Digital Systems Lab 3

< < Lab 2 Lab 4 > >

Lab 3: Troubleshooting

Download Lab 3 (PDF)

Troubleshooting steps: be specific!

It is important to be specific when you list steps to use to debug your circuit. By the end of the semester you should have a flowchart in your head of how to go from start to finish to check the wiring in your circuit. Each step should be specific and give you a concrete item to perform, and a clear determination of what would be an indication of that issue being a problem. (For example: specific step is “check each output signal on the DIP switch and ensure that the signal is LOW when the switch is OFF and HIGH when the switch is ON.” This step is specific and you know that if you do not obtain that specific outcome, that there is an issue with the DIP switch.)

Here is a list of common troubleshooting steps that students list that are not specific, and the reason(s) why they are not specific enough.

Check for broken parts

  • Which parts will you check?
  • How will you check them?

For example: a broken LED can be diagnosed by connecting the anode of the LED directly to VCC and checking to see if the LED illuminates (assuming the cathode is appropriately connected to ground via a current-limiting resistor). This is different from how you would check for a broken DIP switch, a broken logic chip, or a broken wire.

Check that components are in the correct row

  • Which components will you check?
  • Where is this typically an issue?
  • Will you use the logic probe to check, or a different tool? What signals on the logic probe would lead you to find this issue?

For example: sometimes students will place VCC wires on the DIP switch a row offset (typically one row too high). Sometimes you can determine this by looking at the DIP switch (in which case, the specific step is: visually inspect the DIP switch and check for VCC wires in each input row). Sometimes you can determine this by using a logic probe on the DIP switch signal wires (in which case, the specific step is: use the logic probe on the output of each DIP switch signal and ensure that a HIGH and LOW signal is received depending on the position of the switch).

Check if pins show correct signal

  • Which pins are you checking?
  • What tool are you using to check the signals?
  • How will you know if the signals are correct or not?

Make sure everything is wired correctly / inspect wiring

  • Which wires are you checking?
  • How are you checking the wires?
  • Are there resources such a pinout diagram that you will use to help you with this step?

This is not a specific enough step. If you are checking the DIP switch, what are you looking for? If you’re checking the logic chips, what are you looking for? Be specific!

Make sure components are placed correctly

  • Which specific components are you checking?
  • How would you know if they are placed correctly?

For example: an LED that is placed incorrectly (shorted, backward) is different to diagnose than a logic chip that is placed incorrectly (upside-down, not straddling the trench).

Check the LED

  • What aspect of the LED are you checking?
  • How will you check this?

For example: sometimes students place an LED backward. This can either be diagnosed with a visual inspection or by placing the LED in the opposite orientation and seeing if that fixes the problem. This is different from how you would check for an LED that is burnt out, or an LED that is shorted to itself.

Fix the circuit

  • This is not a troubleshooting step, this is what you do after you have found the problem(s) with your circuit.

Use the logic probe / analyze signals

  • Where are you using the logic probe?
  • How will you know if the signal is correct or not?

Check that gates are placed correctly

  • What are the specific ways that gates can be placed incorrectly?
  • How would you know if a gate is placed incorrectly?

Logic gates VCC and GND

  • What tool will you use to determine if VCC and GND are connected properly?
  • Exactly where on the breadboard or logic chip(s) will you probe?

Check for loose connections

  • How will you do this?
  • What components are typically loose in a circuit?

For example: sometimes a DIP chip may not be seated properly in the breadboard. A visual inspection may identify this, but it may not. Loose wires may cause intermittent signal fluctuations, and can typically be diagnosed by moving around the wires and seeing if that changes the output.

Resources

Watch/read the following resources prior to the first lab. This information will teach you how to use lab equipment and provide suggestions and technical tips for successfully completing the lab.

This lab is licensed under creative commons as CC-BY-SA-NC. For more information, visit creativecommons.org.