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 two-hour session of my Interfacing Computer Systems class for this seventh week of the second term, I continued with the description of the 8255A programmable peripheral interface.

But before that I started with a question that somehow recapped what we had taken up the week before. I asked them how many different configurations in Mode Zero the chip could have.

Knowing that each port could be either input or output, and there are three ports, it was clear to them at some point that the answer was a multiple of two. But they kept limiting themselves to six and below despite the fact that I was not acknowledging any of the answers they gave as correct.

When someone finally said eight, I asked why he said it was eight. He couldn’t give me the right answer, because it was not eight, but at least I gave them the hint that just shouting out the correct answer would not do, they had to explain how it became that number.

The correct answer, after all, was sixteen, because the upper four bits of Port C can be set as input or output separately from the lower four bits.

I showed them a sample input phase of the 8255A chip in Mode Zero. Even though it was not even considered simple handshaking, there are still several bits of input that have to be triggered or activated in order for the data to be sent properly.

This includes the chip enable, sending the two-bit address of the port the data bus is supposed to be connected to, and activating the read operation bit. Thus, it is not just sending data. The same, I showed them, is true for a Mode Zero output operation.

For Mode One input, I had tell them about the four bits of Port C acting as the strobe and “input buffer full” or “data accepted” signals of Part A. So in effect, this means that unlike Mode Zero, that could be a twelve-bit signal, for Mode One, it really is just down to eight bits again.

For the same operation using Port B, I showed them that the other four bits of Port C act as the hand shakers for this port, and what the exact pin assignments were.

For Mode One output, strobe is just replaced with acknowledge, and input buffer full with output buffer full.

I left the intricacies of the Mode Two operation and double handshaking signal transmission and receipt for the next lecture session, whenever that will be.

Session 825 receives an input buffer full. Class dismissed.