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PS/2 Model 25-286 ISA Riser

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    PS/2 Model 25-286 ISA Riser

    This started off as a project to reproduce a riser for the Model 25-286 board. The only one I could get my hands on was $40, for a little 2" by 7" PCB, no components at all.

    After some thinking, and some inadvertent prodding by bladamson who is making his own riser, I decided I could add some amount of functionality with the spare board space.

    This is what spawned my tinkering the other night, adapting a Malinov FDC controller PCB into a UMB provider, here.

    After modeling the 120P .100 edge connector and sounding out my stock riser to compare to the schematics, I thought I had a working design. (foreshadowing)

    I added a 3rd ISA slot on the backside of the riser. I had seen this on some of the risers from aftermarket/upgrade boards for the model 25.

    At the far end I added a 128k SRAM chip and a GAL for a little address decoding.

    In my machine and intended use-case, the 3rd slot has an XT-IDE card. Mapped to C800 to be above the video ROM. The SRAM is mapped from CA00 to E000, between the XT-IDE and the BIOS ROM which starts at E000.

    The added 88K of UMB's is enough to hold my entire load of drivers and TSRs, leaving the Model 25 with 625K free conventional memory. I use HIMEM.SYS for the OEM ram, and USE!UMBS.SYS for my 88k of added memory.

    I only had one layout error. The ISA slot on which the IBM interface is based on has four particular pins, in the order WRITE READ WRITE READ. The IBM interface, otherwise just ISA with a number of grounds added was laid out WRITE READ READ WRITE. I thought this was a typo, and I thought I had proved it out with my multimeter. I was wrong on both counts. After no I/O functions worked on ISA cards, I figured out my error. A couple of quick bodges and we were off to the races.

    The physical dimensions match the 8-bit version of the riser from the early Model 25s. The plastic riser from either machine will work, they just need some (a lot) of trimming. I have found with at least one ISA card on the primary slots secured with a screw that the riser is firmly in place and I don't see the need for the riser. You just can't be entirely hamfisted when inserting/removing ISA cards.

    First the bodge, then beauty shots.

    I have 4 more of these PCBs at hand, and Gerbers with the fixed IO signals if anyone is interested. Also the GAL programming files as well. I bought enough ISA slots to fully populate the boards, boy is it fun to solder 98x3 pins all in one sitting, plus I have a couple more of the SRAM chips and 16V8 GALs coming out of my ears!

    Very very nice. Interesting that the XTIDE works properly too.


      The xt-ide was working with the original riser. I have it's eprom mapped to C800 through CA00. My riser doesn't do anything strange to the xt-ide, other than I have it mounted in the upside down slot on the back of the riser.

      The UMB provider is only mapped for 88k from CA00 to E000 which is where the PS2 BIOS begins (it is 128k)


        Sorry, it was more an out loud thought of mine comparing it to the PS1 2011 286. XTIDE wont work unless its a very specific version. Didn't mean to derail the thread.

        Originally posted by TheDrip View Post
        The xt-ide was working with the original riser. I have it's eprom mapped to C800 through CA00. My riser doesn't do anything strange to the xt-ide, other than I have it mounted in the upside down slot on the back of the riser.

        The UMB provider is only mapped for 88k from CA00 to E000 which is where the PS2 BIOS begins (it is 128k)


          No issue from me. No one else has even replied so there is really no thread to derail!


            Pretty snazzy!

            Ya beat me to it, lol.
            -- Lee
            If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
            Looking to Buy/Trade For (non-working is fine): Mac IIci hard drive sled and one bottom rubber foot, Multisync VGA CRTs, Decent NuBus video card, PC-era Tandy stuff, Weird Old Unix Stuff, Aesthetic Old Serial Terminals (HP and Data General in particular)


              which trace width and the lowest space between trace did you use ?


                I will double check the DRC I used, but I believe it was 6/6


                  I found that some Eduquest machines had FOUR Isa slots with an extra on the backside. I have to double check if that will physically fit in my case, but if so I will add the 4th slot to the re-spin with the corrected IO R/W and have new boards made.


                    Not mine, images of an original Eduquest riser showing two slots on the backside.


                      I got out the boroscope and inspected the inside of the case with everything put together. Definitely not enough room for a 4th slot. The chassis on the Eduquest must be different.


                        This is really nice work, @TheDrip! I’ll have to open up my 25sx and check the riser in it.

                        I’d like one of your PCBs, either assembled, kit, or bare, whatever you are interested in doing. I’d also like to see your Gerbers.

                        Have you looked at the model 30-286 riser? I’d assume that they would be very similar or identical.

                        - Alex


                          PM me your email and I will get the Gerbers and GAL files over to you.

                          I will also get your address for sending over bits 'n pieces.


                            As for the 30-286 riser, it's electronically compatible, but physically it won't fit. The model 30 riser is 3 slots tall and runs into the bottom of the monitor casing.

                            Theoretically you can sawzall off the top slot to make it work in the 25 but no one seems willing to do that!


                              Thanks, @TheDrip. I was thinking of the reverse, using your riser in the model 30-286. That should fit fine.

                              Since your riser is superior to the stock riser, it'd be a great upgrade in that machine, too.

                              - Alex