Answer:A 4-input NAND gate is normally high. It can only go low when all four of the inputs have a logical high on them. Assuming the circuit should see a condition where all four inputs go high, then a constant high output indicates a fault does exist.The first step in troubleshooting this failure is to verify the circuit, under normal operation should be seeing a condition where all four inputs do go high. It is completely possible that the circuit is working properly and the condition you were expecting simply does not occur. Verifying what your inputs should be allow you to confirm whether a failure exists in reality.Second, you should verify that the logic probe, multi-meter, oscilloscope, logic analyzer, or monitoring circuit you are using to test circuit outputs is working properly. This is easiest done by touching the probe of the monitoring device to another output you should see changing states. In one system I once had to trouble-shoot, the device had a built-in monitoring circuit and the failure was actually in the monitoring circuit rather than in the main circuit. Once you have confirmed that your test equipment is working properly, you can begin testing the actual circuit to troubleshoot the failure.Given that the circuit should be seeing periods where the gate should go low and that your test equipment is working properly, then a constant high output on this gate means either the output of the gate is open (assuming their is a tie-up resistor somewhere on the output line) or at least one of the inputs is being held low. A constant low input could be a failed gate driving one of the inputs, a short to ground on the circuit board trace connected to the constant low input, or a tie-up resistor on the input that has opened (Technically, this last cause creates a floating input which could intermittently be seen as either a high or low, but in my experience is invariably seen as a low.) The only way to determine which fault exists is to measure the voltages on each input pin to determine which input is failing and then trace the constant low back to its source.729 viewsView 1 upvote1Add a comment...Sponsored by Purdue University OnlinePurdue's 100% Online Master's Now Accepting Applications.Earn an online MS in Electrical and Computer Engineering degree in 12-24 months. View curriculum guide.Learn MoreLoring Chien, Electrical Engineer for 45 years & IEEE Sr. Life memberAnswered Jul 2, 2020
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