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Video RAM Upgrade for Tandy 2500 SX/33

A

andyjibb

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May 13, 2018
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I have a Tandy 2500 SX/33 currently with the stock 256kb of video memory.
It has two empty 20-pin DIP sockets to upgrade the RAM. I have two questions:

1. What is the max video RAM this board will support? I have read conflicting info, some saying 512kb and some saying 1mb.
2. Knowing that info, what kind of chips should I get to max out the RAM?

Any insight would be helpful, thanks!
 
Now, the hunt to find the right chips for this upgrade. 128KB 20-pin DIPS don't look that common on eBay...
 
andyjibb said:
Now, the hunt to find the right chips for this upgrade. 128KB 20-pin DIPS don't look that common on eBay...
Click to expand...

Note that there is no such thing as a “128KB 20 pin DIP”. You *will*, on the other hand, not find it too hard to locate 44256 256kx4 DRAMs.

(Sort of surprised that’s what it uses, but I found a photo of the motherboard and the 256K soldered to it is also a pair of 44256s. They’re the 70ns speed grade, so look for that or faster.)
 
Eudimorphodon said:
Note that there is no such thing as a “128KB 20 pin DIP”. You *will*, on the other hand, not find it too hard to locate 44256 256kx4 DRAMs.

(Sort of surprised that’s what it uses, but I found a photo of the motherboard and the 256K soldered to it is also a pair of 44256s. They’re the 70ns speed grade, so look for that or faster.)
Click to expand...
Thanks for confirming that for me. I had a feeling it might work that way, but wasn't sure.
 
It's an interesting design to use a 256Kx8 RAM array (or 512kx8) for VGA; most early VGA cards used 32 bit wide memory arrays because of the way VGA's planar memory is set up and it let them use relatively slow RAM. Off the top of my head math says 70ns is *barely* fast enough to pull it off, but hey, apparently it works.

VGA cards from the 90's that had RAM expansion sockets usually used big fat 16 bit wide SOJ DRAMS like this in pairs.
 
Eudimorphodon said:
It's an interesting design to use a 256Kx8 RAM array (or 512kx8) for VGA; most early VGA cards used 32 bit wide memory arrays because of the way VGA's planar memory is set up and it let them use relatively slow RAM. Off the top of my head math says 70ns is *barely* fast enough to pull it off, but hey, apparently it works.

VGA cards from the 90's that had RAM expansion sockets usually used big fat 16 bit wide SOJ DRAMS like this in pairs.
Click to expand...
Thanks for the information. I love learning about this stuff.
 
With my video cards, I've been lucky enough to get away with finding the socketed equivalent of the chips already soldered to the board and not have to look for anything exotic. As mentioned already, they're 256Kx8 70ns fast-page DRAM. I found mine for my 1000 RSX on eBay back about 6 years ago. Faster DRAM chips will also work (60ns, 50ns, etc) if that's what you can find, but won't offer any performance boost.
Tandy 1000RSX VRAM.png
 
blackepyon said:
With my video cards, I've been lucky enough to get away with finding the socketed equivalent of the chips already soldered to the board and not have to look for anything exotic. As mentioned already, they're 256Kx8 70ns fast-page DRAM. I found mine for my 1000 RSX on eBay back about 6 years ago. Faster DRAM chips will also work (60ns, 50ns, etc) if that's what you can find, but won't offer any performance boost.
View attachment 1262805
Click to expand...
Thanks for the info! I ended up buying some SAMSUNG KM44C256CP-7 on eBay. Still waiting for them to arrive. I think they should work...
 
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