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Thread: IBM 5155 RAM upgrade questions

  1. #1

    Default IBM 5155 RAM upgrade questions

    Hi all,

    I recently became the owner of a beautiful mint IBM 5155 PC Portable. It came with the usual 256 KB of RAM, and I'm a little confused as to what is the maximum RAM that can be used in the motherboard.

    It's my understanding that the IBM PC/XT had two motherboard revisions - the first of which supports up to 256 KB on the motherboard, and the second up to 640 KB. According to what I've read online, the 5155 only ever shipped with the early revision of the motherboard. However, I've seen several videos on YouTube of 5155's with 512 or 640 KB of RAM, and when I asked the uploader if it was on the motherboard or an expansion card, two of them replied and said it was all on the motherboard. One of them even told me he owns two, both with 640 KB of RAM, all on the motherboard, and he never upgraded them with a later XT motherboard.

    So what's the deal? Is the early XT motherboard capable of holding 640 KB of RAM? Did the 5155 later ship with the revised motherboard? Or are the YouTubers I spoke to mistaken? I guess the big question is, can I expand the RAM on my unit any further on the motherboard itself? My unit indeed has the "64-256K" motherboard.

    Thanks!

  2. #2
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    All of your questions (and more!) are answered at the wonderful minuszerodegrees: http://www.minuszerodegrees.net/
    Offering a bounty for:
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  3. #3

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    I've been there, and that's where my confusion started. It says that 256 KB is the max (at least, it appears to - it doesn't actually outright tell you the maximum for the 64-256K board), but then there are the ones I've seen on YouTube that have more, but apparently without a motherboard swap or an expansion card.

  4. #4
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    At the bottom of this page, there is a link to instructions on modifying the 256KB board to allow 640KB on board.

  5. #5

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    Wow, thanks! I read about a modification being possible before, but I had no idea it was that simple! That should be fairly doable. The hardest part will get getting the memory chips. Looks like I can get 41257 chips on eBay, but Ill be paying almost $40 for 18 of them - 8x what I paid for the computer itself! Anyone know of any equivalents that are more cheaply available?
    Last edited by TheMaritimeMan; March 3rd, 2016 at 08:17 PM.

  6. #6
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    You found an IBM 5155 in mint condition for $5!?
    Offering a bounty for:
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    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  7. #7
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    You could just get an AST SixPack Plus card, it will boost memory to 640K add printer/serial/game ports....all in one slot !

  8. #8

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    Quote Originally Posted by Trixter View Post
    You found an IBM 5155 in mint condition for $5!?
    Yes sir. I found it in the amateur radio station at my university, and when I asked about it they had no problem giving it to me. The only reasons they didn't give it to me for free was that money made the paperwork easier, and they could give me a receipt for it. Plus the money goes directly to the station's funds.

    Quote Originally Posted by mikey99 View Post
    You could just get an AST SixPack Plus card, it will boost memory to 640K add printer/serial/game ports....all in one slot !
    Did a quick look on eBay, and there were only two, for $75 and $200, unfortunately.

  9. #9
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    Quote Originally Posted by TheMaritimeMan View Post
    Yes sir.
    Well, I officially hate you now

    Did a quick look on eBay, and there were only two, for $75 and $200, unfortunately.
    You will find that these prices (well, at least the $75 one) are par for the course, including the memory in your previous post. Your experience (getting a great system in great condition for $5) is not typical of the current state of the hobby, although it is nice to hear people can still stumble onto good finds now and then.
    Offering a bounty for:
    - A working Sanyo MBC-775, Olivetti M24, or Logabax 1600
    - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

  10. #10

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    I will admit that I'm unfathomably cheap. For example, if I had seen this computer in, say, a thrift store, I wouldn't have paid more than $20 for it. Guess I'll suck it up and eventually pay for the chips, then. Although I'm not surprised at the prices of the expansion card, I am surprised at the prices of the chips, since I see 64 Kbit chips for a fraction of the cost of the 256 Kbit ones. I guess that only makes sense.

    I just remembered, my Epson Apex Plus, which has 640 KB of RAM, may have the right chips. This computer totally takes precedence over that one, so I'll gladly pull the chips from that if they turn out to be the right ones. I also have a Sharp PC-7000, but that uses the 10-pin 41464 chips.

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