BackDriving Logic Circuits

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ParkerMike

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What is Backdriving logic circuits mean? you're forcing the logic's output state

Why would you want to backdrive a logic circuit? for what kind of test
 

The maximum allowed output logic low current of a 74xxxx TTL logic IC is 16mA and is 8mA for a 74LSxx TTL logic IC.
If you try to force the logic low output to be a logic high (why?) then you will probably over-heat the output transistor inside the IC and destroy it.

We talked about this about 6 months ago, didn't we?
 

Automated Test Equipment (ATE) relies on the ability to drive and sense responses to measure state or functions per spec.

Back driving is an acceptable method to pulse an output with a lower impedance if there is no other method available. Naturally low energy is used with short durations.

It should be pointed out that line transients on unbuffered Flip Flop outputs can actually change the state of the Flip Flop by back-driving the output and toggling internal inputs.
 
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In my 'youth' we called this "bus busting", when a bus with several driving devices was misbehaving we would briefly (a few mS at most) force current in/out of a bus against it's established logic level. The idea was that you enabled or disabled each device driving the bus, the one that didn't change the force current was the faulty one.
It worked well as long as the circuit designed had made provision for doing so and I believe was the method recommended by several IC manufacturers. It had to be done using automated board testers, it certainly wasn't a "try this at home" method.

Brian.
 

"Bus Busting" is correct:
1) You disable all the devices that are driving the bus. Can ParkerMike or whatever he calls himself today disable all the devices that drive the bus?
2) You feed high current low impedance logic levels onto the bus.
3) The device that blows up is the defective one.
 

The simple process of measuring VOH/VOL consists of "back
driving" to limited extent.

More likely the interest is in the part's ability to withstand
abuse and survive undamaged.
 

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