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need help on using a logic probe TIA

Discussion in 'Electronic Basics' started by Sierra, Feb 24, 2004.

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  1. Sierra

    Sierra Guest

    I'm looking for sufficient knowledge before using a logic probe model AT-301
    (no instructions for using this probe is available) to troubleshoot on a
    monitoring cct board. The board comprises mainly of 40 series logic gates
    and buffers driving small transistors based on input condtions. I have all
    data sheets showing truth tables and all other data of chips.

    The logic probe has a Red led for HI, a green led for LO and a yellow led
    for PULSE. A switch for TTL & CMOS. A switch for MEM and PULSE. and its has
    a Black and Red lead terminated to banana clips.

    Can anyone tell me how to use this probe - like which lead is to be
    connected to where and switch positions while attempting to locate faulty
    chips.

    TIA
     
  2. CFoley1064

    CFoley1064 Guest

    Subject: need help on using a logic probe TIA
    Basic logic probes are easy. Connect the red lead to a portion of the circuit
    that has the + voltage for the CMOS ICs (don't connect this to a logic output
    -- most logic probes require 20 to 40 mA to operate -- be sure the circuit
    under test can handle the extra loading), and connect the negative wire to
    circuit common or GND. If you've got an axial cap at the output of the logic
    power supply voltage regulator, that's just perfect, otherwise, hunt around.
    Make sure not to do this bass-ackwards -- you might let the smoke out. Set
    your switch for TTL or CMOS, depending on what you're testing. Now you're
    ready to use the logic probe.

    Place the metal probe on the IC pin to be tested. If it's logic high (usually
    more than 70% of Vcc if CMOS, more than 2.4V if TTL), the red HI LED will lite.
    If it's logic low (usually less than 30% of Vcc if CMOS, less than 0.8V if
    TTL) the logic LO LED will lite. Every time there's a logic transition
    (usually from LO to HI) the PULSE LED will lite for half a second or so.

    If you don't get any lite, that means the logic output is in an intermediate
    state, which almost always means bad news on the board -- either that, or one
    of the power supply gator clips came loose 8o).

    The only problem I have with most logic probes is that the probe itself is
    usually a little clunky -- not so convenient for SMT small pitch ICs. Watch
    out for shorting pins to each other with the probe, too. It's safer to probe
    with your elbow resting on something to keep things steady.

    If you haven't got a scope handy, the logic probe can be the next best thing,
    and it fits in your small tool satchel, so you can take it anywhere.

    Good luck
    Chris
     
  3. Sierra

    Sierra Guest

    Chris,
    Thanks a lot for guiding me. Still there's a bit I don't know. What is this
    switch on the probe for MEM and PULSE? Is that switch of any use while
    testing?
     
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