Logic Probe That measures HI Z impedance

[Billy Mayo]

Logic Probe That measures HI Z impedance
Which Logic Probe measures HI Z impedance

I can't find one that will display or measure HI Z impedance

Do these? or where can I find one that does?

https://www.amazon.com/Precision-DP52...ds=Logic+Probe

https://www.amazon.com/RSR-LOGIC-PROB...ds=Logic+Probe

https://www.amazon.com/Elenco-Electro...s=Logic+Probes

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Multi-meter Pens have a logic setting, but do they measure the HI Z logic level?

I think they are better than logic probes because they can display the logic voltage level, you can see the voltage display , a logic probe you don't know what HIGH or LOW voltage you have when during with CMOS can go from +5 volts to +22 volts for a High, so an Multi meter pen can display this information

https://www.amazon.com/Range-Pen-Ty...F8&qid=1389065164&sr=8-7&keywords=Logic+Probe

https://www.amazon.com/Dawson-DDM19...F8&qid=1389065201&sr=8-6&keywords=Logic+Probe

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Some logic probes have a MEMORY option button
This is for freezing logic states that are logic signals in the nanoseconds or a quick pulse or trigger signals

What else can u use the Logic Probe Memory option button for?

 

[large_ghostman]

Logic Probe That measures HI Z impedance


What else can u use the Logic Probe Memory option button for?

Its something to sit and click when you are bored, say for example when reading your own posts

 

[Billy Mayo]

It could be any of these

1.) Logic low = Zero volts
2.) Blown output = zero volts
3.) Floating output = zero volts
4.) Tri state output = zero volts
5.) Open output= zero volts
6.) Shorted output to ground = Zero volts

How can I verify which is which if they are all zero volts?

I can't tell the difference because my DVM meter or O-scope is at zero volts when I'm measuring Logic Gates, latches, flip flops, etc. so is it a logic low? Blown output? Floating output? Tri-state output? open output? shorted output? because they all can be ZERO VOLTS , so how can you tell please?

How can you verify , test, check these different malfunctions?

How do I know if the output pin is shorted to ground when it's zero volts? zero volts can be a logic low so you might think the IC chip is working
How do I know if the output pin is Open , when it's measuring zero volts? zero volts
How do I know if the output pin is Tri State output and it's working?, when it's measuring zero volts? it can be shorted to ground , or open because any of these options will measure zero volts


When Logic Signals are BELOW ground, - 5 volts, - 10 volts, - 15 volts, what is this type of logic called? it's not called negative logic

 

[Billy Mayo]

I think for measuring the floating output, you have to measuring it using an OHM meter, not a volts meter to verify that the Tri-state is Hi impedance, you can't measure the voltage of a Tri-state logic because it's NOT a voltage it's a resistance right?

 

[large_ghostman]

1.) Logic low = Zero volts
2.) Blown output = zero volts
3.) Floating output = zero volts
4.) Tri state output = zero volts
5.) Open output= zero volts
6.) Shorted output to ground = Zero volts

Did you think about any of this first???
Lets take a little peek.
1.) not necessarily for example

generally the following holds true
CMOS LOW is 0V to 1/3 VDD. so it may well not be 0V
TTL LOW 0V to 0.8V so again might not be 0V High starts at around 2V
ECL Lets not go there, you will think your meter is broken when it display a - sign

2.) yes 0V

3.) Floating means it could be anything,

4.) Tri state output, total rubbish, you need to say wich state! TRI state means
state 1 Low
state 2 High
state 3 INPUT HIGH Z!
5.) Blown and OPEN the same!

6.)

When Logic Signals are BELOW ground, - 5 volts, - 10 volts, - 15 volts, what is this type of logic called? it's not called negative logic

ECL

How do I know if the output pin is Tri State output and it's working?, when it's measuring zero volts? it can be shorted to ground , or open because any of these options will measure zero volts

Ask someone with rudimentary basic electronics knowledge.

 

[JimW]

Lots of correct info here, and a lot that is wrong.

