Forcing a Logic HIGH, to a low state. How can I do this?

[danny davis]

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.

Is there any other ways?

1.) Can I turn the POWER OFF on the circuit
2.) Inject an external power supply#1 on the VCC to ground rails, of the same supply voltage as when the circuit is ON
3.) Then I can use another external power supply#2 to inject logic High levels on the inputs

Don't you think this approach will work?

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I do need to verify if the Flip flops are working too, which I need to Inject a voltage level to the SET and RESET pins so I can make sure they are storing the logic levels and outputting them compared with their truth table

Bottom line I'm trying to Verify each Logic IC chip one by one with the power off but trying to Inject it with two different power supplies

 

[betwixt]

'Danny' still doesn't read what I wrote earlier.

I think this is leading to one of those "which is the best way to stop a car?" questions, is it to drive it forward into a wall or reverse it into a wall?

Brian.

 

[Audioguru]

There is a much longer thread about this on another website forum where Danny calls himself Billy.
He admitted here and there that he "does not know and has no idea" about the expected logic inputs and outputs and has no Known Good Board to compare with.

 

[chuckey]

Audioguru, can you tell me the name of the other forum, so I can avoid it?:grin:
Frank

 

[Audioguru]

Audioguru, can you tell me the name of the other forum, so I can avoid it?:grin:
Frank

This one: https://www.electro-tech-online.com...n-measuring-on-current-circuits.139371/page-7

 

[danny davis]

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?

 

[betwixt]

A logic low lies on it's belly, close to the ground and looks up at you. A dead output lies on it's back with it's legs pointing upwards and has a glazed look in it's eyes.

Brian.

 

[danny davis]

Does using a logic probe tell when it's a low logic state and a dead output? Can i can't tell using an o scope , would using a logic probe be better?

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what is the difference it millivolts from a low logic level from a dead output at zero volts? because a low logic level is not at zero volts? it's in the millivolts right?

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what is the difference it millivolts from a low logic level from a dead output at zero volts? because a low logic level is not at zero volts? it's in the millivolts right?

 

[Audioguru]

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?

Danny, some of your circuits have TTL and some have Cmos that is COMPLETELY DIFFERENT. Are they mixed on the same circuit board?
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.

 

[danny davis]

What do you mean by "dead"? Shorted to ground? Shorted to the positive supply? Disconnected?

I mean the IC chip internally has a dead output, or dead output stage inside the IC chip which outputs a zero volt potential , it's not a short or open or hi Z , it's just zero volt potential

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.

When A Cmos input is floating , how do you measure it with a DVM meter? or do I use the ohms range?

Cause there is floating low, floating high and inbetween

Floating Low is different than a Logic Low , what's the difference?

Floating High is different than a Logic High, what's the difference?

When Either A Floating Low or High measures a voltage on the input or output of a Gate might trick you because it measures just like a real logic level voltage but it's floating which means it's not referenced to ground or so it's floating the input pins which tricks the Gates to output a result

I have many circuit boards like this at work that I can't find out why something is not working is because there is floating inputs and outputs that trick the logic gates , flip flops ,etc

Hi Z means high impedance so I use my DVM meter set on ohms range right? to measure a logic Hi Z state?

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Because TTL and CMOS have different Logic LOW logic voltages
They do? That is news to me. I've always found both of their outputs to be zero volts. As for telling which family a part belongs to, you just look at it's part number.
TTL logic Low is 0.75 vdc
CMOS logic Low is 15% of VCC or for 7 volts to 18 volts it's 40% of VCC

A Logic Probe detects the difference between the two because a Logic probe has a TTL /CMOS switch

An Oscilloscope for me is to had to tell because i have my O- scope input channel .5 or .1 on the knob and it picks up probe noise, O -scope is displaying so much noise because i'm trying to measuring millivolts when measuring logic LOWS.

When you trying to measure millivolts on the O-scope , it picksup to measuring noise and it display is hard to measure how much voltage is the Logic LOW , millivoltage at

And there is one advantage that a probe has, in that it can differentiate between a LOW and a HighZ state.
THANK YOU for notice this too

On a O-scope I can't tell the difference between a Low state, is the output dead , or a HighZ state = floating

A Floating input or output = High Z = Tri-state

or is a Floating input or output a High Z?
or is Tri-State different than a High Z?

