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Thread: Struggling to build a working 486

  1. #1
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    Default Struggling to build a working 486

    Having working early DOS machines, a decent XP/98 dual boot machine, and of course plenty of current machines, I've found the gap in my collection of functional machines is something of the Windows 3.1 era. So I've decided to build a decent 486 machine.

    But, buying a bunch of used parts, and putting them together, more often than not results in a useless doorstop being built.

    How in the heck am I supposed to figure out how to get a working machine together with nothing but used parts, often sold as is/for parts repair, because sellers can't be bothered to test anything?

    Standard troubleshooting has always told me to start with everything disconnected from the motherboard, and add things one by one. That seems to not be an effective method in this era, though.

    I have a new power supply. It powers up individual components. I know that's good.

    So I started with power supply and motherboard (and pc speaker). Nothing.

    So I returned the motherboard, reported it as dead which was my findings based on what has been standard troubleshooting all of my professional life in computers, and found a listing for one that was tested.

    Same results. Another dead board.

    So I reached out to the seller. He asked if I put RAM in it, said RAM is required for it to turn on.

    Tried that....still nothing.

    Now I need to sort out, is it a bad motherboard, or bad RAM?

    What even is the right RAM for this? The motherboard is dx6900 ver 1.4, found this link about it - https://stason.org/TULARC/pc/motherb...0-VER-1-4.html

    It looks like the better option for ram is the 72 pin slots. So I went looking for 72 pin RAM on ebay. Lots of options. Some say for MAC Apple. Some say for mac and pc. How do I figure out what is or isn't compatible with this board? The link I found is great for pin and jumper settings, but offers no help as far as RAM information.

    Could someone point me in the right direction as far as what RAM to get, and how to obtain tested and known working RAM of this vintage? I understand troubleshooting and testing is necessary when dealing with older things, but once again, I run into nothing but roadblocks around here, and am getting rather sick of acquiring piles of garbage that are absolutely useless to me. It seems every time I try to pivot away from one roadblock to something that should be easier, it's just another roadblock, waste of money, and pile of garbage.

  2. #2

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    If your RAM is bad, you'll still usually hear a post-code over the PC speaker.

    To even get that far, though, you'll need to have the CPU installed and jumpered correctly.

    Unless you are running a DX4/100, you'll want to jumper for 5v. If there is no jumper for that, your mobo either only supports 5v, or autodetects CPU voltage. You can test for this by putting a jumper wire into a certain pin in the CPU socket and then measuring voltage on a certain other one, but let's just forget that for now unless you explicitly need a DX4 mobo.

    Your front-side bus speed can be set to either 25MHz or 33MHz. This is combined with a multiplier setting to get the CPU speed set right. For example, a DX2/50 (50MHz) would have the FSB jumpered to 25MHz, and the multiplier jumpered to x2. A 33MHz CPU would be jumpered to 33MHz and x1. 66MHz CPU jumpered to 33MHz x2. DX/4-"100" would be jumpered to 33MHz x3. If in doubt, try running the machine at 25MHz x1. Most if not all CPUs ought to be able to run at a lower-than-rated speed, but jumpering it too high can cause instability or even immediate crash on boot.

    Once you get that far, insert some RAM, a video card, and connect a speaker. If you are lucky, you'll get a BIOS POST screen. If you are unlucky, you should at least hear beep codes that you can look up in your mobo manual.

    Be careful with 72 pin ram. Most of what you'll find for sale is 60ns EDO ram designed for Pentium systems, and most 486 boards were designed to work with 70ns DRAMs. Some 486 boards will run EDO RAM fine, some will behave erratically with it, and some won't work at all. I think it has something to do with the refresh rate, but I am not 100% sure on that.
    Last edited by bladamson; June 6th, 2020 at 09:28 AM.
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): TRS-80 Model II,12,16,6000, Mac IIci hard drive sled and one bottom rubber foot, Hercules card + mono monitor (preferably IBM 5151), Multisync VGA CRTs, 040 or 601 card for Mac IIci, Decent NuBus video card, Commodore PC(286+), PC-era Tandy stuff, Aesthetic Old Serial Terminals, Amiga 2000 or 3000UX

  3. #3
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    Default

    You do not want Mac RAM. It is non-parity and will not work with a motherboard needing parity RAM (indicated by the x9 or x36).

    Testing would probably be easier with 30-pin SIMMs. Find a set of 4 SIMMs each having 9 chips running at 70ns. Those are the most common type of 1 MB 30 pin SIMM so should be easy to find some that work and are affordable. There were so many varieties of 72 pin SIMMs and that motherboard only works with a narrow range of them. Checking the label of each chip could let you determine if the 72 pin SIMM matches but that is a lot of work. Don't expect to find any SIMM with a larger capacity than 4 MB.

    Guaranteed working RAM? I don't know of a good source for it.

  4. #4
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    Default

    What I bought is parity RAM, but I've not found a straightforward guide for what works with parity, what works with non-parity.

    The seller of the motherboard says the board takes EDO RAM. Don't know what that is, or ...... I'm all confused and just want a clear guide as to what to buy that will work exactly.

    The motherboard came with processor chip, was tested with that, jumpers should be set right for that processor.

