusive, being some weird value between 0v and 5v (usually 1 or 3 something volts). The values of unused outputs and the address bus output doesn't change, whether in reset or not.

    Also, the 5v regulator and the center of the Z80 gets really hot after a couple minutes. If I press my finger on the center of the Z80, I can only hold it there for 10 seconds because it is so hot.

    Is all of this normal, or is something wrong? If you'd like, I can send pictures of the front/back of the circuit, or I can find the output of every pin and list them.

    The schematics are Donn Stewart's Z80 computer. I currently have wired up everything in the following pages:

    Images Copyright Donn Stewart

    I also made a little change to the active pin on the Address LO chip, and attached that to a SPSD toggle switch, with it being connected to ground when off. This will allow me to take manual control of the lower 8 bits of the address line when I want to manually reprogram the RAM. Currently it will be 0v, like the schematics say. Also, the clock input on the Z80 is directly attached to the step button.

    It seems like it is really wrong, and the heat doesn't seem normal.

    Any insight into this would be great! :)

    Last edited: Oct 24, 2009
  2. bertus


    Apr 5, 2008

    The links do not work.
    You can put the images here local using the Manage Attachment button.

  3. beenthere

    Retired Moderator

    Apr 20, 2004
    The heat and indeterminate output levels says something is very wrong. If you simply tied those inputs to +5 volts directly when you
    - that might be part of the problem. The Z80 might have tried to pull one of those inputs down (you missed a pin number), and the direct connection fried the IC.

    It's better practice to use pullup resistors to establish high levels on unused inputs. Something on the order of 20K should work. But you may need a new Z80.
  4. Wendy


    Mar 24, 2008
    Last time I shopped the vendor had them for 50¢ each, but that was over 10 years ago.
  5. dag10

    Thread Starter New Member

    Oct 10, 2009
    Yeah, fried chip makes sense.

    Could you clarify what you mean by pullup resistors? Are you saying that I just use a 20K Ω resistor to connect unused (active high) inputs to +5v?

    Yeah, I got mine for $1.10
    Last edited: Oct 24, 2009
  6. hgmjr

    Retired Moderator

    Jan 28, 2005
    Yes. That is what Beenthere is suggesting.

  7. dag10

    Thread Starter New Member

    Oct 10, 2009
    Thanks, I'll try that. I'll double check my pin wiring too. I'll also order a bunch of Z80's. :p