1. Short the point to ground, and take a chance on doing damage or otherwise invalidating the test.
2. Cut traces to isolate the point from any outputs and then short it to ground.
3. Lift a pin from a chip, similar to cutting a trace.
This one: https://www.electro-tech-online.com...n-measuring-on-current-circuits.139371/page-7Audioguru, can you tell me the name of the other forum, so I can avoid it?:grin:
Frank
Danny, some of your circuits have TTL and some have Cmos that is COMPLETELY DIFFERENT. Are they mixed on the same circuit board?How can u tell between a logic low and a dead output?
i can't tell when using an o-scope, how do u guys tell please?
What do you mean by "dead"? Shorted to ground? Shorted to the positive supply? Disconnected?
A TTL input needs up to 1.6mA to make it low so when it is connected to a disconnected output it floats high.
A Cmos input draws no current so when it floats it could be low, high or in between.
be 'pulled' to one logic level or the other with a resistor
0.5pps means it takes 1 second for half a pulse, so it will take 2 seconds for a complete pulse.
I thought we were talking about output voltage levels, not input voltage thresholds. And a logic probe doesn't tell you which is which. The switch just lets you change the threshold level that the probe uses to decide the high or low state. But the voltage that it is looking at has to come from the OUTPUT of the driving device.
.The switch just lets you change the threshold level that the probe uses to decide the high or low state
I have a pull down resistor connect to a CMOS gate so it defaults at a Logic Low
Are these called pull down resistors?
The output may well be high, as I said, the logic state will be indeterminate, it could be high or low. The OUTPUT is what the logic function tells it to be based on the signals at the input pins. If by chance the input seems to be in the right state for the output to be high, that's what it will be.At work I have circuit boards that have a Floating Cmos inputs, but the outputs of the Cmos gates are HIGH
It's really weird how the circuit board works when there is a Floating input on the CMOS , but it won't work when I apply a logic HIGH or LOW , it only works when I apply a Floating input
How I apply a floating input is i disconnect or lift up a component or pin that is driving the Cmos input , than the circuit board works
Of course.You LOOK ALSO
http://www.barrytech.com/tektronix/probes/tekp6401.html
Low state volts is 0.7 volts
http://www.testmart.com/webdata/mfr_pdfs/INST/INSTEK_GLP-1A.pdf
TTL low logic state is 0.75
NO.TTL circuits typically work on 5V supplies. A probe set to check TTL levels might pick 2.5V as the threshold. The actual levels it would expect to see are more likely to be 0.5V and 4.5V so it can clearly see if these are above or below 2.5V.
Brian.
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