If you have a bus contention fault, you use the current tracer to trace back to the drivers with high currents. Only one of them should be trying to drive the bus, so the others are at fault. Either the at-fault driver is stuck or something upstream is defective.

So I probe every pin that is "tied" to the buss line using a current tracer, which ever pin has the higher current is the bad IC chip?

 

Are you looking for the Universal Truth for All Circuits?

 

Are you looking for the Universal Truth for All Circuits?

No, To increase my troubleshooting skills

 

Why do you keep making unlimited claims about tiny bits of information that make them look like Universal Truths that apply to All Circuits Everywhere? Do you think that improves your skills?

 

I was just asking how you guys use a logic pulser and current traces so I can increase my troubleshooting skills from your experiences of using a logic pulser and current traces. examples would be helpfull

 

Examples of which equipment you have and which circuit you are working on would help, too.

 

BK pulser , TTL and CMOS circuits from the 70's and 80's

 

So I probe every pin that is "tied" to the buss line using a current tracer, which ever pin has the higher current is the bad IC chip?

I forgot an important detail. You need a logic pulser to repeatedly briefly toggle the bus to LOW so you can trace the current from the point that you pulse back to the drivers that are pulling it HIGH. The HP logic pulser automatically determines correct pulse polarity.

There are other ways to find bus contention problems, and all of them require work. I've used voltmeters and freeze spray before the tracer and pulser were available. I also unsoldered driver pins, after using other methods that indicated they could be at fault.

 

You need a logic pulser to repeatedly briefly toggle the bus to LOW so you can trace the current from the point that you pulse back to the drivers that are pulling it HIGH.

So I "force" the buss line low using a logic pulser? the buss line stays low as you're using the current tracer to probe each IC chip pin that is "tied" to the buss line?

I've used voltmeters and freeze spray before the tracer and pulser were available.

So you had your voltmeter probe on the buss line and would freeze spray one IC chip at a time to see if the voltmeter would change voltage?

 

So I "force" the buss line low using a logic pulser? the buss line stays low as you're using the current tracer to probe each IC chip pin that is "tied" to the buss line?

I'm assuming you have two drivers trying to force a bit on the bus high, only one is correct; but you don't know which one besides the one that should be driving the bus. By using the logic pulser to pulse the net LOW, you can use the current tracer to find which drivers are trying to pull it HIGH.

You don't probe points with a current tracer, you trace the net to find the area with the highest current.

The December 1976 HP Journal describes the HP current tracer, logic pulser, logic probe, and logic clip and gives some usage information. I posted a link in a thread last month. If you search as I suggested, you'll find the link. Otherwise, you can use Google; there are many sources for HP journals on the web.

So you had your voltmeter probe on the buss line and would freeze spray one IC chip at a time to see if the voltmeter would change voltage?

For the voltmeter, you could pull the bus (assuming it was being driven HIGH by multiple drivers) low through some resistance that won't kill the drivers. Then you measure the voltage on all of the drivers and the ones with the highest voltage are most likely the ones trying to drive it HIGH.

Freeze spray only works when currents are high enough that you have sufficient temperature gradiant on the board to isolate a stuck node. I used that technique to find power shorts more than stuck at logic.

 

BK pulser , TTL and CMOS circuits from the 70's and 80's

Could you please give us a little more information than that? Specifically what type of circuits from the 70's and 80's.

 

With no equipment except a BK pulser, how do you measure any outputs?

 

you trace the net to find the area with the highest current.

So you would just move the current tracer along the PCB trace until you would find an area that had high current draw?

Freeze spray only works when currents are high enough that you have sufficient temperature gradiant on the board to isolate a stuck node. I used that technique to find power shorts more than stuck at.

Power short, what is this? +VCC to ground short you mean

 

So you would just move the current tracer along the PCB trace until you would find an area that had high current draw?

Yes. In the hypothetical situation I was describing, you pulse the net LOW at some point (repeatedly) and find the segments of the net with the highest current with the tracer. If you have 2 drivers trying to drive the net high, one will be the one you expect to be driving the net and the other will be the at-fault driver. You need to use some technique to isolate a part of the net to start troubleshooting; there is no solution that will deal with all situations. In the end, you need to use your knowledge of the circuit.

Power short, what is this? +VCC to ground short you mean

Yes. I once worked on a line that assembled HP 3000 Series III computers. One of the possible PCB manufacturing defects was shorts; usually on an inner layer (our boards were 4-8 layers). If the short was on the top or bottom layer, we could sometimes salvage the board. If it was on an inner layer, we usually cranked up the current to see if we could clear the short. We usually ended up burning up the board and scrapping it.

A voltmeter can also be used for this, but it's generally very tedious on a square foot PCB.

 

we usually cranked up the current to see if we could clear the short. We usually ended up burning up the board and scrapping it.

Yes i have heard of that technique

Another Technique I know is:
Turn off the power to the circuit board and use an external power supply and inject the +VCC or inject the bussline to 500millvolts at 1 amp current limit and measure the voltage on each node until you find the largest voltage drop. This is your area of where the short is