Re: PIC, Keyboard, and USART

On Thu, 23 Mar 2006 15:48:39 -0600, "Abstract Dissonance"
<Abstract.Dissonance@xxxxxxxxxxx> wrote:

"Jonathan Kirwan" <jkirwan@xxxxxxxxxxxxxx> wrote in message
news:4a4622p34mgk71nr4f1oolk66pga0g1aip@xxxxxxxxxx
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.

Sheesh... that would be helpful to have. I wonder if one can download these
online now?

Not that I'm aware of. I would suppose that IBM still holds the
copyright to it -- it's not _that_ old, yet. I'd be happy to scan
them in and post them up, if permission to do so could be found. It's
irreplaceable. The set also includes _complete_ BIOS listings, as
well. It's fairly complete, as I said.

+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.

What is pin 1 and pin 2? The pins from the connectors? i.e., Data and Clock?
Is that an inverter(buffer I guess)? (i.e. a bidirection link with a
pullup and LP filter?)

As I understand the connector from the schematic (no, I haven't done
keyboard experimentation on my own, so this isn't first hand), the
5-pin round DIN connector has pin 1 as clock and pin 2 as data. 3 is
no-connect, 4 is GND, and 5 is the +5 supply.

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.

What I have found on the net is not completely technical but just what
someone says works. Although they did point out certain issues they do not
go into the details.

Bad news.

Thanks for the info. Its good to know there are technical specifications
out there for this... now I just have to get my hands on it.

Yes. It should be available somewhere. I've not looked around. But
it should be out on the net, I'd hope.

Jon
.

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