XT/AT Keyboard interface design

[Kenneth Keeley]

Hi,
I am building a small 68000 computer from the ground up. I would
like to build a keyboard interface that would be able to use a pc
keyboard. Does anybody know of a simple circuit design that I could
use. I did find a design some time ago for an XT keyboard interface
using a serial to parallel convertor. Would this still work with an AT
keyboard. A link to a Circuit Diagram or at least a write up on a
circuit diagram would be great.

Thanks
Kenneth

 

[Jonathan Kirwan]

I am building a small 68000 computer from the ground up. I would
like to build a keyboard interface that would be able to use a pc
keyboard. Does anybody know of a simple circuit design that I could
use. I did find a design some time ago for an XT keyboard interface
using a serial to parallel convertor. Would this still work with an AT
keyboard. A link to a Circuit Diagram or at least a write up on a
circuit diagram would be great.

I have the IBM docs on the electrical and software on the AT keyboard and I
think there are a few pages on the web with this, also. Do I gather you don't
actually want to do the electrical interface design, though? (It's not hard.)

Have you already looked around on the web? If so, what have you found that does
NOT meet your needs but does seem to get kind of close?

Jon

 

[CFoley1064]

Subject: XT/AT Keyboard interface design

From: [email protected] (Kenneth Keeley)
Date: 10/3/2004 9:14 PM Central Daylight Time
Message-id: <[email protected]>

Hi,
I am building a small 68000 computer from the ground up. I would
like to build a keyboard interface that would be able to use a pc
keyboard. Does anybody know of a simple circuit design that I could
use. I did find a design some time ago for an XT keyboard interface
using a serial to parallel convertor. Would this still work with an AT
keyboard. A link to a Circuit Diagram or at least a write up on a
circuit diagram would be great.

Thanks
Kenneth

Hi, Kenneth. The standard PC/XT keyboard is much different than the AT and up
keyboard. The PC/XT used a proprietary Intel keyboard interface IC, and
required quite a bit of overhead from the poor overworked 8088. On AT and up
PCs, there's an 8031 uC embedded in the keyboard to do the scut work, and the
trick is just to do serial interface with the 8031.

If you're building one from scratch, that would probably be the easiest way to
go. Make it a software project instead of a hardware one, with a very simple
hardware interface.

You might want to look at this link -- there's a lot of good information there,
links to other sources, 6502 coding for controlling an AT keyboard, as well as
a guest appearance from a frequent poster to this group:

http://members.tripod.com/~ilkerf/c64tower/F_Keyboard_FAQ.html

Don't re-invent the wheel (unless you need to)!

Good luck
Chris

 

[Tim Wescott]

Hi,
I am building a small 68000 computer from the ground up. I would
like to build a keyboard interface that would be able to use a pc
keyboard. Does anybody know of a simple circuit design that I could
use. I did find a design some time ago for an XT keyboard interface
using a serial to parallel convertor. Would this still work with an AT
keyboard. A link to a Circuit Diagram or at least a write up on a
circuit diagram would be great.

Thanks
Kenneth

The AT keyboard is yesterday's news -- the new big thing is to use USB
for everything. But the AT keyboard is still less expensive and easier
to apply, and will probably be available for years to come.

So you have the option of complicating your life with a USB interface,
which will let you do lots more than stick on a keyboard, or you may
want to keep things simple with the AT.

 

[Spehro Pefhany]

The AT keyboard is yesterday's news -- the new big thing is to use USB
for everything. But the AT keyboard is still less expensive and easier
to apply, and will probably be available for years to come.

So you have the option of complicating your life with a USB interface,
which will let you do lots more than stick on a keyboard, or you may
want to keep things simple with the AT.

Implementing the host controller side of a USB interface would be
quite difficult.


Best regards,
Spehro Pefhany

 

[Norm Dresner]

From: [email protected] (Kenneth Keeley)
The best collection of PC interface info is at
http://www.beyondlogic.org/
and I know they have keyboard information.

Norm

 

[Rich Grise]

Implementing the host controller side of a USB interface would be
quite difficult.

Once you wade through the morass of descriptions of how the PS2 keyboard
and its interface operate, it's almost trivial. Two open-collector lines,
clock and data. Whichever unit wants to initiate communication pulls the
$BIT low - I'd have to go back and look it up - I got a half-way idea
how it works while doing a keyboard hack to play MAME games. I only studied
enough to y-connect two keyboard controllers - it's almost an ordinary
wire-or, except a little bit bidirectional. You just have to make sure
that your keyboard really is in a reset, idle, ready state when the
computer expects it to be, and it turns out that's not hard at all. In
fact, the only reason not to hotplug keyboards or mice is because the
crappy connector could apply power-supply voltage levels at signal pins
before the circuit is powered up to handle them.

