Many 1 Bit DRAM to Fewer 4 Bit?

Thread Starter

apprenticemart2

Joined May 7, 2013
67
Hi,

I have a system that uses 256 x 1 DRAM (12 of them to give 12 bits)

The system supports up to 1M total with 4 x ROW ADDRESS STROBES, 1 strobe for each 256K x 12 bank
and I want to try and build a memory expansion board using 256 x 4 DRAM to save on chip count.

The 256 x 4 has an OUTPUT ENABLE that the 256 x 1 doesn't.

How would I generate this, using an inverter on the WRITE ENABLE so that when WRITE ENABLE is 1 OUTPUT ENABLE is 0 and vice versa?

Any tips on using 4 bit DRAM instead of 1 bit?

I could also try to use larger 1 Bit DRAM like 1M x 1?
(I would only be using 768K of the whole 1M)

How would I use the extra ROW ACCESS STROBES to control the extra address lines if this is how I would do it?

No I don't really know what I'm doing. I'm stubborn like that :)

Any help is better than none.

Thanks
 

takao21203

Joined Apr 28, 2012
3,702
Very confusing writeup, its not clear why you need or want to use DRAM, and if you use it, dont you know they need refresh, and lets say you cater for that, but now you are some how stuck how to use a different DRAM chip?

Or do you want to rebuld/repair an old circuit?
 

Thread Starter

apprenticemart2

Joined May 7, 2013
67
Very confusing writeup, its not clear why you need or want to use DRAM, and if you use it, dont you know they need refresh, and lets say you cater for that, but now you are some how stuck how to use a different DRAM chip?

Or do you want to rebuld/repair an old circuit?
I want to make an addition to a circuit I already have.

I have no knowledge of anything other than to try to copy the mechanisms that the device already uses in attempt to understand how it works.

I have a schematic for a memory expansion card that I already have which gives me 512K total. I want to learn how I can upgrade this to 1Mb.

There is 256K on the mainboard, and 256K on the expansion card.

If there are better ways to do it than to try to copy the original design then I would be interested to know.
 

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takao21203

Joined Apr 28, 2012
3,702
as far as I understand in the circuit Q and D are simply connected, while most memory chips only have IO, and use /OE to enable the output for reading, otherwise put the data on the databus, and dont enable.

I think in the schematic its /W, which you'd have to connect to /OE.

And dont forget /CS too, there could be one or two.

Anyway, 256 bits is very small these days, you could replace it with static RAM probably.
And in the schematic there are 9 address lines.

12bit width is odd too.
 

Papabravo

Joined Feb 24, 2006
16,481
Many DRAM controllers from that era (256 x 1) were made from MSI logic. You might have more than a little bit of trouble adapting such a controller to your purposes. On the other hand if there is an IC DRAM controller you might have trouble finding the part as it might be obsolete. In either case you might want to consider a new design of your own rather than trying to adapt someone else's design.

Tell us what you know from the chips you see on the existing board.
 

Thread Starter

apprenticemart2

Joined May 7, 2013
67
Yes. This was the second option in my original post.

It may be easier as the 1M x 1Bit chips will use the same signals as the 256 x 1Bit chips. The only difference is that there are more address spaces than the 256k chips. I would tie the unused addresses to +V?

So, would I just connect the extra ROW Address Strobes up to the extra addresses directly or would some signal conditioning/alteration be required before hand?

I'm reading these at the moment
http://n8vem-sbc.pbworks.com/f/SRAM+Replacement+for+TMS99x8+VRAM.pdf

http://bitcycle.org/retro/spectrum/SRAM_replacement/
 

Papabravo

Joined Feb 24, 2006
16,481
Yes. This was the second option in my original post.

It may be easier as the 1M x 1Bit chips will use the same signals as the 256 x 1Bit chips. The only difference is that there are more address spaces than the 256k chips. I would tie the unused addresses to +V?

So, would I just connect the extra ROW Address Strobes up to the extra addresses directly or would some signal conditioning/alteration be required before hand?

I'm reading these at the moment
http://n8vem-sbc.pbworks.com/f/SRAM+Replacement+for+TMS99x8+VRAM.pdf

http://bitcycle.org/retro/spectrum/SRAM_replacement/
The verbal descriptions are getting way too confusing. I suggest a schematic of the existing board and your proposed schematic of the replacement board.
 

Papabravo

Joined Feb 24, 2006
16,481
Replacing DRAM with SRAM will work if you want to eliminate the controller and the multiplexing altogether. We can't hep very much without a schematic because we CANNOT see what you are working with!!
 

Thread Starter

apprenticemart2

Joined May 7, 2013
67
OK. In that case I have to use DRAM. The signals I have to work with are the same ones that the expansion board uses that I already posted so I'm just going to have to duplicate the ram expansion board another 2 times and stack them on top of each other.

I found an old website that used to have a schematic of a board that could be used to replace 4off 256x1bit dram with 1off 256x4bit dram but the site has been abandoned and the download link doesn't work. 24 extra chips is a lot. I would like to cut down on that by using 4 bit chips but they have an output enable signal which is in addition to all the signals the 1 bit drams have.

Thanks for your help.

The ram expansion board connects to the connector P205 I have marked with a sticky in the PDF comments and the data lines are up in the top right corner.
 

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Thread Starter

apprenticemart2

Joined May 7, 2013
67
Sorry was just updating it in case anyone got confused as to what the mission objectives were :)

Have a nice vacation and don't bring any Ebola back with you.
 
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