Logic Probe that measures HI Z impedance

[betwixt]

Re: Logic Probe That measures HI Z impedance

Oh dear - back to square one. :|

Yes, you can do that but only if the power is turned off - so how do you know if the device is being driven or not?

Please listen to what everyone is telling you:
1. There is no such thing as a floating input. It would be a design error, not a fault condition.
2. The ONLY circumstances where an output should ever float is if the pin is tri-stated. That is an intentional state, it does not happen by accident. You should see if the conditions to make the pin tri-state are present to see if the device is working properly or not. If it is being told to be tri-state yet still producing output the device is faulty. If it isn't being told to be tri-state and it is producing no output the device is faulty.

An Ohm meter is not a logic analyser and never will be, in fact connecting one to a live circuit could damage that circuit or the meter.

Also consider WHY the output of a device may have a built in tri-state output. It is so more than one output can be joined together without them "back driving" each other. In that case, it isn't unreasonable to assume that somethng else is connected to the tri-state output and you could be measuring that instead of the pin you think you are.

Brian.

 

[danny davis]

Re: Logic Probe That measures HI Z impedance

You should see if the conditions to make the pin tri-state are present to see if the device is working properly or not.

A tri-state Condition will measure the same as an OPEN , BLOWN, SHORTED , it will be zero volts, so how do I know if it's a logic low state, or an OPEN, or BLOWN, or shorted condition?

How do you know if the Tri-state is working properly? what do you do to verify this? what tests and check do u do?

If it is being told to be tri-state yet still producing output the device is faulty. If it isn't being told to be tri-state and it is producing no output the device is faulty.

A tri-state Condition will measure the same as an OPEN , BLOWN, SHORTED , it will be zero volts, so how do I know if it's a logic low state, or an OPEN, or BLOWN, or shorted condition?

How do you know if the Tri-state is working properly? what do you do to verify this? what tests and check do u do?

Also consider WHY the output of a device may have a built in tri-state output. It is so more than one output can be joined together without them "back driving" each other. In that case, it isn't unreasonable to assume that somethng else is connected to the tri-state output and you could be measuring that instead of the pin you think you are.

Yes true, they do BUSS or have a BUSS LINE tieing all the tri-state outputs together or join tri state outputs together

So how would u troubleshooting this? without lift each output pin or cutting each buss line one by one until you find the fault?

 

[betwixt]

Re: Logic Probe That measures HI Z impedance

A tri-state Condition will measure the same as an OPEN , BLOWN, SHORTED , it will be zero volts

Nonsense!

So how would u troubleshooting this? without lift each output pin or cutting each buss line one by one until you find the fault?

Sometimes that is the only way. In general, if a bus voltage appears to be wrong and you are not sure which of the devices driving it is responsible, the strategy would be to enable each device in turn and drive it's output high and low. There would normally be some change in the bus voltage as the good devices did their job but no response from the one that was 'stuck'.

Brian.

 

[danny davis]

Re: Logic Probe That measures HI Z impedance

A tri-state Condition will measure the same as an OPEN , BLOWN, SHORTED , it will be zero volts
Nonsense!

It's true, what am I doing wrong or how can i tell the different , or how can i tell if it working please?

the strategy would be to enable each device in turn and drive it's output high and low. There would normally be some change in the bus voltage as the good devices did their job but no response from the one that was 'stuck'

How do you drive it's output high or low? using a Logic pulser? or what would u do or use?

 

[betwixt]

Re: Logic Probe That measures HI Z impedance

If you have an IC in plastic box or a metal can with an output wire sticking out of it, and the voltage on that wire is not as expected for the conditions at the ICs inputs, it is faulty or something else outside the IC is responsible. If you really need to know what has gone wrong INSIDE the IC you have to dismantle it and view it through a microscope.

You drive the output of a logic gate high or low by providing it with the appropriate conditions at it's input pins to give that output. I can't understand your obsession with pulsers and probes, they can be useful tools but 99% of time you can diagnose digital faults without them.

Brian.

 

[danny davis]

Re: Logic Probe That measures HI Z impedance

Y

You drive the output of a logic gate high or low by providing it with the appropriate conditions at it's input pins to give that output

Ok just to make it clear, I'm not testing or checking the LOGIC HIGH state/condition, I"M testing, checking, verifying the LOGIC LOW state and also the TRI-STATE output

I Supply the Logic conditions on the input pins to test the LOGIC low state output, but how do you know if it's a logic low? a short to ground? an open? blown output?
What can I do to the logic conditions on the input pins to test the logic low states output to know it's working?

Let me ask you this:

1.) If I or you supply the logic conditions on the input pins to get a

LOGIC low

on the output and the IC chip had a SHORT to ground on the output pin, what would the output MEASURE in DC voltage and it's output resistance?

2.) If I or you supply the logic conditions on the input pins to get a LOGIC low on the output and the IC chip had a OPEN on the output pin, WHAT WOULD the output MEASURE in DC voltage and it's output resistance?

3.) If I or you supply the logic conditions on the input pins to get a LOGIC low on the output and the IC chip had a BLOWN on the output pin, WHAT WOULD the output MEASURE in DC voltage and it's output resistance?

4.) If I or you supply the logic conditions on the input pins to get a

TRI-state condition

on the output and the IC chip had a

SHORT

on the output pin, WHAT WOULD the output MEASURE in DC voltage and it's output resistance?

5.) If I or you supply the logic conditions on the input pins to get a

TRI-state condition

on the output and the IC chip had a OPEN on the output pin, WHAT WOULD IT MEASURE in DC voltage and it's output resistance?

6.) If I or you supply the logic conditions on the input pins to get a

TRI-state condition

on the output and the IC chip is WORKING, nothing wrong with it on the output pin, WHAT WOULD the output MEASURE in DC voltage and it's output resistance?

 

[betwixt]

Re: Logic Probe That measures HI Z impedance

OK, lots of silly questions but here is the answer:

1. a short to ground implies a connection between the logic signal and the ground so the answer is 0V.
2. could be anything. It depends ENTIRELY on the circuitry connected to the pin.
3. depends on what you mean by 'BLOWN', again it could be anything.
4. current through a short would override anything the IC could produce so the answer is the same voltage it is shorted to.
5. could be anything. It depends ENTIRELY on the circuitry connected to the pin.
6. could be anything. It depends ENTIRELY on the circuitry connected to the pin.

PLEASE,PLEASE,PLEASE, absolutely, completely, utterly and forever - FORGET MEASURING LOGIC SIGNALS WITH AN OHM METER. It is not the right tool and never will be. It's a bit like using a measuring jug to test air pressure, it gives no meaningful results whatsoever and the voltages it uses to test resistance can damage the circuit you are connecting it to.

Brian.