 

[Audioguru]

Billy or Danny.
As I said a minute ago on the other website forum, your spec's for TTL and for Cmos logic low voltages are wrong. Look at their datasheets then learn this simple stuff.

 

[danny davis]

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

 

[betwixt]

Billy/Danny - firstly, can you give us a real name to call you and please tell us which country you are in, it will help us give advice on local suppliers.

You seem to completely misunderstand logic levels and how logic circuits work. I know you turn to us so you can learn about them but even when we tell you, you ask the same questions again and again.

There are ONLY two logic levels, high and low, sometimes called '1' and '0'. The third condition, which we call 'floating' is not a logic level, in fact it means the level could be anything because nothing is driving it.
Logic gates produce a logic output level which depends upon the logic levels at their inputs. The different types of gates use different combinations of input levels to produce ther output level. For more on this, look up "truth tables" which will explain how the inputs and outputs are related to reach other.

The different families of logic ICs have different voltage thresholds at their inputs and produce different voltages at their outputs. There is no particular voltage which is a high input or a low input, there is a middle voltage and anything below it is seen as a low and anything above it as a high. This threshold voltage is different from one device to another but the idea is that whatever drives the input will make it go much lower than the threshold or much higher than the threshold so there is never any doubt about the logic state it is supposed to be.

A floating input to a logic gate will make the output unpredictable because it can't tell if the pin is high or low. The voltage on a floating input tends to drift and pick up noise which makes the device unsure about what it is supposed to do. This is why the input to a logic gate should never be left unconnected, it should always be driven from another IC or be 'pulled' to one logic level or the other with a resistor.

The output of a logic gate can sometimes be made to float. This is possible when, and only when, the device has an 'output enable' pin. That pin works like an isolating switch inside the IC package, it either connects the logic output to the pin (enabled) or disconnects it so the pin has nothing driving it (disabled). The reason we need that facility is for circuits where more than one logic output pin is connected to one logic input pin. If the outputs were enabled at the same time and one was driving high while the other was driving low the resulting 'fight' would result in an unpredictable level. A well designed circuit would make sure that only one device was enabled to drive that point in the circuit at any time so avoiding any argument.

Brian.

 

[danny davis]

be 'pulled' to one logic level or the other with a resistor

I have a pull down resistor connect to a CMOS gate so it defaults at a Logic Low

Are these called pull down resistors?

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

It doesn't make sense to me how a floating input will make the circuit board work but not a Logic High or Low

The Cmos Logic Gate does go to a Cmos Decoder to a Cmos Flip Flop to a LIGHT

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0.5pps means it takes 1 second for half a pulse, so it will take 2 seconds for a complete pulse.

How do you measure the PPS of a pulse? so you measure Half the pulse that is in 1 second? how do u do that and why do you want to measure half the pulse in 1 second i don't get it

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.

So does the Logic Probe TTL/CMOS switch do anything, since the driving device output voltage levels are not the threshold levels?

The switch just lets you change the threshold level that the probe uses to decide the high or low state

.

But the Driving Device output are not the threshold levels so the Logic probe switch does nothing to tell you if it's thresholds are good or not right?

 

[betwixt]

I have a pull down resistor connect to a CMOS gate so it defaults at a Logic Low

Are these called pull down resistors?

Yes, and if the resistor is connected to the supply instead itis called a 'pull-up' resistor.

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

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.

If the circuit ONLY works when the input is disconnected (floating) there is either a design problem or more likely, the signal you isolated is a clock and instead of being the correct waveform, it is now random but nevertheless able to drive circuits further down the line. They won't work properly but the presence of changing logic states they will do something rather than nothing.

PPS = Pulses Per Second, it's measure of frequency like Hz is cycles per second. 0.5 PPS means that 1 PPS takes two seconds. You can't measure half a pulse in one second because within that second you can't tell when the pulse finishes. You can still say it's 0.5 PPS if you look at the bigger picture of the whole pulse taking 2 Seconds.