Low level logic outputs have the ability to sink current. TTL inputs (including logic probes) that are connected to your output under test source a small amount of current. So a working low level output pin will sink that current down close to ground. Depending on the logic family and the amount of current to be drawn low, this voltage should be 0 to .7 Volts. Some exceptions to this, but a reasonable place to start.

Hi Z outputs, open outputs, unpowered outputs will not have the ability to sink the current so the probe will not (or should not if it is a reasonable design) indicate a logic low.

An output that is shorted to ground will be indicated as a logic low. No good way to know the difference unless you want to supply a higher current into the output and carefully watch the voltage. Pretty dependent on the logic family that you are working with.

A good logic probe also requires a high level output to source current before it indicates a high. So a Hi Z (tristate) or blown output and no pull up risistor should not cause the logic high LED to light. So a good probe on a tri state output should have neither that logic high or logic low LED lit.

Not every logic probe is designed well (although it is tough for even a reasonable digital design engineer to screw this up). Your mileage may vary.

JimW

 

[kubeek]

An output that is shorted to ground will be indicated as a logic low. No good way to know the difference unless you want to supply a higher current into the output and carefully watch the voltage. Pretty dependent on the logic family that you are working with.

A better way would be toadjust the inputs to the logic element in such way that it should produce logic 1 on the output. If the output then still stays at 0 then you know that something is wrong with that gate. It could be shorted output or a miriad of other possibilities inside the chip which you can´t simply distinguish.

 

[Billy Mayo]

A better way would be to adjust the inputs to the logic element in such way that it should produce logic 1 on the output. If the output then still stays at 0 then you know that something is wrong with that gate.

But I'm not checking the LOGIC HIGH, I'm checking the logic LOW

How do you adjust the input logic so know if it's a logic low? a short? an open? Tri-state or HI Z? they all will measure 0 zero volts

So how do you adjust the input logic to get a tri-state output?

 

[Billy Mayo]

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?

 

[JimW]

Sure, I'll keep playing this game.
1) IF it is a true short to ground, then you will measure zero volts relative to ground. The resistance to ground will be zero ohms, the resistance to the IC supply voltage will be the same as the resistance from ground to the supply voltage.
2) A true open to the output is quite rare, usually only part of the output driving circuit is open. But I will play along with a pure open pin on an IC. If that pin is connected to nothing else in the circuit, and the test equipment you are using is a pure passive test device, then you will measure zero volts. The resistance will be infinite to both the ground and supply rails. If the output is connected to another logic device input pin or to a pull up resistor (like most any device in a working circuit), then the voltage will be somewhere beween 0 and the supply voltage depending on the logic family. This is the same as when you connect it to a logic probe.
3)"Blown" doesn't mean anything definitive. The output is "bad", it "just doesn't work", it "has failed" all mean the same thing. Some or all of the output drive components in the IC are damaged. That may mean it can drive high but not low, or low but not high, or anything in between. I would expect some resistance to ground and to the supply plane, but not a direct short to either.
4) You didn't define where the short was to. A short to ground is a short to ground, regardless of whether it was in a tri-state condition or not. A short to another signal line is completely different (and is sometimes intentional when using tri-state. So without knowing what it is shorted to, I can't tell you what you want to know. Based on your other questions, I will assume you mean a short to ground, and then answer 1 applies.
5) An output in a tri-state condition is approaching an open circuit already. So it is close to answer 2). There is always some impedance to ground an the supply voltage, but it could easily be in the 10's of Megohms and may not be readable by common ohmmeters.
6) As in answer 5, a tri-state output is essentially an open circuit already. So answer 2 applies.

As others have tried to say, you can't know the state of an output by looking at a single static case. You will need to create test conditions and observe what happens. Put a weak pull up on the output and monitor the voltage. Put a strong pull up on the output and watch the voltage. That will help differentiate between dead shorts to ground and the other cases. Better still would be to inject short voltage pulses into the output and watch the frequency response, although this would again depend on what other components are connected to the same output.

