Model M XT compatibility? Hmm...

[kishy]

Perhaps this has been brought up before, but I'll bring it up again...

I'm reading an eBook copy of Scott Mueller's Upgrading and Repairing PCs, 6th ed. On pg 20, it says the following about keyboards:

With many keyboards, you can alter the way that the keyboard interfaces by flipping a switch on the bottom of the keyboard. Others, such as IBM's Enhanced 101-key Keyboard, detect which type of system they are plugged into and switch automatically.

Now, I know about auto-sensing keyboards; I have one or two myself. That's nothing new.

However, forgive me if I'm mistaken, but wasn't the original "101-key enhanced keyboard" the Model M itself? Wouldn't this indicate that the Model M, at least in its original version, was auto-sensing AT vs XT?

Hmm...

[quadibloc]

My understanding was that the very first Model M keyboards only worked on a PS/2, and did not use anything but Scan Code Set 3.

IBM soon corrected this, and they brought out 101-key keyboards that would connect to an AT, and which could be told by the computer to switch from one scan code to another. If your computer was just an XT, you might have had to settle for a keyboard from KeyTronics or Mitsumi with an XT/AT switch on the back. But I could be completely wrong.

If the keyboard that came with the AT was compatible with the XT, because that's when the switch from Scan Code Set 1 to Scan Code Set 2 took place, then the AT Bios would have had the ability to tell the keyboard to switch to Scan Code Set 2, while the XT, which was designed before there were multiple scan code sets, would have used the keyboard with its initial setting. As long as the AT keyboard could work with an XT or an AT, this ability would have been retained by the model M.

But if the AT keyboard could not connect to an XT, because the AT didn't tell the keyboard to switch over, so it had to default to Scan Code Set 2, then the Model M would have not been able to do so either; everything I've read about the Model M notes that it could be told by the computer to switch scan codes, but it did not have any special ability to detect the kind of computer it was connected to.

[kishy]

I have two (wanting to sell one, a Mitsumi, the other is a Laser) AT/XT switchable keyboards and one auto sensing BTC, and an XT motherboard that wants a case.

As I understand it, "going backwards" to XT compatibility is 100% the responsibility of the keyboard, and that the keyboard would be AT compatible and have technology to sense the computer it was attached to (perhaps the presence of power but lack of signals within a given timeframe, since it's a unidirectional interface, is how this is done).

That change would most likely involve two things: changing the clockrate of the keyboard (since I think it's different), and changing scancode set to something set-1 compatible (basically what you said).

The problem, as far as testing goes (I'd set up that XT board and go for it) is that my XT motherboard might itself support AT keyboards...it has a date of 1989 somewhere on it so it's at LEAST that young, possibly younger. If it supports AT keyboards, then I don't know if a connected Model M is switching to XT mode or if the motherboard is switching to AT mode (I do know it works with XT keyboards).

Bleh, tired mind, complex thoughts. Or so they seem to the tired mind.

And wasn't the first Model M on the XT 286? Or at least that's where the layout appeared, I know that much, and the XT 286 was before the PS/2 and even the AT.

edit:
point 1 - I've discovered confusion in re-reading my post and edited as appropriate, for this thread let's use full words - motherboard and keyboard
point 2 - this is obviously an XT-class motherboard, not an actual XT motherboard.

[quadibloc]

My original reply to you was correct in everything it stated, I have now been able to confirm. But in addition, I have found out the answer to your question.

When the IBM PC AT was introduced in 1984, it cost about $10,000. As a result, the IBM PC XT continued to be sold. (Later, IBM even made the IBM XT 286 computer.) XT computers made after 1984 had a BIOS upgrade that allowed them to work with an AT keyboard.

So it's only the original IBM PC, and early specimens of the XT, that will only work with a PC keyboard, or a third-party keyboard with a physical XT/AT switch. Later specimens of the XT will work with an AT keyboard (and a PS/2 keyboard with an adapter) just fine. The fact that not all XTs work the same naturally leads to confusion.

[skriefal]

The problem, as far as testing goes (I'd set up that XT board and go for it) is that my XT motherboard might itself support AT keyboards...it has a date of 1989 somewhere on it so it's at LEAST that young, possibly younger. If it supports AT keyboards, then I don't know if a connected Model M is switching to XT mode or if the motherboard is switching to AT mode (I do know it works with XT keyboards).

When you state that this is an XT motherboard, are you referring solely to the fact that it has the older, larger round keyboard jack -- sometimes referred to as an "XT" jack?  Use of this keyboard jack does not imply that the board uses the XT keyboard protocol.  The same is true of keyboards that use the larger plug.

Anything made in 1989 is unlikely to be an XT board, regardless of labeling or type of keyboard jack.

