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

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

    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.
    With kind regards / met vriendelijke groet, Ruud Baltissen

    www.baltissen.org

    #2
    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...
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs Also: Blogspot

    Comment


      #3
      I was under the impression that the "Baby AT" had the same form factor as the 5160 XT board.
      Reach me: vcfblackhole _at_ protonmail dot com.

      Comment


        #4
        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.

        Comment


          #5
          Originally posted by Chuck(G) View Post
          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.
          My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs Also: Blogspot

          Comment


            #6
            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.)

            Comment


              #7
              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.'

              Comment


                #8
                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.

                Comment


                  #9
                  Originally posted by modem7 View Post
                  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.
                  With kind regards / met vriendelijke groet, Ruud Baltissen

                  www.baltissen.org

                  Comment


                    #10
                    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.

                    Comment


                      #11
                      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.
                      Last edited by tipc; August 17, 2020, 11:36 PM.

                      Comment


                        #12
                        Has anyone asked the question why he wants to build a "baby AT" mobo? Nobody even knows what one is (except in the earliest sense). Is this like a Grail quest LOL? To be safe, best to stick with an well establshed standard.

                        Comment


                          #13
                          Originally posted by tipc View Post
                          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.
                          With kind regards / met vriendelijke groet, Ruud Baltissen

                          www.baltissen.org

                          Comment


                            #14
                            Originally posted by tipc View Post
                            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
                            With kind regards / met vriendelijke groet, Ruud Baltissen

                            www.baltissen.org

                            Comment


                              #15
                              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.
                              My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs Also: Blogspot

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