Digital circuit inputs turn on thresholds, turn on voltage?

[KeepItSimpleStupid]

OK, rise time is usuaslly defined from 10% to 90% of the steady state excursion.

When there is an RC circuit, the waveform is exponential.

When a cap is charged with a constant current source the result is a linear ramp.

The "straight line" you are seeing may just be an artifact of the scope set at Auto trigger or the sweep speed isn't slow enough, so it isn't a line.

Rise-time/Fall time and reset pulse are probably all appropriate.

 

[Billy Mayo]

The "straight line" you are seeing may just be an artifact of the scope set at Auto trigger or the sweep speed isn't slow enough, so it isn't a line.

What trigger should I use than?

So you're saying I will see a Slope? instead of a straight line?

 

[Billy Mayo]

I hit the AUTOSETUP button on my digital oscilloscope

do you want me to change the trigger to what and change the time speed to slower on the oscilloscope?

 

[KeepItSimpleStupid]

Most scopes have what's called auto-trigger. The purpose is in the absence of a valid trigger signal, the scope triggers. So, if you had a waveform that went from 0 V to 5 V in 10 sec and were set to the 1 ms/div setting horizontal and there were 10 div across the screen, then every 10 mS+, the scope would trigger a line.

Put a voltmeter to the signal and attempt to time it and then pick a time base for that.

Hopefully, if you slow the timebase down, you will actually see the time varying ramp.

 

[Billy Mayo]

Put a voltmeter to the signal and attempt to time it and then pick a time base for that.

What kind of stop watch does milliseconds or microseconds?

So, if you had a waveform that went from 0 V to 5 V in 10 sec and were set to the 1 ms/div setting horizontal and there were 10 div across the screen, then every 10 mS+, the scope would trigger a line.

Is the formula 10 secs x 1ms/div = 10ms?

 

[Billy Mayo]

How is a logic pulsar different than a function generator? Does a logic Pulsar do anything different that it's ment for logic troubleshooting? how do you use a logic pulsar for troubleshooting TTL and CMOS logic IC Chips please?

 

[KeepItSimpleStupid]

slow rise time

You said it took a while, something observeable. So if it took a few seconds, you might see that on a voltmeter. If not, my mistake. there exists in many frequency counters a timer/counter mode of operation, but I think that's beyond the scope here.

Can you operate the DSO in X-Y mode? Turn off the time base in the X-direction.

auto-trigger

Hypothetically, I just said suppose your scope had 10 major divisions across the screen and each MAJOR division was 1 ms.
So, without a valid trigger, the scope would create a trigger because it can't trigger on slope or level or external.

Logic pulser
Let's take a look at one: https://www.electro-tech-online.com/custompdfs/2013/03/LP425K.pdf

It's A LOT different than a function generator.

Think of it as a device that can override an input very briefly which means it can "force" an input to briefly change state.

A function generator would end up frying the components unless used at a real input.

If you, had say a simple NAND gate and you wanted to know if it were functioning, you could look at the output with a logic probe which can capture the pulse and inject one with the pulser.

 

[Billy Mayo]

Thank you for you info

It's A LOT different than a function generator.

How it is different than a function generator? you can set the function generator has a square waveform or pulse at 10us just like a logic pulsar right? a pulsar is set at a very low frequency 500hz at 10us I think

Think of it as a device that can override an input very briefly which means it can "force" an input to briefly change state.

How can It override an input? do you mean if there was an input already there also?

Why can a Logic Pulsar override an input and force an input to change states but a function generator can't?

A function generator would end up frying the components unless used at a real input.

Why would a function generator fry the components? and a logic pulsar wouldn't

 

[KeepItSimpleStupid]

Ok, take a look at fig 6 here: https://www.electro-tech-online.com/custompdfs/2013/03/TTL.pdf

Think of a fig 6 feeding another fig 6 feeding another fig 6 and you want to inject something after the first fig 6.

Now back to fig 6 again, picture running an AC source (function generator) into Vout. It might not be pleasant for the function generator or the gate. The function generator doesn't normally like to see DC at it's input and the output isn;t going to take continuous current in the wrong direction. The output devices will heat up.

I'll use an analogy. A fuse. Let's even pick a 1 A fuse. So, let's pick a 3 V voltage source, a 1 ohm resistor in series with a 2 ohm resistor. Let's pretend I can short out the 2 ohm resistor. That would cause a 3 A current to flow and the fuse would pop using conventional wisdom.

Now, suppose I have this fancy circuit (logic pulsar in our analogy) and i can short the resistor for only 1 millisecond. Will I blow the fuse? No. Would an oscilloscope detect a voltage change across the resistors? yes.

Now if I took a 3V Voltage source (The function generator in the analogy) and placed it across the 1 ohm resistor for a long time, what might happen? the function generator and/or the circuit would fail.

I hope that makes sense.

IC's, transistors and resistors typically fail when their substrates can't take the heat away fast enough. You could probably put a 1/4 Watt resistor into liquid nitrogen and have it dissipate 1 W through it because there is sufficient cooling with no damage.

