schmitt trigger question

[G.T. W.]

Hello,

I was attempting to compare input / output of an MC7414 chip on a pcb.
The board was in the device (centerfuge) operating under its own power
supply. The circuit board is on a floating ground, but I measured 5.1
Volts (RMS) between pins 14 and 7 with a Fluke meter.

Unfortuneately, the only o'scope available to compare input/output was
a digital model I've never seen before, there was no instructions on
how to confirm its settings. Everything was set to 'auto'.

The first probe was set on the 7414, pin 1(input). A positive pulse was
seen there. A second probe was put on the output (pin 2) to observe
the re-shaped pulse. There was no wave-shaping, no squared pulse.
Both traces appeared identical, a positive-going pulse, rounded at the
half-power points. Further, the output was not inverted.

Due to my ignorance with a digital o'scope, I can not say what the true
pulse amplitude was. Using pin 7 as a ground reference for the probe,
the scope insisted the input pulse to pin 1 was 174 volts (I looked
very hard for a decimal point, there wasn't one).

My two questions are:

1. What was I doing wrong?

2. What condition could cause a schmitt trigger to pass a signal
unchanged, instead of re-shaping it?

PS. I had the same result with every gate on the 7414. Thank you.

 

"G.T. W." wrote:

Hello,

I was attempting to compare input / output of an MC7414 chip on a pcb.
The board was in the device (centerfuge) operating under its own power
supply. The circuit board is on a floating ground, but I measured 5.1
Volts (RMS) between pins 14 and 7 with a Fluke meter.

Unfortuneately, the only o'scope available to compare input/output was
a digital model I've never seen before, there was no instructions on
how to confirm its settings. Everything was set to 'auto'.

The first probe was set on the 7414, pin 1(input). A positive pulse was
seen there. A second probe was put on the output (pin 2) to observe
the re-shaped pulse. There was no wave-shaping, no squared pulse.
Both traces appeared identical, a positive-going pulse, rounded at the
half-power points. Further, the output was not inverted.

Due to my ignorance with a digital o'scope, I can not say what the true
pulse amplitude was. Using pin 7 as a ground reference for the probe,
the scope insisted the input pulse to pin 1 was 174 volts (I looked
very hard for a decimal point, there wasn't one).

My two questions are:

1. What was I doing wrong?

Using a bad set of probes? Using a bad scope? Something is wrong
with the setup. Essentially, it boils down to not knowing how
to use the tool so that you have confidence in what it tells you.

2. What condition could cause a schmitt trigger to pass a signal
unchanged, instead of re-shaping it?

See the answer above - you don't know if the indication on the scope
is accurate or valid, so you don't know if the signal passed
unchanged or not. Heck, channel 2 could be inverted.

If you get a chance to use the 'scope again, bring a known
square wave generator with you and scope it. Since you
know what the generator does, you can verify the scope setup
and what it displays, before trying to use the scope to diagnose
the circuit in question.
> PS. I had the same result with every gate on the 7414. Thank you.

 

[G.T. W.]

In article <3F63DF4E.8B5C6A0@bellatlantic.net>,
<ehsjr@bellatlantic.net> wrote:

If you get a chance to use the 'scope again, bring a known
square wave generator with you and scope it. Since you
know what the generator does, you can verify the scope setup
and what it displays, before trying to use the scope to diagnose
the circuit in question.

That's good advice, and I will do just that. There is a wave generator
in the shop, but being a new intern I can't just go poking around and
grabbing things without a reason.

What I was hoping, is that the scope was really indicating 1.74 volts,
because that would sort of make more sense if the 7414 doesn't go high
until 1.8 volts.

What I am not sure of, if the schmitt is not 'triggered', would it
then see the 1.74 as a "0" or would it pass unchanged? Thank you.

 

"G.T. W." wrote:

In article <3F63DF4E.8B5C6A0@bellatlantic.net>,
ehsjr@bellatlantic.net> wrote:


If you get a chance to use the 'scope again, bring a known
square wave generator with you and scope it. Since you
know what the generator does, you can verify the scope setup
and what it displays, before trying to use the scope to diagnose
the circuit in question.

That's good advice, and I will do just that. There is a wave generator
in the shop, but being a new intern I can't just go poking around and
grabbing things without a reason.

What I was hoping, is that the scope was really indicating 1.74 volts,
because that would sort of make more sense if the 7414 doesn't go high
until 1.8 volts.

Put a 1.5 volt battery from scope ground to the probe and see
what the scope indicates - that'll tell you if the display is
really ~ 1.74 volts.

What I am not sure of, if the schmitt is not 'triggered', would it
then see the 1.74 as a "0" or would it pass unchanged? Thank you.

I don't know - grab a breadboard and try it with known test equipment,
or a datasheet and see for yourself.

 

[Lewin A.R.W. Edwards]

What I was hoping, is that the scope was really indicating 1.74 volts,

What I am not sure of, if the schmitt is not 'triggered', would it
then see the 1.74 as a "0" or would it pass unchanged? Thank you.

It would "see it as a 0". That's the purpose of a Schmitt trigger.

I'm going to take a wild guess, BTW, that the scope was configured
with AC-coupled inputs.