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Thread: Z80a reset pin question

  1. #1
    Join Date
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    Default Z80a reset pin question

    When unit has been off for several hours I can get a, pin#26 reset line, LOW pulse on start up followed by the line going high then nothing. On power recycle I get a fast flash of high then nothing.

    Am I right to assume I have a problem with the reset signal between the power supply and the CPU? Or could this be a bad CPU?




    The unit was able to boot on cold until yesterday but could not be turn off and started again. Now cold I get a start then video garbage slowly appear on my crt.

    Thanks

  2. #2
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    I do have exactly 5v at the PS and constant 4.44v at the reset pin.

  3. #3

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    The reset pin needs a low pulse at start, followed by high thereafter.

    The low pulse is generally done by having a resistor to 5v and a capacitor to ground. Because the capacitor takes a little while to charge, the pin will be low until charging is nearly complete.

    So, you only have 2 components to check out. Some computers like to complicate things with extra gates and so on, so you might need to consult the schematic.

  4. #4
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    If it is a 'commercial' piece of equipment you have, then the reset circuitry will be a bit more complex than a CR network.

    As a minimum, it should have some schmitt trigger inverter logic to make a nice clean reset signal. The problem with a CR network is that the voltage tends to stay in the 'invalid' region too long between 0.8 V and 2.5V (or so).

    In addition, it is usual to place a small diode across the resistor to discharge the capacitor more quickly when the power supply is switched off.

    If the Z80 contains dynamic RAM, then a reset circuit like this could destroy the contents! Most commercial machines therefore will only permit a RESET to occur at the appropriate time within the Z80 operation to preserve DRAM contents.

    But, yes, the /RESET pin should be held LOW for a short period at start-up and then go HIGH. If the /RESET line is not going high, then it is likely that the reset circuitry is at fault and you need to (a) locate the schematic for your particular machine and (b) trace the reset logic either backwards or forwards to find out why it is not working correctly.

    What machine have you got?

    Dave

  5. #5
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    All I get is a flash of HIGH and no pulse then nothing on the logic probe. The line shows 4.4v on it. It was a commercial disk video interface for a TRS-80 Model 100.

    Yes I have a low resolution schematic not great but it will help.

    Thanks for the info.
    Last edited by framer; May 18th, 2020 at 08:15 AM.

  6. #6

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    Quote Originally Posted by framer View Post
    All I get is a flash of HIGH and no pulse then nothing on the logic probe. The line shows 4.4v on it. It was a commercial disk video interface for a TRS-80 Model 100.

    Yes I have a low resolution schematic not great but it will help.

    Thanks for the info.
    Your logic probe may not catch a pulse on powerup.
    What IC is driving reset\ ?
    Dwight

  7. #7
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    I've order a assortment of caps. I did try a hard wire reset and it worked perfect. I checked the voltage check point for the reset line as instructed in a trouble shooting guide for the unit and the voltage was right on 5V.

    I have a strong suspect to replace.

    I'll post after the operation is completed.

    framer

  8. #8
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    Hi Dwight,

    A guess is sn74ls14n The only thing in the trouble shooting guide was to check the voltage on pin 10 it was right on 5V. I have not yet probed the chip.

    I do have a direct line from pin 26 on the cpu to a 1000pf cap to ground. That's what I'm planning on replacing when I get the part.

    framer

  9. #9

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    One thing that can happen is that the oscillator circuit gets old and doesn't take off as fast as it used to, from power on. Most circuits that people use do not consider a slow oscillator. The processor need a number of clean cycles under reset. This can often be thought to be a reset problem. I had a pinball machine like that.
    Dwight

  10. #10
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    It's fixed. Started this morning cooling chips with ice. I seemed to boot if I iced two chips together. But had no effect if I ice one at a time. But I noticed I was also cooling a 4.7uf electrolytic capacitor. I discharged it and the interface booted. Tried it 4 times and it worked each time. I replaced the capacitor and everything works perfect.

    It was only 3/8 of an inch away from the reset line check point and I never noticed a fine trace over to the 74LS14N.

    I got a bit of an education on this fix. I thank everyone for the help.

    framer

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