As quadiblock states, the term "XT" is confusing.  It could refer to several PCs or PC lines from IBM, to a type of keyboard connector and plug (which IIRC is actually an incorrect usage of the term), or to a keyboard protocol.  And these three may or may not be present at the same time.

[kishy]

My original reply to you was correct in everything it stated, I have now been able to confirm. But in addition, I have found out the answer to your question.

I didn't intend to "correct" you if you thought I did, I was expanding on my own thoughts about what you were explaining.

As for the question of if PS/2 Model Ms can auto sense being connected to an XT (to be clear, the actual PC and XT keyboard interface, not the later one you describe later in reply), I don't feel it has been...you have answered that yes, it will work on some actual IBM XTs (late-model ones), but not if it can be used on a true XT (by which I mean the original design).

When you state that this is an XT motherboard, are you referring solely to the fact that it has the older, larger round keyboard jack -- sometimes referred to as an "XT" jack?  Use of this keyboard jack does not imply that the board uses the XT keyboard protocol.  The same is true of keyboards that use the larger plug.

Anything made in 1989 is unlikely to be an XT board, regardless of labeling or type of keyboard jack.

It's an 8088 with 8-bit ISA slots and 512kb RAM, with DIP switches for system configuration (and integrated floppy controller, but doesn't work with high density disks). I believe, if I recall correctly, the motherboard has a date of 1987 somewhere printed directly on it and the keyboard controller (or floppy, again I forget) chip is dated 1989.

The motherboard DOES definitely work with XT-protocol keyboards (switchable ones in XT mode) and also an auto-sensing one...but I was unable to test with an exclusively AT-compatible keyboard for some reason (I forget why I never did).

edit:
here you go, I found some old pictures:
http://s528.photobucket.com/albums/dd323/ml_89/8088/

[Hak Foo]

My understanding was that the very first Model M keyboards only worked on a PS/2, and did not use anything but Scan Code Set 3.

The first Ms predate the PS/2's 1987 introduction.

My '87 1391401 works well on a decidedly non-PS/2 Phenom II.

Not entirely believing it.

Also, the timeline was

AT-> XT 286 -> PS/2

(I grew up reading those old Scott Mueller reference books instead of comics)

[kishy]

The first Ms predate the PS/2's 1987 introduction.

My '87 1391401 works well on a decidedly non-PS/2 Phenom II.

Not entirely believing it.

Also, the timeline was

AT-> XT 286 -> PS/2

(I grew up reading those old Scott Mueller reference books instead of comics)

Oh that's a good point...1985 are the earliest known Ms, PS/2 was a 1987 thing like you said...but AT was only 1984 right?

The XT 286, I imagine, used the AT interface? (being one of the later ones quadibloc mentioned)

Oh and in a sense you could say I'm "growing up reading (antiquated) technology references"...though I keep forgetting 19 isn't that young.

[Hak Foo]

The XT 286 was basically an AT in an XT case, as best I gather, with a smaller size (baby-AT) board.

[ch_123]

I had read somewhere that the 1390120 supported both XT and AT, but I'm not really sure about this. Besides, why would you want to use a Model M with an XT, it's a waste of an opportunity to use an old Model F!

My understanding was that the very first Model M keyboards only worked on a PS/2, and did not use anything but Scan Code Set 3.

I think they planned to use it with the PS/2, but never quite got around to it.

[quadibloc]

I didn't intend to "correct" you if you thought I did,

No, I wasn't thinking that at all. In my first post, I was unsure of some of the things I noted from memory; while generally confident, I didn't know some of the essential facts I needed to answer your original question. So, after I found an authoritative reference (Upgrading and Repairing PCs, Sixth Edition, QUE) I was happy to report that I hadn't made any mistakes in the things I originally stated as fact... and I had additional information which did answer the question.

Incidentally, of course, the AT itself had the larger five-pin connector, the smaller six-pin one being called the PS/2 connector for a reason. That later XT clone boards probably were designed to work with contemporary keyboards makes sense.

[quadibloc]

The first Ms predate the PS/2's 1987 introduction.

Yes, but do they work on a PC? IBM introduced the model M style of keyboard for one of its ASCII terminals that predated the PS/2, and then also used it with the RT/PC, but they never sold it for the purpose of attaching it to a PC until the PS/2 came out.

I think they planned to use it with the PS/2, but never quite got around to it.

Maybe you're right, and my recollection here is not quite accurate. It could be that the only thing that came after the introduction of the PS/2 was Model M keyboards with the full-size five-pin DIN connector for use with previous models of the computer, and not a change in scan code sets, so that even the earliest PS/2 keyboards only needed an adapter.

