Student "edition" found at {csi dot journalspace dot com}.

Maybe I shouldn't have started this blog now, not with everything that's been going on.

In the second meeting of Computer Systems Organization lecture class for the eighth week of the third term, I discussed multiplexers and de-multiplexers.

Before going into the particulars, I first asked the students about where they might have heard the term multiplex before. One of the students answered karaoke machines and recordings, where if the user turns the knob completely in one direction, a singer’s voice can be heard to accompany the instrumentals, while if it is turned completely in the other way the singer’s voice disappears.

In the same way, an n-bit address multiplexer has two raised to n number of input lines or bits, but only one output. So two address bits would have four input bits accompanied, and depending on the value found in the address bits, the correspondingly numbered input bit is now connected to the output line.

In a way this is just like the encoder, except that the output is not one or high for just one bit out of the several lines available. Besides, it’s also possible that the output could be high or low depending on the value of the input bit chosen by the address bits.

This circuit also introduced the enable bit, which, besides the power up and ground pins of a chip, in effect is a second “switching on” of the circuit for use.

Just to be sure that the students understood the concept I just relayed, I gave then a three-bit address multiplexer with values in the address and in the inputs, and I asked them what the value of the output was.

They only had to say whether it was one or zero, and almost half of the class still gave the wrong answer. But apparently those students got clarified with the workings of the circuit after that example.

The de-multiplexer is just opposite of the multiplexer in that there is only one data input and n-bits for address, and two raised to n bits for output that again depend on all the input for their values.

Lastly I told them the day’s lesson would not be included in their twice-postponed test for the next meeting.

Session 1025 was not able to answer the checkpoint correctly. Class dismissed.