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Pulses on breadboard supply lines

Discussion in 'Electronic Design' started by bonzer, Dec 12, 2005.

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  1. bonzer

    bonzer Guest

    I have been put together a circuit on solderless breadboard that links
    a z80 to my pc serial port. I ran a program that continually sends data
    from the pc and the z80 circuit echoes it back so I can see that it
    works ok. The problem is that characters being sent are sometimes being
    corrupted. If I put my logic probe on the ground lines off the
    breadboard, a pulse on the ground line is heard at the same time a
    character is corrupted. I'm assuming it's these pulses on the ground
    lines that are causing a mis-read of data. The data is only being read
    and transmitted at 9600bps.

    How can I get rid of these glitches?
  2. Is the Z80 supply bypassed with a capacitor very close to the
    processor? What other chips are involved in this circuit? What is
    the source of power?
  3. bonzer

    bonzer Guest

    The z80 does have a capacitor across the line near it. There are two
    breadboards that are linked. The first has the Z80, an eprom, some ram
    and a few logic ic's and the second board has a Zilog SIO/0, a CTC and
    max232 ic on it. The source of the power to both boards is a PC ATX
    power supply (5v).
  4. Tim Williams

    Tim Williams Guest

    Idunno, put on more caps? LOL

    Electrolytics with low ESR, like removed from switching supplies, might
    help. Not that contact resistance helps that any, but that shouldn't matter
    too much. (I've pulled 20A through them protoboard rails and it seems okay
    with it!)

  5. Is the supply clean if you load it with just a resistor that draws the
    same average current as your circuit?

    A picture or three of your assembly might be helpful. Do you have
    place on the web to post a few? Do you have an ISP that allows you to
    post pictures through the newsgroup,

  6. bonzer

    bonzer Guest

    John, the supply is clean most of the time EXCEPT when there's a LOT of
    switching activity - ie, transferring lots of data on the serial line.
    I will try to get some pics of the board(s).
  7. A picture of the scope trace that shows the supply pulse might help, also.
  8. Rich Grise

    Rich Grise Guest

    For something like this on protoboard, I've been known to use three
    or four bypass capacitors on one processor. Like a 1 uF tant. in
    parallel with a .1 ceramic from Vcc to Gnd, and another .1 ceramic
    from each power pin to the opposite bus. And the buses themselves
    usually get at least 3, preferably about 5 or 6 of different values.

    I'm sure this will start flames, but I don't believe it's possible for
    there to be too many bypass capacitors.

    I don't think I'd try to do anything faster than a typical Z80 on a
    protoboard, however.

    Good Luck!
  9. Donald

    Donald Guest

    Use a PCB !!

    Signals over 100Khz is to high for a solderless breadboard.

  10. Tim Williams

    Tim Williams Guest

    Idunno, I've ran PWM on the order of 25-50W on mine at that frequency. ;-)

    At higher RF, signals tend not to stay in the wires anyway so I don't mess
    with it to begin with ;-)

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