logic probing questions

Discussion in 'General Electronics Chat' started by ZWhooshdoo, Jul 16, 2011.

  1. ZWhooshdoo

    Thread Starter New Member

    Jul 16, 2011
    I am troubleshooting a Ms PacMan PCB and I have come across two scenarios that do not make sense to me. Any help would be greatly appreciated. Thank you in advance.

    1. What does it mean when the logic probe shows pulsing, and both the high and low LED's are lit simultaneously? Since the voltage obviously cannot be both high and low at the same time, would this simply mean that the voltage is oscillating too rapidly for the LED's to visually represent the changes, making them appear to be lit at the same time?

    2. What does it mean when the logic probe shows pulsing, but neither the high nor low LED is lit? How can there be pulsing when the voltage is apparently "floating" in between the high and low thresholds for the particular IC, therefore creating no high or low logic states? I red somewhere that BOTH LED's MUST be lit along with the pulse light flashing in order for the clock circuit to be functioning properly, but the game has actually functioned just fine, with neither of the LED's lit as I mentioned above. I am very confused.
  2. SgtWookie


    Jul 17, 2007
    Did you make the logic probe yourself?
    Commercially made probes frequently have a single red/green LED. If the signal is high, the LED would be red, if low, green, and if pulsing at around 50% duty cycle, appear amber.
    If you have two separate red and green LEDs, and they are both lit, you probably have a signal that approximates a square wave at greater than 50Hz (too fast for you to see the blinking).
    If it's indicating pulsing but neither LED is lit, it may be that a single pulse is at a duty cycle too low for you to see it, and at a high impedance state the rest of the time. That could occur on a tri-state bus.