Pages created and updated by Terry Sturtevant Date Posted: May 12, 2017

Digital Debugging

You will use the debugger board (see below) to test whether your circuit is working or not.

debugger board
[click image to get a larger image]

Another version looks like this:
debugger board
[click image to get a larger image]

The debugger board is set up with either two or four banks of eight signals. Here's a closer look at a single bank:

one bank
[click image to get a larger image]

Each bank can be configured to either "display" logic levels in your circuit or to "control" logic levels in your circuit. In "display" mode, you can connect a signal to its corresponding connection on the debugger board and the LED will be ON if the signal is HIGH and OFF if it is LOW.

Here's what the debugger board looks like connected:
debugger board

These are the power connections:
debugger board power 

This is a bank in "control" mode (i.e. you can use it for inputs to a circuit):
debugger board 
    control mode bank
Hint: If you're not using all of the pins in one bank, spread them out so it's easier to tell which is which.

This is a bank in "display" mode (i.e. you can use it to see the outputs from a circuit):
debugger board 
     display mode bank

Note: The debugger board has TTL gates on its input. These gates, like most TTL gates, tend to float HIGH if unconnected. Thus, if you have nothing connected to a pin in a bank set for "display" mode, the LED will usually be ON. No connection does not mean LOW; pins must be tied LOW if you want to ensure that they are LOW.

In "control" mode, the DIP switches for a bank of connectors will control the logic level on the pins.

Note: Make sure that you do not try and have two different things trying to control the same signal. For instance, only use the debugger board in "control" mode to connect to inputs of your circuit which are not connected to anything else.

Switching a bank from one mode to the other is accomplished either by a single DIP switch or by a jumper. (You can tell which mode you're in by whether the bank of DIP switches change the LEDS. If so, you're in "control" mode. If not, you're in "display" mode.) The circuits on the debugger board need power and ground to work, so make sure they are connected.

Here are a few important rules to follow when handling ICs during debuging:

  1. Always use red wires for power and black wires for ground. Don't use either colour for anything else!
  2. Always turn off the power on your breadboard before removing/inserting any ICs.
  3. Be careful to avoid reversing power connections. Typically we only need +5 V and 0 V (ground) connections for digital circuits. Ask your IA for help setting up power busses on your breadboard if in doubt.

Troubleshooting Suggestions

Your circuit will probably not work the very first time. Therefore, trouble-shooting is a basic skill that you will have to learn.

The followings are a list of helpful suggestions:


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