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Looking for the dimensions or form factor of the baby AT board

Ruud

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Hello,

I'm looking for the form factor of the baby AT card so I can reproduce it in Eagle. I found the ones for ATX at various places but the ones for the baby AT or XT seems hard to find.

If not to be found, I think I have to do the job of measuring everything myself :) Of course I will the results.

Why do I want them? I want to design my own motherboard, just for the fun of it. Yes, I know, there already exists very nice designs like Sergey's XI8088 (in fact I have two of them) but again, just for the fun having my own one.
 
I don't think there ever really was an "official" specification for Baby AT? (If you look at archive.org's old dumps of formfactors.org, which was the "open" standards board that pushed ATX and BTX and now redirects to intel.com there is nary a word on it.) More or less I think the various players in the industry just individually slapped the motherboard from an IBM 5162 (strictly speaking the closest thing to an official "Baby AT") on the photocopier, measured it, and said "close enough".

(Pentium era Baby AT motherboards in particular were really bad about things like random case and slot interference, there's a reason why ATX needed to happen.)

That said you'd think that *somewhere* on the web there must be a decent cad rendering of an existing AT board you should be able to get dimensions from...
 
I believe there were several variants of the Baby AT form factor. IBM primarily narrowed the AT in one dimension whereas my ASUS P2B-B boards labelled as "Baby AT" were narrowed in both dimensions to 8.66" x 8.97".

Perhaps the closest defined format is uATX at 9.6" x 9.6" or 24.4cm x 24.4cm for which there are plenty of small tower cases.
 
I was under the impression that the "Baby AT" had the same form factor as the 5160 XT board.

In theory, but I think the truth is more complicated than that when you're talking about generic boards. It's been a long time, but it's my recollection that most "generic" Baby AT motherboards in the 80's and 90's tended to be just a *little* bit bigger than a 5160 board. (I hot rodded a few 5160s I got for nothing at government surplus sales into 486 and Pentium machines back in the day. Usually had to cut out some of the left drive mount to clear the CPU, SIMMs, or whatever else sat in that area, but the main thing I'm thinking of is most boards were maybe... something under half an inch? longer front-to-back.) This of course doesn't count those tiny "half-baby" boards that were pretty common near the end of a given CPU's life; I used to have a 386SX board that was hardly deeper than a 16 bit slot connector (barely reached the second row of standoff mounts).

Another thing you see on generic Baby AT boards is they'll have mounting hole peculiarities, which I think are there because they're a compromise between the XT and AT standoff positions. One that's really common is an oblong-shaped standoff hole in the upper left quadrant. Like this:

standoff_hole.png

(This is from a picture from an eBay listing for a 386SX motherboard, but it's *very* common on all the generic motherboards.)

If you were going to make a board that you want to fit the most cases it might be worth copying some of that weirdness.
 
Mueller's information on the baby-AT form factor is shown at [here].

(Of course, Mueller's 'Upgrading and Repairing PCs' book is not infallible.)
 
Conflicting with Mueller's book, in the motherboard manual at [here], the motherboard maker's definition of 'baby AT' is 12" x 8.6"

'Different things to different people.'
 
As Chuck said, early "baby AT's" mobos had the same mount points as a 5160 mobo. As you move towards the pentium era, maybe the physical size was smaller, but mobos were made to conform to AT moint points? I once had a bog standard AT clone case that wouldn't take an 5170 mobo, not without a hacksaw. Years later I bought an AT case (something tells me it wouldn't have taken a full sized AT board either), and installed a pentium mobo inside. Something tells me it didn't conform to the 5160 specs.
 
Mueller's information on the baby-AT form factor is shown at [here].
I found that one as well, but it is puzzling me. I have various XT clones and they have nine holes in three rows of three holes each, nicely spread. This picture is missing the hole at bottom/right and the one at middle/right is much higher. The second one from left in the top row does ring a bell.

What was the nice part of the pictures of the form factors of the ATX boards that I found: they showed you the position of the slots. Now I have to do all the "hard" work. Why "hard"? I live in meter land and all dimensions are most probably in inches. If I measure something, convert it into inches and I get a nice number, fine. But if it isn't a nice number...... What is most important is that I get the position of the slots relative to the holes right. I think that I have to dig up an original IBM XT board and use that as a main reference. Then I will check the results against various 286(+) boards I have laying around as well.