It would help us to know what you are really trying to get answered. Is is a quiz question? Do you have a component in a circuit you are trying to test? Are you trying to build a test device? Or are you trying to test a component by itself in a static environment?

JimW

 

[Billy Mayo]

Better still would be to inject short voltage pulses into the output and watch the frequency response

,

How do u test the frequency response in Logic circuits? using what?

Inject short voltage pulses? at what voltage? what time? nanoseconds

What do i use to measure the frequency response on the output pin of a Logic IC chip?

What does testing, checking or watching the frequency response tell a electronic technician when testing logic IC chips?

5) An output in a tri-state condition is approaching an open circuit already. So it is close to answer 2). There is always some impedance to ground an the supply voltage, but it could easily be in the 10's of Megohms and may not be readable by common ohmmeters.

The Tri-state output voltage is ZERO right? but the output resistance is 10 megohms?

10 megohms from ground to the VCC of the logic IC chip? only when the output pin is OPEN or Tri-state

A short to another signal line is completely different (and is sometimes intentional when using tri-state.

Why would they use a Tri-state as a short? intentionally short u mean?

I have seen when they TIE together multiple IC tri-state output pins on a BUSS line , is this what u mean by intentional short?

 

[JimW]

You are asking questions that there are no simple answers to. Every logic family has its own characteristics of output capacitance to ground and internal impedance to both ground and other logic pins, and that will change depending on the circuit conditions that the chip is in. You still haven't said what you want to do.

You might be able to do what you want with a off the shelf time domain reflectometer. You may be able to do this with a one shot, some sort of FET drive circuit with an adjustable output drive and a digital storage oscilloscope. Depending on the logic family and the circuit will determine what you will see. Try something easy like connecting a simple oscillator through a transistor driver and connect it to the output of a chip and look at what you see with a scope. You will see a difference when connecting it to a good output vs a bad output, vs a tri-state output. The rise and fall times will vary based on the logic family and the circuit. But the answer is that YOU will have to build some test jigs, try different drivers, and make a bunch of notes. Look for the rise/fall time variations due to the output capacitance, the same test into a chip with a bad output should show something different in voltages and rise/fall times.

Tri-state outputs are never completely open circuit. So it is unlikely that the voltage is exactly zero, I would assume there is going to be some leakage that results in some small voltage (unless of course the output is actually connected to another IC). The output impedance could easily be 10 megohms isolating the output pin from the internal circuitry (ground, VCC, other outputs, all of the inputs, everything). Or depending on the logic family, it may not.

Designers short Hi Z outputs to other signals all the time. For lots of reasons. Sometimes they are bussed signals with mulitple sources and destinations. That is as good an example as any.

-JimW

 

[large_ghostman]

Sure, I'll keep playing this game.


It would help us to know what you are really trying to get answered. Is is a quiz question? Do you have a component in a circuit you are trying to test? Are you trying to build a test device? Or are you trying to test a component by itself in a static environment?

JimW

Mod Edit.... You promised.... LG

 

[Billy Mayo]

time domain reflectometer.

Never used one, what do u use this for when testing logic circuits? can you give some examples of how u use it to test and troubleshoot with it?

Look for the rise/fall time variations due to the output capacitance, the same test into a chip with a bad output should show something different in voltages and rise/fall times.

I have a BK logic pulsar , but i don't know what i can use to look for the rise/fall time variations

What do you use to INJECT on the output pin to look for the rise/fall time variations ? to test the output capacitance?

Designers short Hi Z outputs to other signals all the time. For lots of reasons. Sometimes they are bussed signals with mulitple sources and destinations.

Any other examples of using a HI Z output? or tri- state output? how it is used?

 

[4pyros]

How do u test the frequency response in Logic circuits? using what?

Inject short voltage pulses? at what voltage? what time? nanoseconds

What do i use to measure the frequency response on the output pin of a Logic IC chip?

What does testing, checking or watching the frequency response tell a electronic technician when testing logic IC chips?

You dont need to know this, It tells you nothing.