logic probes

[digital]

Why do commercial logic probes have two alligator clips protruding for power why can't they just use small batteries instead ?

Why must some use a switch when testing between TTL or CMOS ?

 

[fsahmed]

Logic probes need to be run on the supply that is given to the IC for proper logic signal detection.

 

[Dean Huster]

Although they're quick, small and handy, logic probes are a lousy substitute for an oscilloscope. They have trouble with undefined logic levels, will often tell you that a point is LOW when it's really 0.5 volts (which is totally unacceptable) and may tell you if there's a pulse, but nothing about the pulse. I'm only mentioning all this to say that if a person were choosing to spend $100 or more on a high-end logic probe, he might instead consider putting that and a little more into a a used oscilloscope for not that much more.

And the simple fact that the voltage on the supply leads determine the logic level points that the probe will use to determine LOW, HIGH and undefined says that there's a heck of a lot of compromise going on there!

Dean

 

[zevon8]

While I have used logic probes, 99% of the time I will use a scope, or if the situation requires or allows, one of my meters with min/max or state recording.

I have to agree with Dean, I mostly find logic probes not much better than an LED and a piece of wire. The most aggravating feature is the supposed "pulse" capture. I swear, a piezo lighter 5 feet away can trigger the pulse indicator. You spend more time clearing the pulse memory and guessing if a pulse really happened, than gaining any ground testing.