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IBM 5155 RAM upgrade questions

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themaritimegirl

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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!
 
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.
 
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?
 
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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 !
 
Trixter said:
You found an IBM 5155 in mint condition for $5!?
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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.

mikey99 said:
You could just get an AST SixPack Plus card, it will boost memory to 640K add printer/serial/game ports....all in one slot !
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Did a quick look on eBay, and there were only two, for $75 and $200, unfortunately.
 
TheMaritimeMan said:
Yes sir.
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Well, I officially hate you now ;-)

Did a quick look on eBay, and there were only two, for $75 and $200, unfortunately.
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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.
 
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.
 
I just cracked open the Epson Apex Plus. It has 9 socketed M41256A-12 chips, which appear to be a compatible part. It has 21 more chips, but they're soldered to the motherboard. That's a good start though - just the one socketed bank will bump me to 448 KB. If I successfully perform the mod and get it to work with the socketed bank, I may unsolder enough chips to get a full 640 KB. Now I just need to get a hold of the TTL logic chip, which should be cheap and easy. My university's tech shop may have some.
 
TheMaritimeMan said:
I just cracked open the Epson Apex Plus. It has 9 socketed M41256A-12 chips, which appear to be a compatible part. It has 21 more chips, but they're soldered to the motherboard. That's a good start though - just the one socketed bank will bump me to 448 KB.
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I am not sure that is possible. Remember that the 41256 chips has to be installed in the first banks.
 
If this is really a near mint condition machine, you should really consider not modifying the motherboard. Is there any reason a ram expansion card could not be used with this specific machine (It is not totally filled up already is it?) Of course, to each their own.

Also, I would not put unsoldered chips in to a socket for something like this as the legs would be all torn up, might make poor contact, and may even slightly damage the sockets.
 
pearce_jj said:
It can run 640KB, my RAM PCB will do the job :)
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That looks like a great solution, but I have no way of soldering surface-mount components. Is there anywhere I can buy an assembled unit?

SomeGuy said:
Is there any reason a ram expansion card could not be used with this specific machine (It is not totally filled up already is it?)
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Simply because I assume they go for lots of money. The SixPackPlus does, at least. At least those Apple IIGS cards look like they can be gotten reasonably cheap for the chips (I didn't look at this thread and see vwestlife's link before the listing ended).

SomeGuy said:
Also, I would not put unsoldered chips in to a socket for something like this as the legs would be all torn up, might make poor contact, and may even slightly damage the sockets.
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Good thinking, I didn't think of that when I wrote that.
 
You should consider THIS MOD to put 1 MB on the motherboard. The thread linked HERE explains the process. It's a long thread but worth the reading in my opinion. I convinced Chuck(G) to help program the GAL to make it work some years ago. I would be glad to help you with the mod if you are interested. I can supply a programmed GAL and adapter for cost of postage. Just PM me.
 
Really, there's a few ways to accomplish what you're after.

Personally, I'm a fan of the backfilling EMS approach (which can be done using an EEMS or true EMS 4.0 board, or it can be done using an XMA board and XMA2EMS.SYS if you want genuine IBM hardware). With backfilling EMS, most board manufacturers typically recommend 256k be on the motherboard, and then the RAM board takes care of the rest. The reason you'd want to do this is for more flexible memory allocation (want to map a 16k page into arbitrary address space that's unused? Just do it), as well as better support of things like Desqview (which really, really wants backfilling EMS to manage task switching for it).

The trick is finding one, though, and there's no AST SixPakPremiums (what I'm using - although I could use a PremiumPak to upgrade to 2 MiB of EEMS) on eBay right now.

The official IBM approach to upgrading RAM on an XT motherboard does present the problem that, if you have soldered RAM on the motherboard, you WILL have to desolder it. (IIRC the first bank is sometimes soldered.) Another approach is similar to HIMEMV2's approach of replacing the U44 decoder PROM, but instead of having mappings to go beyond 640k, it maps things so the memory layout can be 64/64/256/256 in the default map (which guarantees that nothing has to be soldered), instead of IBM's 256/256/64/64 in the E2 jumpered 1-2 map. That's what someone did on the XT board I've got... but I don't know what settings are in the other pages, so I can't easily run 256 on my board without replacing the PROM with a stock one. So, I'm running 128k on the motherboard in my 5155, which reduces available EMS by another 128k...
 
TheMaritimeMan said:
Simply because I assume they go for lots of money. The SixPackPlus does, at least. At least those Apple IIGS cards look like they can be gotten reasonably cheap for the chips (I didn't look at this thread and see vwestlife's link before the listing ended).
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I think with the SixPack plus, that is a more well known and feature rich card. With an expansion card like this it may also have a clock chip, serial ports, parallel ports, or provide EMS (bank-switched) memory beyond 640k.

So it depends on what all you want to add. I'm not sure what they would normally sell for, but I'd expect with some patience a simple no-name 384K ISA expansion board might come up for cheap. The main problem with no-name cards is figuring out the DIP switch settings.
 
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