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Basic Logic Gate Troubleshooting
22 questions By Tony R. Kuphaldt
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Question 19 of 22
The output of the following gate circuit is always low, no matter what states the input switches are in. Assume that CMOS logic gates are being used here:
Identify which of these possibilities could account for the output always being low:
- Output of U1 stuck in a high state
- Output of U1 stuck in a low state
- R1 failed open
- Switch C failed open
- Switch B failed open
- Switch A failed shorted
Reveal answerOnly these two possibilities could account for the output always being low:
- Output of U1 stuck in a low state
- Switch C failed open
Follow-up question: determine what each of the other faults would do to the circuit.
Notes:Questions like this help students hone their troubleshooting skills by forcing them to think through the consequences of each possibility. This is an essential step in troubleshooting, and it requires a firm understanding of circuit function.
If a student suggests switch B failing open could cause the output to remain low, they are either misunderstanding the operation of an OR gate, or they are assuming switch A has also failed open (or something else happening that keeps input A low all the time). I find this kind of assumption frequently in students new to troubleshooting: assuming multiple faults. While multiple faults are not impossible, they are less likely than single faults. Therefore, the good technician first looks for single faults capable of accounting for all observed states before looking for (less likely) combinations of faults. This is a practical application of Occam’s Razor.
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Question 20 of 22
The output of the following gate circuit is always high, no matter what states the input switches are in. Assume that CMOS logic gates are being used here:
Identify which of these possibilities could account for the output always being high:
- Output of U1 stuck in a high state
- Output of U2 stuck in a high state
- R1 failed open
- R2 failed shorted
- R3 failed shorted
- Switch A failed open
- Switch B failed shorted
- Switch C failed shorted
Reveal answerOnly these three possibilities could account for the output always being high:
- R1 failed open
- R3 failed shorted
- Switch B failed shorted
Notes:Questions like this help students hone their troubleshooting skills by forcing them to think through the consequences of each possibility. This is an essential step in troubleshooting, and it requires a firm understanding of circuit function.
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Question 21 of 22
The following gate circuit has a problem:
When tested, it is found that the circuit does not respond in the same manner as its (ideal) truth table predicts. Here is a comparison of the ideal and actual truth tables, as predicted and tested:
A B C Output (ideal) Output (actual)
0 0 0 1 1
0 0 1 0 0
0 1 0 1 1
0 1 1 1 0
1 0 0 1 1
1 0 1 1 1
1 1 0 1 1
1 1 1 1 1
The first thing a good electronics technician would do, of course, is set up either a voltmeter or a logic probe and begin testing for logic levels in the circuit to see what is wrong. However, the settings of the input switches are very important as part of the diagnosis. Based on the design of the circuit, and the truth table results shown, in what states (open or closed) would you first set the input switches, and then what logic level would you first test with the logic probe or voltmeter?Reveal answerSwitch settings:
- A open (0)
- B closed (1)
- C closed (1)
Then, measure the logic state of the lower input on the NOR gate (coming from the “B” switch).
Notes:Ask your students to explain what logic state is supposed to be at that point in the circuit, and what logic state they suspect might be there that could account for the aberrant output. Also discuss why this particular choice of switch settings is the best for a first test.
If students do not immediately grasp why the switches should be set as the answer indicates, pose the following scenario. Suppose they were asked to troubleshoot a simple light bulb circuit using only a voltmeter. The problem is, the light bulb does not come on when the switch is closed. Would it be best to proceed with their voltage measurements with the switch on or off? It should be easy to understand that having the switch in the off position would only interfere with the diagnosis, and that turning the switch on is the best way to reveal the fault (so that one could use the voltmeter to see where voltage is not present, but should be). Likewise, it is wise to set up this faulted logic circuit in such a way that the output ought to be doing something it isn’t. This way, one may easily compare logic states as they are versus as they should be, and from there determine what type of fault could be causing the problem.