    When I put in one of my parity ram chips, connect power, and turn it on, power stops shortly after. I've connected the fan on the processor as well. With only the fan connected, turning on the power supply results in the fan spinning. When I connect the motherboard, turning it on results in the fan spinning briefly and immediately stopping. Turning it off and back on results in no fan movement, until I disconnect and reconnect either the ram or the power supply.


    I just want a clear instruction as to what parts to buy to build a working machine so I can stop acquiring garbage piles and stop throwing my money away on more and more projects that turn into garbage piles. I am beyond frustrated at this point. I just want to build a working 486, and I am willing to work at building it and troubleshooting, I don't just want to buy a working machine, building it is part of it, but I want more than just non working piles of garbage.

    Can I please for the love of god get some guidance in the right direction to buy a working set of parts to build a working machine?

  5. #5
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    I'm told here I want parity RAM. The parity RAM I have doesn't work.

    The seller tells me I want EDO RAM. All 72 pin EDO RAM I search for on ebay is non-parity.


    Can ANYONE just give me a straight answer.....what in the heck RAM works with the motherboard which model and web link info I've posted?

    Does anyone know how to determine what RAM to buy?

    Why is it so hard to get a clear answer about this? I've given the exact motherboard model, posted the link I found.....but is the information about what type of RAM this requires lost to time forever? Why is it so impossible to get a clear answer to just about anything around here?

  6. Default

    If the AT power supply is kicking off after you switch it on that would indicate a short circuit. Is the motherboard installed in a case? Sometimes mounting plates or standoffs can contact a solder joint or a trace running next to a mounting hole and cause a short. Something to look for.

    Other things to think about: Maybe the board was jumpered correctly at one time but jumpers fell off during shipment? Double check that the PC speaker is connected on the right pins? Connect a power LED and see if that comes on?

    edit: I wouldn't assume the RAM is bad just yet. But it's tough to know whether parity or FPM/EDO even matters without knowing what chipset the motherboard uses
    Last edited by bakemono; June 6th, 2020 at 11:26 AM.

  7. #7
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    The only source I can find mentioning the motherboard model claims it uses FPM memory not EDO. I expect that it should work with FPM 30 pin SIMMs. https://www.memoryx.com/generic-memory-30-pin-simm.html choose the 1MB 9 chip 70ns design. Those were the most common form of memory in the early 90s and useful in early 486, most 386, and even some 286 systems and memory expansion cards. Double check the status of the listed supplier. I think I ordered from them many years ago. They may not be as reliable now.

    I know you want the 72 pin but it is difficult to find memory that matches and even harder since a lot of memory is incorrectly labeled.

    Note: If you have the fake cache version of that motherboard, throw it out. It is going to be slow. It was also not very reliable.

    http://www.elhvb.com/webhq/specs.htm
    https://www.memoryx.com/fpm.html

  8. #8

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    Most PC RAM iirc is non-parity. IIRC it's (some of) the Macs that need parity ram, various old Unix workstations, and some server boards. If the mobo is supposed to use EDO RAM, you'll be fine with EDO RAM then I guess. The deal with EDO RAM is that it has a burst mode that the older 72 pin RAMs didn't, but EDO is *supposed* to be backwards compatible.

    Surely a board that is new enough to have VLB will have 72 pin SIMM sockets, not 30 pin? If you can see any pictures of the board, zoom in on the SIMM sockets. If it takes 72 pin SIMMs you'll see a divider thing in the middle of each socket. If 30-pin, there won't be that divider (and the contacts will be spaced further apart and fewer).

    Edit: Went back to the page you linked. Looks like that board will take both 30 pin and 72 pin RAM. Interesting. I bet you can't install both at the same time though, or if you do I bet the 72 pin ram slows down to 30 pin speeds.
    Last edited by bladamson; June 6th, 2020 at 12:01 PM.
    -- Lee
    If you get super-bored, try muh crappy YouTube channel: Old Computer Fun!
    Looking to Buy/Trade For (non-working is fine): TRS-80 Model II,12,16,6000, Mac IIci hard drive sled and one bottom rubber foot, Hercules card + mono monitor (preferably IBM 5151), Multisync VGA CRTs, 040 or 601 card for Mac IIci, Decent NuBus video card, Commodore PC(286+), PC-era Tandy stuff, Aesthetic Old Serial Terminals, Amiga 2000 or 3000UX

  9. #9

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    Make sure you have the AT power connectors in the right positions on the motherboard. Black wires should be in the center and red/orange wires should be on the outside.

  10. #10
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    Default

    From talking to the seller of the motherboard, I'm not too confident in whatever he said. He recommended checking the pin configurations, because he doesn't remember what processor he tested it with. Seems like when he listed it as tested, he means, at some point in time I tested all these parts, and they worked some time ago, but now I"m gonna mix and match parts and list them as tested without so much as details that they work together or have been tested recently or in this configuration. So....so much for relying on listings that claim they are tested, and receiving things ready to use after testing.

    Meanwhile, the seller of the RAM suggested another listing of his RAM that should work with the board, so I'm working an exchange there.

    "fake cache version" ????? How do I determine this?

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