 

[audioguru]

It's not a slope/ramp, its a straight Line ( DC voltage )

When you press the active low reset button, the straight line ( DC level ) goes down to zero volts straight line, when you let go of the active low reset button, the straight line ( DC level ) starts rising up still is a straight line rising up at a different voltage very slowly until it reaches + 5 volts

A slope is a line that rises up.
The voltage of a charging capacitor rises up, making a slope.

 

[Billy Mayo]

The logic pulsar current output is 0.4mA i think? The function generator would have to my current on the output that would damage TTTL and CMOS chips right?

 

[KeepItSimpleStupid]

I think the datasheet of the kit I referenced said 100 mA, but the pulse is very brief.

The whole point is that the pulses are not continuous.

 

[Billy Mayo]

So continuous pulses damage TTL and CMOS chips? The logic pulsar has pulses that are not continuous? The pulses look like they are a pulse train

 

[KeepItSimpleStupid]

The pulses from a logic pulser are of a very short duration and of high enough drive to override an output.

We are talking about applying a signal to the output of something. Generally, that's not a good thing to do. Examples: Paralleling two identical power supplies to double the current. Grid tie inverters also tie and output voltage to an output voltage.

Same deal here. A special case. Function generators are OK at the inputs, but not the outputs of a logic circuit.

I guess you can say that a "logic pulser" is the poor man's equivalent of a Mixed Signal Oscilloscope or Logic analyzer.

Just like you can use an ohmmeter to find a shorted transistor junction and the "diode test" is better and a curve tracer out of circuit is the best.

What can be a very useful tool for fixing large TTL based systems is a tone milli-ohmmeter. Just picture one TTL IC shorted out of 30 or so on a board. How might you find it?

 

[JimB]

Just picture one TTL IC shorted out of 30 or so on a board. How might you find it?

Been there, done that! (on a smaller scale).

After all these years I cannot rmember the exact function of the equipment, but the +5v rail was being pulled low, to 2 or 3 volts.
The problem had to be in one of five or six ICs.
So after carefully removing them all from the circuit board, the problem turned out to be the +5v regulator!
Damn!!!

JimB

 

[KeepItSimpleStupid]

JimB:

I think my worth TTL troubleshooting was on a Auger Spectrophotometer which had a few wire-wrapped commercial boards. The color on the display screen started to get intermittent and unfortunately one of the colors that comprised an indicator.

Turned out that the +5 and ground pins were soldered on the IC's. On one IC it wasn't soldered at all and it took like 7 years to show up.

I do remember using my tone ohmmeter for one particular problem. That's it. One. It was invaluable though.

Rule one: Check the power supply and ripple. OK, maybe rule #2. make sure it's plugged in. I do remember a support call for a modem/DecWriter deal and i asked if it was plugged in and got a response of "No!" I saw the results of not removing flux from a transistor too. You learn from someone else's mistakes.

 

[audioguru]

I think my worth TTL troubleshooting was on a Auger Spectrophotometer which had a few wire-wrapped commercial boards.

My first digital circuits job was installing and repairing Philips "office computers" in about 1965 (I think). The circuits were all wire-wrap and the logic was DTL.
The program was fed in with punched cards. The RAM memory was many ferrite donuts on many wires (core memory). No microprocessor.

I designed a simple circuit to fix a BUG in the machine and Philips awarded me with a Philishave electric shaver.
The shaver is 48 years old and still works fine. I will use it in a few minutes so I look civilized at my daughters Easter feast.
Happy Easter!

 

[Billy Mayo]

Why is it bad to put +5 volts on the output of a TTL or CMOS chip? don't they output +5 volts HIGH signal

So I should Inject the +5 volts NOT on the output but a near by component after the output?

What you mean by the opposite voltage level? it's either zero LOW or +5 HIGH

If I apply +5 volts to a LOW signal that will cause a SHORT?

 

[audioguru]

You should look at the datasheet of an old TTL IC or LS TTL IC.
Its output never goes as high as +5V, it goes high to a minimum of +2.4V for TTL or +2.7V for LS TTL but typically to +3.4V.

The low output of TTL goes down to about +0.4V with a max current of 16mA. If your "logic pulser" tries to force a TTL low output to +5V then the output might be destroyed unless the pulse is for a very short duration.

If a Cmos logic IC has a 5V supply and has no load or if it drives another Cmos input then its output goes to +5V.

 

[KeepItSimpleStupid]

audioguru is saying exactly what I said: "the output might be destroyed unless the pulse is for a very short duration.

For fun, I'll bring in a Math concept of Average: https://www.electro-tech-online.com/custompdfs/2013/04/Lesson8.pdf

The issue is Average power of the pulse. You know P=V*I and all of the other definitions of power. Energy is the definate integral of ∫P dt from t-0 to t=n. or the sum of V * I over time. Suppose V = 5 V and I = 100 mA and the duration was 0.1 mS and let's say Normally the voltage is 4.5 V at 5 mA.
That little blip for 0.1 mS won't contribute to the temperature rise of of the output device or the energy into the output.

A function generator applied continuously or a short for that matter applied at the output will mean destruction.