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Thread: IBM PS/1 Model 2121 CPU Upgrade (386 to 486)

  1. #1

    Default IBM PS/1 Model 2121 CPU Upgrade (386 to 486)

    Hey all! I recently attempted to follow the example set forth on this blog: http://www.zonadepruebas.com/viewtopic.php?t=6184 and upgrade my IBM PS/1's CPU from an Intel 386SX to a Ti 486. See attached photos of my solder-in replacement. However, when I turn the computer on, I do not seem to be getting the stellar test results that blog post seems to be showing. In fact... I am showing some very strange results indeed. CheckIt 3.0 states that my CPU is a 164 Mhz 486 (yikes! I wish...) and CheckIt Pro seems to think my CPU is 20 Mhz (instead of the 33 Mhz designation on the chip.) What might be happening here?

    OLD
    OLD386.jpg

    NEW
    NEW486.jpg

    CheckIt 3.0
    CheckIt3.jpg

    CheckIt Pro 1.11
    CheckItPro1.jpg

  2. #2
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    On the bottom left side of the TX486 chip, do I see a solder bridge on the leftmost pins? Sure looks like one.486SLC.jpg
    Last edited by Chuck(G); February 17th, 2019 at 02:54 PM.

  3. #3

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    Quote Originally Posted by Chuck(G) View Post
    On the bottom left side of the TX486 chip, do I see a solder bridge on the leftmost pins? Sure looks like one.486SLC.jpg
    I concur. Sure looks like a bridge or a bent pin of some sort.

  4. #4

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    Just because the 486SLC/E chip is rated for 33 MHz doesn't mean it'll automatically upgrade your system to 33 MHz when you install it. Unless you also changed the clock crystal on the motherboard, it'll still be running at either 16 or 20 MHz, depending on what the original speed of the 386SX chip of your PS/1 was.

  5. #5
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    There are 5V versions of the clock-doubled TI 486 chip as well that might make more of a difference. Generally, 3.3V TI 486 chips have a "V" in the part number.

  6. #6

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    We were discussing these TI chips in another thread. They are called TI486SXLC or SXLC2. They are both capable of clock doubling despite the different labels.

    There are actually three different versions I am aware of.
    -The standard "unlettered" version is a purely 5V chip. (ex. 40MHz part would end in -40 or -040)
    -There is a "5V tolerant" 3.3V version with the G designation (ex. -G40)
    The manual is not clear on what 5V tolerant means. Some people think this means it could work at either 3.3 or 5V, but maybe this means the core runs at 3.3V while the I/O runs at 5V? Regardless, some people report these chips running in 5V systems without problems (other than getting really hot).
    -There is a purely 3.3V version with the V designation (ex. -V40).

    Clock doubling is probably the way to go unless you can swap in a faster crystal without pushing the motherboard timings way out of spec.
    "Will the Highways on the internets become more few?"

    V'Ger XT

  7. #7

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    Quote Originally Posted by T-R-A View Post
    I concur. Sure looks like a bridge or a bent pin of some sort.
    Good observation. it was a bent leg, I tested with the multimeter and saw no evidence of a short. In addition, I went back in and fixed the bent leg anyway and saw no differences.

  8. #8

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    Quote Originally Posted by vwestlife View Post
    Just because the 486SLC/E chip is rated for 33 MHz doesn't mean it'll automatically upgrade your system to 33 MHz when you install it. Unless you also changed the clock crystal on the motherboard, it'll still be running at either 16 or 20 MHz, depending on what the original speed of the 386SX chip of your PS/1 was.
    Looks like the guy in the blog post i linked replaced the oscillator. I ordered this oscillator on DigiKey yesterday: https://www.digikey.com/product-deta...1-5-ND/5980077. Will try it out as soon as I receive it and report back.

  9. #9

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    Hi.

    I also got an 2121 and want to upgrade the CPU and RAM. Are there any news from you about the CPU?

    Regrads
    Andreas

  10. #10

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    Andreas,

    Sorry about the late reply. I have made it work by upgrading the oscillator to the 66 MHz one I bought on Digikey (link is above). If you have any questions, feel free to ask.

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