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4116 DRAM replacement

Posted: Thu Jul 27, 2017 9:29 am
by jonb
Hi

As you know, I have been fighting a rearguard action against a number of Superbrains. The biggest PITA with these machines, and others, is the flaky 4116 DRAM, which is getting harder (more expensive) to find. I need a modern equivalent in the shape of a plug in board.

For example, there are devices that replace the lower 16K of the Spectrum's memory and I'd like to understand their design to the Superbrain (and other machines, perhaps). Is there a schematic for such a device anywhere?

Cheers
JonB

Re: 4116 DRAM replacement

Posted: Thu Jul 27, 2017 9:56 am
by danielj
You should be able to just use static RAM instead...

http://sindik.at/data/vram/

d.

Re: 4116 DRAM replacement

Posted: Thu Jul 27, 2017 10:17 am
by jonb
That looks tasty.. I'll have a read, see if I can come up with something appropriate.

There is also a means of hacking 4164 DRAMs to act as 4116 here: http://www.amibay.com/showthread.php?67 ... eplacement

..and I considered using 8x4164s to create a single 64k bank, but this is not going to work on the Superbrain as it selectively disables individual 16k memory banks at different times by disabling the RAS signal.

Re: 4116 DRAM replacement

Posted: Thu Jul 27, 2017 11:26 am
by 1024MAK
The replacement RAM modules for the ZX Spectrum do use SRAM.

As DRAM has the address lines multiplexed, any replacement circuit that uses SRAM must de-multiiplex these address lines.
There is a further possible complication, 4116 DRAM has separate data in and data out pins. It's not easy to use SRAM if a system uses these as separate lines. But not a problem if they are linked together as a data in/out line.

There is a schematic somewhere that the ZX Spectrum modules are based on. I'll see if I can find it... Ahh I see that Daniel has already linked to Alwin's page :D

The slightly easier solution is to use 4164 (or equivalent) DRAM chips. You just need to isolate the -5V and +12V tracks IIRC (I'll check...). Then the 4164 chips can just plug in in place of the 4116 chips. Nice and neat 8)
Okay, details here
So, as I say, isolate the tracks that supply +12V and -5V to the 4116 chips. If replacing them all, you can cut these two power feeds up stream of all the DRAM chips/sockets. Then connect the former +12V track to a +5V supply rail (one that can supply a fair mount of current). Connect the former -5V track to 0V. Put in lots of 4164 chips and your done :wink:

Mark

Re: 4116 DRAM replacement

Posted: Thu Jul 27, 2017 10:24 pm
by stevendt