basic digital troubleshooting

[BradtheRad]

Why should I tied the inputs to ground? or the supply +? I just don't get it

Shouldn't I lift up all the input pins and inject a high or low to monitor the output of the chip?

An input left unconnected is likely to pick up ambient 60-cycle fields (or other magnetic noise). This is amplified internally, possibly causing spikes on the supply pins or misbehavior in other sections of the chip, etc.

Even in normal operation, digital chips of the earlier TTL family (as opposed to the low-power families) are known as needing precautions due to their switching higher currents.

To troubleshoot chips by themselves, it might pay to make your own testing rig. Arrange some dip switches around an IC socket, so you can install various IC's, then apply various input combinations, and observe the resulting outputs.

Be careful to drain off static charge at all times when working with IC's.

 

[walters]

If i tie the inputs to ground or V+, how can I inject any kind of input or logic on the inputs if they are tied to ground or V+?

 

[D.A.(Tony)Stewart]

Consider the chip on its own and see if it behaves as expected. WIth Cmos you can temp. ground with a resistor on an input and create a state change and look at at an output. THe resistor is just in case it is hardwired to V+ instead of correctly terminated with a 10KΩ pullup. But you basically read the Specsheet and walk around the pins to see what are inputs and outputs and see if it makes sense . YOu need to be able to read though.

 

[walters]

If you have the Datasheets for each IC chip

What are some In circuit test that you can do? since you have the datasheets

What in circuit tests can I do having the datasheets

---------- Post added at 08:07 ---------- Previous post was at 07:58 ----------

The way I do it is when i have the datasheets for a IC chip
I put the oscilloscope probe on the output pins to monitor the output
With the inputs I use a square wave function generator set to +5 volts
I use a Seconded Function generator set at square waveform for the clock input
And I use a pushbutton for the Reset input

I will try each input and output compared with the Datasheets with that IC chip

Most of the Time the IC chips will check out GOOD, but when logic is turned on it circuit doesn't work

Because I test each IC chip like in the circuit or stage to give you a result like turn on a motor or light,

But how do you find out which chip is bad if they all test good like the way i do it?

 

[BradtheRad]

Most of the Time the IC chips will check out GOOD, but when logic is turned on it circuit doesn't work

This is the predicament that makes you tear your hair out.

As chips multiply, so does the complexity. A logic gate has a truth table with 4 rows and 3 columns.

Add another 5 logic gates and soon your truth table has so many rows and columns, it becomes unmanageable.

Does that describe your circuit? The chips can operate properly, but some unexpected combination of states defies the up-until-now flawless logic of your flow chart.

It's like computer programming. The logic gates do exactly what you tell them to do... no more, no less.

You might find you need to draw up a truth table with 20 rows and 20 columns. Then test all possible conditions by switching inputs by hand, before you can track down one little glitch.

You may wish you could have an 8 trace oscilloscope. Or computerized diagnostic octopus.

Etc.

Your ingenuity and persistence will be called upon for a time, before you can track down the glitch.