Re: PIC, Keyboard, and USART

I assume that you are talking about PC/AT keyboards here (or simply
known as "AT keyboards.") I think the original PC keyboard was
incompatible with the AT keyboard, but that all clones since that
advent are now following the AT keyboard arrangement and I doubt you
could even find a PC (non-AT) keyboard these days. Of course, I have
long since forgotten, for sure. Do check on what I'm saying.

Regarding AT keyboards, there is a very detailed electrical spec of
the hardware to be found in the IBM Technical Reference, Personal
Computer AT set (4 volume set, which I will protect with my life!)
That spec is the schematic itself, on page 17 of 22. They use '07
open-collector drivers tied to an RC filter, with 10k and 47pF.

+5
|
/
\ 10k
/
|
|\ |
----| >-----+--------------+---O (either connector pin 1 or 2)
|/ | |
| |
--- |
--- |
| 47pF |
| |
--- |
/// |
|
|
<---------------------------'

That spells out the exact details of both pin 1 and pin 2. You should
be able to note that the IBM PC can use either line is input or
output. There is a reason for that design. If you intend to operate
the keyboard correctly, you will need to mimic it.

You will _also_ need to fully understand how the keyboard is properly
operated from the PC. It is discussed on pages 1-31 to 1-44 and all
of chapter 4 of that Technical Reference I mentioned. But in
particular there is the capability to observe aborts in transmission
and to deal with conflicts in securing the interface from one side or
the other.

I think that the technical details on the AT keyboard were fairly well
considered by IBM before laying them out and the proper operation of
the interface isn't entirely trivial, if you plan to get it right.
There are very specific timing issues you need to be aware of. Have
you searched the web for all this info? You should be aware of
something called the "BAT" and the power up exchanges, graceful
operation in the face of communication aborts, how to deal with
moments when both you and the keyboard decide to talk to each other,
etc.

Jon
.

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