CSC270 Lab 4 2011

--D. Thiebaut 10:42, 14 February 2011 (EST)

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This lab deals with decoders, and in particular the 74LS42, and will have you look at using the decoder as a building block of combinational circuits, or as a dedicated decoder.

The 74LS42 Datasheet

Experiment #1

Use a 74LS42 to implement a 3-to-8 decoder with enable and active-low outputs. The challenge here is that you only have 4 LEDs to show outputs. You cannot use one LED connected to two different outputs, or one of the outputs will get damaged in the process. You'll have to switch back and forth between LEDs and signals to show one set of outputs, for example the lower 4 output bits, and the other set of outputs, i.e. the upper 4 output bits.

Experiment #2

Use a 74LS42 to implement a majority voter of 3 input signals.

You can only use one extra chip, in addition to the 74LS42!

Experiment #3 (A view of things to come...)

Wire up the following circuit that uses NOR gates (be careful, NORs do not have the same pinout as the other gates). Connect the two inputs (on the left) to switches, and the outputs (on the right) to LEDs.

This is a very unusual circuit. Play with the inputs a bit and figure out its behavior...