And I haven't looked into which end actually should have the pullup.

Hope This Helps!
Rich

 

[petrus bitbyter]

Hi,
I am building a small 68000 computer from the ground up. I would
like to build a keyboard interface that would be able to use a pc
keyboard. Does anybody know of a simple circuit design that I could
use. I did find a design some time ago for an XT keyboard interface
using a serial to parallel convertor. Would this still work with an AT
keyboard. A link to a Circuit Diagram or at least a write up on a
circuit diagram would be great.

Thanks
Kenneth

So you're building a small 68000 computer. Then you'll have to build some
I/O interfaces anyhow. Unless you want to spare the processor, you can make
the 68000 do the job. You will only need the address decoding and two or
three I/O lines using open collector buffers. The interface of the AT
keyboard is well known an available on the net. Look at
www.beyondlogic.org
for instance. The XT keyboard interface differs from the AT one and these
keyboards are only left in attic- or basement corners. There are much more
AT keyboards around and they will stay for quite some time.

If you want another interface you'll have to choose one. You can use a micro
to convert the AT serial code to RS232 serial or parallel or even USB. But
you'll also have to build that interface in your 68000 computer which will
make it less small. For the first two interfaces mentioned, you can find
enough interface chips, ACIAs and PIAs in Motorola jargon. Building a USB
host controller in that computer is much more interesting but far from
trivial. If you succeed you can connect a USB keyboard at once.

petrus bitbyter

 

[Kenneth Keeley]

I have the IBM docs on the electrical and software on the AT keyboard and I
think there are a few pages on the web with this, also. Do I gather you don't
actually want to do the electrical interface design, though? (It's not hard.)

Have you already looked around on the web? If so, what have you found that does
NOT meet your needs but does seem to get kind of close?

Jon

I have looked around the web but haven't found any well shown designs
all of the ones I have found have talked about using a PIC or micro
controller to interface with the keyboard. Do you know where I could
find the IBM docs that you referred to.

Kenneth

 

[Jonathan Kirwan]

I have looked around the web but haven't found any well shown designs
all of the ones I have found have talked about using a PIC or micro
controller to interface with the keyboard.

That should help, I'd imagine. But it doesn't give you the theory, I suppose.

Do you know where I could
find the IBM docs that you referred to.

Yeah. On my shelf, in the sometimes coveted, 4-volume set called the "IBM
Technical Reference." I'd just figured that someone out there must have
documented all the important details, already.

Jon

 

[petrus bitbyter]

I have looked around the web but haven't found any well shown designs
all of the ones I have found have talked about using a PIC or micro
controller to interface with the keyboard. Do you know where I could
find the IBM docs that you referred to.

Kenneth

Didn't you find www.beyondlogic.org ? All you need to know about the
AT-keyboard interface is available, even a design using a Motorola micro to
convert the AT keyboard codes to RS232 serial. To use it you need to build a
serial RS232 interface at the side of the computer. Which needs address
decoding, an ACIA and some level converter. (Unless you decide skip these
levelconversion on both sides.)

You can use the same address decoding to control some buffers that give you
plenty of I/O lines to make the 6800 handle the keyboard itself. The address
decoding controls busbuffers like LS254 for input and LS364 as an output
register. LS14s were commonly used input buffers and LS125 simulated open
collector output buffers (But you can use real open collector buffers or
even transistors as well). Another description can de found at
http://panda.cs.ndsu.nodak.edu/~achapwes/PICmicro/PS2/ps2.htm
A full AT keyboard specification can be found on
http://www.electronic-engineering.ch/microchip/projects/keyboard/v2xx/keyboard_v2xx.html
A schematic at
http://members.lycos.co.uk/leeedavison/6502/atkey/
This last one is mented for a 6502, but you can connect it to a 68000
processor the same way, even simpler then my proposal above.

If you can't find info like this or can't work with it *I* think you'll have
a long way to go before you have build a 68000 computer.

petrus bitbyter

 

[Mark Zenier]

That should help, I'd imagine. But it doesn't give you the theory, I suppose.


Yeah. On my shelf, in the sometimes coveted, 4-volume set called the "IBM
Technical Reference." I'd just figured that someone out there must have
documented all the important details, already.

It was in a couple of the columns in Circuit Cellar Ink magazine, back
about 10 years ago. Both Ed Nisley and John Dybowski did keyboard
interfaces, and Ed Nisley's stuff talked about all three modes that
an AT keyboard would run. (Stuff I'd not seen anywhere else).

Nisley did that as part of a series on using the PC in embedded
applications, I think it got made into a book.

There's an index on www.circuitcellar.com, as I've heard.

Mark Zenier [email protected] Washington State resident