Digital circuit inputs turn on thresholds, turn on voltage?

[Billy Mayo]

How would a tech troubleshoot unexpected state transitions, or the failure to perform state transitions as expected? because of the input logic threshold voltage level?

Why would a troubleshooting tech need to know about each Logic IC chips, Logic Threshold Voltage Levels?

What kind of troubleshooting would a test technician be doing to have to know the logic threshold voltage levels?

When are they used for what use?


Here is a Chart of them:
http://www.interfacebus.com/voltage_threshold.html

[pebe]

You have put this query in the wrong section. Because it deals with components, rather than a circuit, you should have posted it under 'Components'.

That said, the link you give is pretty comprehensive. It shows the spreads of the various parameters which result from the manufacturing process. In a normal design, the various logic bits within a logic family are all arranged to blend in, and the levels you encounter should fall within the limits given for each family.

You only need to know the spread of levels if you are designing logic chips to interface with discrete transistors or other devices whose input or output levels may not match.

[Billy Mayo]

But is the Logic IC chips are causing unexpected state transitions, or the failure to perform state transitions as expected? because of the input logic threshold voltage level?

What can a troubleshooting tech do to check or test?

How do a troubleshooting tech narrow down or track down to the component that the Logic IC chips have input logic threshold voltage level has problems?

The Logic IC chips can have the same part number but they have different input logic threshold voltages, this causes unexpected state transitions or failure to perform a state transitions

But how does a troubleshooting tech find which IC chip is doing this or has this kind of problem?

[Billy Mayo]

Troubleshooting examples:

how would a tech know if the

1.) low logic state was at -1 volts with a DC offset at +4VDC , logic HIGH state was at +9 volts?
2.) low logic state was at 2 volts with a DC offset at +6VDC, logic HIGH state was at +11 volts?
3.) Low logic state was at -5 volts , Logic High state was at +5 volts?
4.) Low logic state was at -3 volts, with a DC offset at +3 VDC, logic High state was at +6 volts?

How would a tech know this, since the logic LOW state is not zero volts

Where have you seen circuits or what kind of circuits have low logic states like this with DC offset or no DC offset

[pebe]

[quote="Billy Mayo"]But is the Logic IC chips are causing unexpected state transitions, or the failure to perform state transitions as expected? because of the input logic threshold voltage level?

What can a troubleshooting tech do to check or test?

How do a troubleshooting tech narrow down or track down to the component that the Logic IC chips have input logic threshold voltage level has problems?

The Logic IC chips can have the same part number but they have different input logic threshold voltages, this causes unexpected state transitions or failure to perform a state transitions

But how does a troubleshooting tech find which IC chip is doing this or has this kind of problem?[/quote]
Checking for a faulty level is simple. Refer to your link. It shows 'good' values of Vin high (higher than Vih and = logical 1) as a green area, and shows the 'good' values of Vin low (lower than ViL and = logical 0) also as a green area. Therefore measured values in those areas are OK. Voltage levels falling in the red or blue areas are suspect or faulty. If it causes a malfunction it can either be a faulty output of one IC or a faulty input of another IC. The simple answer is to replace both components.

[Billy Mayo]

The problem is that the outputs and the logic high state is +5 volts, how can i get the voltage lower so i can test the IC logic states threshold voltage? Because +5 volts is to high to test the inputs threshold voltage. The list shows the input voltages to be very low and if the outputs are 5 volts high i can't test if the inputs threshold is in spec and working

[pebe]

[quote="Billy Mayo"]Troubleshooting examples:

how would a tech know if the

1.) low logic state was at -1 volts with a DC offset at +4VDC , logic HIGH state was at +9 volts?
2.) low logic state was at 2 volts with a DC offset at +6VDC, logic HIGH state was at +11 volts?
3.) Low logic state was at -5 volts , Logic High state was at +5 volts?
4.) Low logic state was at -3 volts, with a DC offset at +3 VDC, logic High state was at +6 volts?

How would a tech know this, since the logic LOW state is not zero volts

Where have you seen circuits or what kind of circuits have low logic states like this with DC offset or no DC offset[/quote]
Are these hypothetical cases? Because I have never encountered negative offsets in logic circuits.

Let me make it clear. The only voltage points of reference are the negative and positive supply rails for the logic ICs. Their inputs and outputs must ALWAYS be within those limits. Actual in/out voltages will normally be referenced to the negative rail, so you should never encounter negative voltages. If you think about it that way, then the charts in your link will make sense.

I have removed your second thread because it is not a separate topic. It is only a continuation of this topic.

[pebe]

[quote="Billy Mayo"]The problem is that the outputs and the logic high state is +5 volts, how can i get the voltage lower so i can test the IC logic states threshold voltage? Because +5 volts is to high to test the inputs threshold voltage. The list shows the input voltages to be very low and if the outputs are 5 volts high i can't test if the inputs threshold is in spec and working[/quote]
I assume you are testing 'in-circuit' in which case there will be both logic 1 and logic 0 present in the data. What I have already outlined should enable you to troubleshoot the circuit.

If you are talking about testing an IC removed from a board, then the manufacturer's datasheet will normally give test conditions for each IC.

[Billy Mayo]

[quote]Different logic threshold levels are usually not a problem since normal logic output levels always comfortably exceed the threshold requirements for a logic 0 and a logic 1 input.[/quote]

This problem is that the outputs and the logic high state is +5 volts, so how can i get the voltage lower so i can test the IC logic states threshold voltage "in circuit" ? Because +5 volts is to high to test the inputs threshold voltage. The list shows the input voltages to be very low and if the outputs are 5 volts high i can't test if the inputs threshold is in spec and working.

Example:

Good Logic IC output is +5 volts going to a BAD input threshold voltage IC chip which will cause the output to be On all the time

To test the Bad IC chips input threshold voltage , I need to lower the output of the IC chip before the Bad IC inputs
To test at which voltage threshold it turns ON the BAD logic IC chip

[Billy Mayo]

**I'm trying to test and check the input threshold voltage level for each IC chip "in circuit" but how can I do this please?

**I need to find out and write down at what voltage level the IC chip turn on or switches the outputs state

[pebe]

[quote="Billy Mayo"]**I'm trying to test and check the input threshold voltage level for each IC chip "in circuit" but how can I do this please?

**I need to find out and write down at what voltage level the IC chip turn on or switches the outputs state[/quote]
Quite simply - you cannot check that parameter in-circuit.

I find it more than a coincidence that you are asking exactly the same questions as Danny Davis is asking on another forum.
Could you and he be same person?

[Billy Mayo]

[quote]Quite simply - you cannot check that parameter in-circuit.[/quote]

Oh ok thanks

Is Pull down better or is Pull Up better?

Pull down sinks current, Pull up sources current?

When is it best to use pull down resistors? and when is it better to use pull up resistors?

Why would a designer choose pull down resistors instead of pull up resistors or vise versa

If a pull down or pull up resistor , shorts or opens , what happens?
1.) if a pull down resistor shorts it shorts the input or output to ground, which can change a high state to a low state
2.) If a pull down resistor OPENS , it causes a floating input or output, but what else does a pull down resistor that is OPEN do?
3.) If a pull up resistor is shorted, it shorts the input or output to the + supply voltage
4.) If a pull up resistor is OPEN, what happens? beside that the input or output is floating or has a high impedance state

[pebe]

I think you are asking trivial questions for the sake of asking, and I have better things to do that type answers to them.
Why not put your questions to your alter ego, Danny Davis?

[Billy Mayo]

it would help me out a lot for troubleshooting

[pebe]

I have given you all the information that is relevant - - - QED!