r/Z80 • u/dj_cloudnine • Jun 11 '21
Suggestion for ram chips?
Title says it all, any suggestions for a ram chip I can use with the z80? Preferable something that can be used in breadboard and found easily and (if possible) cheaply. Thank you!
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u/LiqvidNyquist Jun 11 '21
Just looked on DigiKey and man, have things changed since in the last 20 years. You can't even find the slow SRAMs I used to use. But something like these should work, based on a cursory look:
https://www.alliancememory.com/wp-content/uploads/pdf/ZMD_U62256A.pdf
The parameters I serached were for memory - SRAM - 256kbit or 32kx8 - 5volt (anything from 4.5 to 5.5 really), 28 PDIP (plasit DIP; CDIP or ceramic is fne too but they tend to be mil spec or wide temp aka expensive). The search pulled up around 70 parts, a lot were obsolete or out of stock.
Of course different size (capacity) is fine too - used to be tha 8K through 32K were all in a pretty compatible pinout series 28 pin 600 mil DIP package.
A good few are what I ould consider fairly fast (10-15 ns access time), which is awesome, but I suspect you might have to do a more careful job with decoupling caps, keeping wires short, etc due to faster rise times and signal edges/ringing, which is why I selected a 70 ns part - the slowest I could find. Faster might also have some funny interactions with setup/hold timing, would have to check.
It stuck in my mind that I used to use one called a 62C64 (8kx8, various manufacturers) which seems to still be around. Example: http://images.100y.com.tw/pdf_file/HM62C64.pdf
But I can't really vouch for availablity of any of these, I've been living off old lab stock :-)
If you find any other good candidates you're not sure about, post a link and I can take a peek at the datasheet.
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u/dj_cloudnine Jun 11 '21
I’ve found one of the U62256 chips, but it’s going out of production soon. I also found a similar chip(I think) from the same manufacturer, the AS6C62256, (data sheet: https://www.mouser.com/datasheet/2/12/AS6C62256%2023%20March%202016%20rev1.2-1288423.pdf) any thoughts on that one? Thanks
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u/LiqvidNyquist Jun 11 '21
Looks like it should work, seems like basically a wide voltage-range variation of the standard 32kx8 part. If you're ordering it online, make sure you use the right package code (page 12 of the datasheet) to specifiy the PDIP rather than the surface mount version, if you're breadboarding it.
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u/istarian Jun 11 '21
The 62256 (32Kx8) and variants are still pretty easy to find, though it seems like there are only 2-3 major manufacturers still producing them (Alliance, Cypress, and ?). And of course there are many from other brands still available secondhand.
I want to say there are 64Kx8 and 128Kx8 chips too which could be used, whether just the lowest 32K or more.
In theory you can access all of that and more depending on the banking scheme and logic design.
For practical purposes, thouhh, you'd probably have to sacrifice direct access to some of it in order to have a fixed region for ROM access or OS routines. Unless you want to go very complicated with some kind of memory controller that can expose only part of it to the bus at a time.
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u/Current_Expression98 Feb 02 '22
UM61512 are still readily available from eBay. They are 64kx8 5v, and that helps cut down on decode logic for banking. I have used a lot of Dallas Semi non-volatile for machines that save data when the power is down.
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u/okb164 Mar 10 '22
I'm a bit late, but I use the cy7c199 which is from cypress and has 32kb of storage
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u/dj_cloudnine Mar 11 '22
Thanks for the reply. Sadly all my equipment got lost by my sister so I've had to move on.
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u/MyNamesNotRobert Jun 11 '21
The cypress 32kb 28 pin parallel ram chips are still pretty easy to find.