Of course I will share the results and I hope that you are willing to share them on your site. Once I have finished the package of the board in Eagle, I'll share it as well.

FYI: if interested, the ATX one is already available. Just contact me. It is able to hold eight ISA slots although only seven have been outlined in the package.
 
I'm not sure there can be said to be a single set of AT / baby AT mount points. I've found that particularly with the central line of screws, there are at least two possible positions, about a centimetre apart. There's an Escom case beside me as I write this, which has fixed mount points matching an Intel Advanced/ZP motherboard. I found a couple of 486 boards with a similar form factor had the central screw in a different place, so that particular mount point couldn't be used.
 
It comes down to the case, what it supports. If it's an old baby at mobo, it's xt, as it'll fit in a 5160 case (had to, as many boards were made to be upgrades for ibm oem. A few were even specified as 5150 upgrades, the Bullet-286 was one). I can't say whether any full size clone at cases would take a baby at mobo, but I kind of want to say yes. I have no specific recollection to that effect though. Later 486/pentium mobos were definitely designed for full sized at cases, but generally I'd have to say very few were full size boards, near the size of a 5170 mobo. I would have to suspect anything specific to the xt form factor wasn't made much past 1990 if that far. Until atx came around, it was all AT.

Maybe share photos of whatever cases you have, and even dimension. I have an actual IBM AT, and something called an ATjr. I should know, but can't remember if it's 5 or 8 slots, probably 8 though (and hence an xt or baby at). I'll scope all that out tomorrow night.

I also have photos somewhere of the MBE-XT Canadian bare board I sent to Mike Mruzek years ago (we actually traded). It seemed to be a 2 sidwd board, iow no intervening layers. He never got it working. Currently I believe it's lost somewhere on the European sub-continent.

Are you planning a layered board Ruud? There's another bloke on here who has an 80186 mobo. You should rattle his cage and glean the knowledge to build one of those. That would be exceptionally cool.
 
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Are you planning a layered board Ruud?
To be honest, I'm not sure what you mean. My goal is to create my own 8088 XT compatible board that should fit into a IBM XT (compatible) case OR one that should fit in an TX case. If possible a two layer one. If that is not possible, one with more layers.
 
Has anyone asked the question why he wants to build a "baby AT" mobo?
When I was looking for the form factors for the XT board, I hardly found anything, just some dimensions. But one site mentioned that XT was the same as "baby AT" and when I used that as search term I found a lot more.

For the "why", see previous and first post.

But it seems I have to do things in the hard way :)
 
As I mentioned, I'd probably recommend looking up as high-res photos as you can find of various late-80's through mid-90's motherboards to get an idea of what they were doing with mounting hole placement. I mentioned the "oval hole" thing in the upper left, but I also recall things like John Elliott mentioned, IE, different hole spacing in the middle row, and there were also variances in where the screw/mounting hole closest to the keyboard connector lived.

Again, I'm pretty sure the initial intention was to allow these boards to be stuck in *either* a 5160 or a 5170 case (which of course was initially designed to take a larger motherboard; the holes *mostly* match the 5160 in the slotted area, but not entirely), but an unintended consequence of this was the mounting positions in clone "Baby AT" cases themselves started becoming an unpredictable mix of 5160 and 5170. This is what you get when you have a whole diverse manufacturing industry basically playing "telephone" instead of actually having standards.

If you *only* want your board to fit XT cases then just copying a 5160 board exactly will probably do. But if you want it mountable in a 90's vintage case I'd definitely recommend adding a few special holes.
 
When I was looking for the form factors for the XT board, I hardly found anything, just some dimensions. But one site mentioned that XT was the same as "baby AT" and when I used that as search term I found a lot more.

For the "why", see previous and first post.

But it seems I have to do things in the hard way :)

Does this help as it shows the standard dims for the AT & XT? The mounting holes would be a prime consideration. http://geekscomputer.blogspot.com/2008/07/xt-and-at-motherboard.html

P.S. Nice home page.
 
Basically unless you had a specific mobo in mind you wanted to emulate to whatever degree, and one that was baby ATish, I recommend just utilizing the well established standards, that is 5160/5162 or 5170. You have a slop factor regarding the actual dimensions. What I'm talking about is the mounting hole locations. If possible, make so it'll mount in either an xt or at case. I can understand therefore choosing a baby at form factor, as vacant at cases are (possibly) more plentiful. I used to have at clone cases, no longer though, only the 5170. The ATjr I mentioned, which could stand to generate some excitement in this thread if nowhere else (ok I'm getting excited if no one else!), is in reality just an xt in virtually all respects. That and a fugazzi. But an xt in a baby AT case, really unrelated to this discussion totally.
 