Just as Linux on today's PCs tries to use Scan Code Set 3 by default, thus leading to problems with some of the cheapest keyboards, IBM software could still have selected Scan Code Set 3 on the PS/2, and I do know this was done in some cases, although I forget the precise details - so whether it was the PS/2 BIOS, or just OS/2, or 3270/5250 emulation software from IBM which I had heard of that did this, I can't be certain.

[ch_123]

The first Model Ms were sold with AT and XT286s, so yes, they work.

[kishy]

I had read somewhere that the 1390120 supported both XT and AT, but I'm not really sure about this. Besides, why would you want to use a Model M with an XT, it's a waste of an opportunity to use an old Model F!

If you read it somewhere, it's probably correct (nobody has anything to gain in making up a story like that, and it's a fairly random one to make up, so I'd suspect it is true if it was ever written anywhere).

As for Model Fs...I'm looking for one, but my price range, as always, is $0 though about $0. You can see the problem. I don't mind (and in some cases much prefer) heavily beat on old keyboards in questionable condition, but even those cannot be found since everyone in my area seems to be nuts about throwing out everything.

No, I wasn't thinking that at all. In my first post, I was unsure of some of the things I noted from memory; while generally confident, I didn't know some of the essential facts I needed to answer your original question. So, after I found an authoritative reference (Upgrading and Repairing PCs, Sixth Edition, QUE) I was happy to report that I hadn't made any mistakes in the things I originally stated as fact... and I had additional information which did answer the question.

Incidentally, of course, the AT itself had the larger five-pin connector, the smaller six-pin one being called the PS/2 connector for a reason. That later XT clone boards probably were designed to work with contemporary keyboards makes sense.

Ah, understood (@ first paragraph). I did not feel the question was answered though, but to be fair I never made the actual question very clear at all.

@ second paragraph: Yes, it wouldn't surprise me at all to know that XT-like clone computers would use the AT keyboard interface/protocol if they were released a significant time after the AT computer was introduced.

The first Model Ms were sold with AT and XT286s, so yes, they work.

Well..."so yes, they work" on an XT 286 and an AT.
Problem is, the XT 286 appears to have used the AT keyboard interface, which is itself already compatible with PS/2, so most PS/2 keyboards should work on an XT 286 if that's true.

Was the reference to XT compatibility in your earlier post on paper or online (if you remember)? If online it may still be out there, floating around.

[Great Hierophant]

The IBM Model M keyboard for PCs came originally in two basic varieties, the 1390131 and the 1390120.  The 1390131 you would purchase with the IBM PC AT Model 339 (released on April 2, 1986) or the IBM PC XT/286 released on September 9, 1986).  The 1390120 you would receive with the IBM PC/XT Models 268, 278 and 89, all released on April 2, 1986.  (The XT Model F works fine in all IBM PCs and XTs, including these XT Models.)  1390131 has the LEDs, 1390120 does not.  They all used a black detachable cable with an 5-pin DIN connector.  

The PS/2s, first released on April 2, 1987, came with the 1391041.  The only difference between the 1391041 and the 1390131 is that the later keyboard uses a greyish detachable cable with a 6-pin mini-DIN connector and the square silver faceplate has given way to the beige oval logo.  (I know there are other models of IBM Model Ms made around this time, but these were the main ones).  The 5 and 6 pin cables are swappable.  

All of these Model Ms should work with an IBM XT motherboard with an updated BIOS.  All will work with an IBM AT motherboard with an updated BIOS or an XT/286.  Some have been reported to work in an IBM PC or XT (without a BIOS update), but most will not.  The LEDs on these Model Ms, if any, will not function when connected to an IBM XT.  The extra keys of a Model M may not register correctly on an IBM AT without an updated BIOS.

It is believed that when the Model Ms lost the detachable cable functionality, so when the XT compatibility.

[ch_123]

Would an AT Model F be backwards compatible with the PC or XT if given a BIOS update?

[Great Hierophant]

Would an AT Model F be backwards compatible with the PC or XT if given a BIOS update?

With a PC, no, because its last BIOS is dated 10/27/82, well before the IBM AT was released.  

Also, what I said for the XT will work with IBM Portable PC because they use the same motherboard.  But you will need to make a custom adapter, because the Portable uses an RJ-type keyboard connector, not a DIN.  

The IBM PCjr. may also work with a Model M and a DIY custom adapter, but your chances of obtaining a keyboard that will work are about the same as getting one that will work with a PC, or XT without a BIOS upgrade.

Whether an AT Model F will work with an late-BIOS XT is a very interesting question.  While I doubt it, I would be happy to test it out if I ever get an AT Model F.

[kishy]

Oooh, good info, thank you Great Hierophant