CGS XOR/XNOR logic: Difference between revisions

'''CGS54''' the '''CGS XOR/XNOR logic''' board is an add-on for the [[CGS pulse divider and Boolean logic]] module, adding XOR and XNOR functionality. Unlike a true digital gate, the module is not sensitive to specific levels, but rather to the difference between them. If the two inputs are within approximately approximately 1.2 volts of each other, they are regarded as being the same logical value. If they are more than approximately 1.2 volts apart, they are regarded as being different. This means that while this module will work quite satisfactorily with any logic levels, it can also be used to compare two analog signals, either positive or negative in value. If only one input is connected, the output will be the same as if two identical inputs were present.

Normally the two inputs of each op-amp are held (approximately) 1.2 volts apart via the pull-up and pull-down resistors, and the four diodes in the input circuit. Connecting two similar input voltages to the inputs will not affect this replationshiprelationship, other than to move the absolueabsolute voltages up or down. The outputs of the op-amps, which are wired as comparators, will reflect this. On the XOR, the inverting input will be higher than the non-inverting input, resulting in the op-amp output being negative. The opposite will occur with the XNOR comparator. As long as the difference between the two inputs is less than approximately 1.2 volts (two diode voltage drops), the relationship between the comparator inputs will remain, and the outputs will not change.

Once the two input are taken more than 1.2 volts apart, the voltage at the diode- pull-up resistor junction will be lower than that at the diode- pull-down resistor, and the relationship between the comparator inputs will be reversed, i.e. on the XOR, the inverting input will be lower than the non-inverting input, resulting in the op-amp output being positive, and the opposite occuringoccurring for the XNOR comparator.

The outputs of the op-amp comparators are passed througthrough a diode so that only positive going voltages reach the output. Any negative going voltages (logic 0) are blocked, and the output held at 0 volts by the pull-down resistor at the output. The 1k8 resistors in line with the output, when combined with the pull-down resistors, limit the positive (logic 1) output level to around 5 volts.