The ATjr I mentioned, which could stand to generate some excitement in this thread if nowhere else (ok I'm getting excited if no one else!), is in reality just an xt in virtually all respects. That and a fugazzi.

My impression, which admittedly may be fuzzy because I was only a teenager at the time, was "ATjr" was sort of an informal name for the first crop of "Baby AT" cases. Which, as you say, were essentially just slightly taller XT cases that had a top that was styled more like an AT. (IE, the drive positions were only exposed on the far right, the "middle" bays were hidden.) It was a little later that the fully XT-derived ATjr morphed into "Baby AT" in the sense of the diverse family of mini/mid-tower cases and small desktops we had up until ATX took over in the later 90's.

The first "AT" my family owned lived in an "ATjr" case, the result of an attempt at upgrading our PC/XT clone going pear-shaped because the 286 motherboard we mail ordered ended up having a component just barely too tall to fit under the drive bays. That resulted in a run to the local computer store to buy *just* the "AT Junior" case, which in addition to the styling changes had the maybe 1/2" of additional clearance under the drive bay needed; didn't even get a power supply, an XT power supply would fit those cases so I used the one we had. (A "real" AT power supply is of course taller and has an "L"-shaped indent to fit around the wider motherboard.)

(I would have taken a hacksaw to the XT case to solve the height interference problem except then our full-height hard disk no longer would have fit along with two floppy drives.)

Apparently some people did advertise computers under the "ATjr" name. In this case, ironically enough, an XT class machine. If they advertised them as such I suppose they must have also slapped labels with that name on them. (The case I bought back then was blank, of course, so the computer store could slap their own badge on it):

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But an xt in a baby AT case, really unrelated to this discussion totally.

Why? If we're talking about making a motherboard for people to play with it might be useful if it fits as many cases as reasonably possible. (Unless, yeah, it's specifically about making a reproduction motherboard. There was someone selling a reproduction 5150 motherboard a few years ago, in the vein of those Apple II reproduction boards; you can probably count the number of clone cases with the 5150's slot spacing on one hand, so if the goal is to do the same for the 5160 then by all means copy that exactly.)

Basically the dimensions that really matter are the slot spacing, the distance between the slots and the back, the position of the keyboard connector relative to the slots, and the relative positioning of at least *some* of the mounting holes to those landmarks. After that as long as you're in the ballpark you'll probably be fine in terms of gross dimensions; if you make the board the same size as a 5160's that's probably the safest bet because it's *slightly* on the small side as full size "Baby AT" motherboards go. The only thing I'd suggest, again, is looking at adding some of those extra holes. There are high-res photos of 5160 and 5170 motherboards out there, if you can find some that are sufficiently dead-on that you can overlay them just chart any differences between the hole locations, put holes that match both into your design, and that will probably be good enough.

(You can probably get pretty good measurements from those photos if you're sneaky and, again, you can find a dead-on view. Remember, the pin pitch in the slots and most of the DIP chips is going to be ten pins to the inch. Use that to calibrate a ruler to the photo and go to town.)
 

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I didn't read the your entite reply, I'll do that later. But by ATjr I was specifically referring to the Advanced Micro Technologies ATjr, which ruffled IBM's panties, and made them change the name (to AMTjr). It's a mostly unexciting xt clone in a diminutive AT style case. I hadn't realized others used the term. My AMTjr, when it was in better shape and worked, still said ATjr at boot, so in yer eye IBM! It does sport a Sony V20 clone. It's in horrible shape presently but will have a complete makeover soon.
 
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AMT and Micro Express were selling their "ATjr"'s at the same time (1986-1987 ballpark). Undoubtedly they were both just slapping their labels on the same made-in-Taiwan clone. Like I said, that same/similar "AT/XT hybrid" cases were around for at least a while afterwards before everyone started abandoning the XT power supply form factor and moving to those "baby AT" size ones we had all through the 1990s.

Judging from news stories about the lawsuit IBM initiated it looks like they were more pissed off about the kid in the Charlie Chaplin suit than the name, but when you're a lawyer you throw everything at the wall you can, I guess.
 

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