7 April Lab in CSCI 274

Lab goals

• See how multiplexers and tri-state drivers implement buses

Starting off

As usual

• mkdir csci/274/lab10
• chmod go-rx csci/274/lab10
• cd csci/274/lab10
• cp /usr/local/csci/274/lab10/*.lgf .

These commands will place two partial diglog circuits, multiplx.lgf and tristbus.log, into your directory.

Remember! Under the CDE (Alpha's and Sun's) you can not start diglog while netscape is running!

The problem

These circuits are partial solutions to the following RTL specification:

• P: A <- A+2, B <- B+3
• Q: A <- B, B <- A
• R: A <- 0, B <- 0

The diglog file multiplx.lgf, show below, is about 80% of the completed solution. It sets B correctly, but there are some missing wires need to set A.

The chips labeled "84" are 4-bit full adders. The right one computes A+2; the left one, B+3. The chips labeled "175" are 4-bit registers. There is no quad 4×1 multiplexers chips available in diglog, but you can build one with three quad 2×1 multiplexers. This construction is done twice in this partial implementation. Finally, there a priority encoder, labeled "148", is used to set the selector inputs to the multiplexer. This is the tricky part of the circuit and the one you need to which you need to pay the most attention. Turn the control inputs, P, Q, and R off and on, and see how the selector inputs change. Study this carefully!

The second partial circuit is an implementation using three-state drivers on a bus. This implementation is stored in tristbus.lgf and is shown below

The "245" chips are tri-state buffers. When the input on the bottom (actually, the left the way the chip is rotated) is 1, the chip drives the bus. This circuit is not protected against short-circuits. Try pressing the P and Q inputs at the same time and see what happens.

Your task

Finish it up!

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