Logic probes do not test the actual threshold voltage, you need special eqipment for that. All they tell you is whether the voltage is much lower or much higher than some mid-range voltage. For example, 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. A similar probe set for 15V CMOS levels would be useless on TTL as it would assume anything below about 7.5V (15V/2) is logic low, even a TTL high at 4.5V would be be below 7.5V and it would always see both logic levels as being low. Probes designed to work on different logic families therefore have adjustable (or pre-set) thresholds to cater for the differences in expected high and low level voltages.

Still haven't told us your name or whereabouts.

Brian.

 

[Audioguru]

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

Of course.
Because the datasheets for TTL ICs say that their maximum input low voltage is 0.8V.
The maximum output low is less at 0.4V to allow for 0.4V of noise.

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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.

NO.
The minimum valid logic high for 74xxc TTL is 2.0V. The maximum valid logic low input is 0.8V. You cannot say there is a threshold voltage that is halfway between them.

 

[betwixt]

I was giving an example!

True, for most bipolar 74 series TTL testing it should be half way between VL(high) and VH(low) or about 1.4V. Actual logic levels would normally exceed that anyway. It varies from one family to another as I stated and in many commercial TL probes, a 74xx gate is used as the input sensor anyway, so it would adopt the levels appropriate to that device.

Brian.

 

[danny davis]

why would anyone want to know the pps of a pulse? when is this pps measurement used mostly?

so to measure the pps of a pulse , i measure 2 seconds of a pulse waveform? isn't 2 seconds the period? or how many cycles in seconds?

 

[danny davis]

1.) At work I troubleshoot Logic circuits that are hard for me because if you lift up an IC Pin or lift up a leg off of a resistor, cap, transistor or anything , it will the circuit in different stages or branches "floating "

So you can't lift up a resistor , cap, transistor , pin to an IC chip without turning the stages and branches FLOATING , plus it changes the DC voltages or logic levels

What kind of problem is this called?

2.) Also when I take out a IC logic chip and put a new IC logic chip in the logic circuit board it will turn on different lights and give different voltages through out the circuit , If I put in a different IC logic chip with the same part number and everything, the Logic circuit is very sensitive when removing and putting in the same part number IC chip.

What kind of problem is this called?

3.) When I put in an New IC logic chip with the same part number , it will cause more problems, problems maybe that was there before the IC chip was bad, you turn on the circuit board and there is 3 more problems than before with the Old IC logic chip.

What kind of problem is this called?

4.) For Logic Circuits that has Busses, Enable busses, Reset busses, Set busses,

a.) I have a buss at work that when you turn on a switch , it makes the Logic Buss for +15 volts to -13 volts back to +15 volts, What is this called? What kind of circuit is it called that flips the voltage back and forth like this in steps?

b.) Also the Bussing problem is about, there is resistors and diodes connected to this buss that goes to different channels to turn on different lights .
My question is that when I lift up a resistor or diode that voltages on the buss will changes and also the voltages on the different channels that are connected to the buss line will have different DC voltages.

What is this called when a circuit does this? and what kind of problem is this called?

 

[betwixt]

1. The components are there for a reason, it's hardly surprising something changes when you disconnect them. It isn't a problem, you are creating a situation that the designer prevented by fitting the parts in the first place.

2. Assuming you are fitting the same type as a replacement, it could be two things - either the IC has different characteristics, they do vary from batch to batch and from one manufacturer to another or, the IC is a programmed part (ROM, PLD, PAL etc.) and the one you are fitting hold different coding.

3. It's called bad design. The circuit is intolerant of small parameter changes.

4. We can't really tell without seeing a specification but those voltage do seem excessive. Most logic bus structures use the logic levels of the ICs so in most cases today it would be 5V or 3.3V. I have never seen a logic bus that changes polarity in the 40+ years I've been working with logic circuits.

I'm concerned about how you are taking the voltage measurements, as almost everything you measure seems to be unusual. Please explain exactly what instruments you are using and how you have them connected when you take these readings.

Brian.