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64-256k --> 256-640k

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josephdaniel

Experienced Member
Joined
Jul 3, 2012
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Location
Florence, Texas, United States
I have an XT from November of 1984 with a 64-256k motherboard in it that I want to modify to the newer 256-640 motherboard. I ran into two small problems where can I get the 256K RAM chips and where can I get the multiplexer chip that goes in u84? I am fine with soldering (Antique electronics is another hobby of mine) pin 1 and 11 on u44
Note also where might I find the later BIOS that includes support for 101 key enhanced keyboards and boots faster?
 
You'll be able to download the binary file of that ROM image from minuszerodegrees

http://www.minuszerodegrees.net/bios/bios.htm
Thanks to modem7.

You'll want to have an EPROM writer, UV eraser, and some spare chips handy (unless someone here volunteers to send some pre-written).
Although I've never needed to upgrade the BIOS on an XT, and I use 101 key clone XT keyboards without problems - perhaps mine go in to '83 key mode' or something.

For the memory situation, have you considered just buying an AST SixPakPlus?
Replace your serial/parallel card with it - gives you serial/parallel/384KB RAM/battery backup clock - in one slot.
One slot is key, because you basically replace your current serial card with it, so no space is lost internally and you get a 640KB XT that can remember the time!

But if you want to go ahead, 41256 chips are easily found on ebay (36 of them + I'd get a few spares). Not sure about U84 sorry, I'm sure someone else here will know.
 
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Thanks guys,
If I can't find a pre-made BIOS I am probably fine without it... I don't really want any multi I/O cards as I just bought all of these cards and then I saw this internal modem that I *had* to have. Famous last words) if I ddin't see it I woulden't be having this problem... Thanks for the tip on the RAM chips though
 
ok so I figured out that I already have 10 256k chips on my expansion board (I will get the other 8 that I need fom ebay) the rest of the chips in my system are all 64k chips. I found the chip that goes in u84, it is the 74LS158 multiplexer chip which is avalable very cheap
 
Why stop at 640K? With 36 256k chips (all 4 banks) and a custom programmed GAL with adapter in u44, you can have 640K of base memory and up to 256k(depending on video card etc...) of Upper Memory that can be used as UMB to LOADHI drivers to free up conventional memory. If interested, I can supply the GAL and adapter for the cost of shipping. READ HERE FOR MORE INFO
 
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I have this mod on my remaining XT-Board, works great, but my 8-bit card doesn't like it very well, so i roll CGA and it's flawless

ibmapc said:
Why stop at 640K? With 36 256k chips (all 4 banks) and a custom programmed GAL with adapter in u44, you can have 640K of base memory and up to 256k(depending on video card etc...) of Upper Memory that can be used as UMB to LOADHI drivers to free up conventional memory. If interested, I can supply the GAL and adapter for the cost of shipping. READ HERE FOR MORE INFO
Click to expand...
 
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