Using the XOR testing method applet

The applet initially looks as in the figure below.

As can be seen, the circuit canvas is empty. The only exception is that the right-most gate is an AND gate. This gate is also known as the output gate and it has to exist (the circuit must have an output). Keep in mind that no other gate can serve as the circuit's final output.

Changing gate properties

In designing a circuit, you may change one of the seven available gates by clicking on the gate itself. This would bring up the Gate Properties dialog box, which permits 4 kinds of actions:

1) Clicking on the gate itself will change the type of gate by rotating through all types, including having no gate at all.

2) Clicking any of the "Add new" buttons will create a new input to the circuit and connect it to a corresponding input on the gate.

3) Clicking "Add existing" will connect the input that is selected from the choice box into the gate.

4) Clicking "Remove" will remove the node connected to the gate.

It is important to note that in order to change the gate into a NOT gate, the first and third inputs must be empty. This is because a NOT gate cannot have more than one input. In order to remove a gate, it must have no inputs or outputs connected to it.

Connecting two gates

To connect one gate's output to another gate's inputs, drag the mouse from the source gate to the destination gate. Since this applet allows no feedback, one must connect a gate to another that is more towards the right.

Please note that node names and input names are automatically generated and you cannot change them directly.

Switching to test mode

The applet will only go into test mode if the circuit is valid. Valid circuits are ones where all gates contribute somehow to the output, and have at least two inputs (except for NOT gates, which must have one input).

Testing for fault coverage

In test mode, you may choose to see or hide the fault table. Each of the fault table's rows represents one of all possible input combinations (input test patterns). One column is used for each of the circuit's inputs. The other columns represent all the possible stuck-at faults in the circuit. Each bit in the table represents the result of the XOR test. If the bit is 1, it means that the input pattern on the corresponding row can test for the fault at the corresponding column. In order to start the process of filling in one column, you will need to select a fault from the choice box and press "Go". A message will appear in the sky-blue message box. In order to go to the next step, you can click next repeatedly. As the program goes from step to step, messages are written to the message box, entries in the fault table are filled, and the nodes in the circuit are labelled according to not only their name, but also their value. This process goes on until the entire column for the fault has been filled. In order to complete the entire column in one shot, press "Finish fault". You may then select another fault to test. Filling in the entire table in one shot can be performed by clicking "Test all". To view the history of messages for all the tests performed on the circuits since the applet was first run, the user can click on the “Text history” checkbox. A pop-up window will appear showing all messages in which the user can scroll through. To hide the history message box, the user can click on the “Text history” checkbox once again to uncheck it and it will disappear. Note that the applet’s message box contains the messages for the current testing session and that the messages are always cleared once the user returns to “Design Mode”.

Finding the Fault Detection Test Set (FDTS)

Once the table is filled, press "FDTS" to go through the process of finding the minimal fault detection test set. You will enter a multiple-step process through which you may navigate using the "Next" button. Rows with the highest weights will be highlighted in green. Bits in the columns of the already- covered faults will be highlighted in cyan. Once all the testable faults will have been tested, you will get a message informing you of the FDTS.