Hello Forum Chums
Another Amibay Exclusive for you...
Dedicated to Rob and Andy
Having been put to the task I have engineered a very nice neat and cheap solution to banish Ram pack wobbles for ever and provide a nice upgrade path for std System owners unable to source a fully working reliable 16K Ram Pack for Sir Clive's original ZX81...
The modification involves the removal of the on board 2x2114L Ram chips which provide the ZX81's std 1K of RAM and populating the existing 28 Pin DIL socket layout that the boards already have provisioned with a 32Kx8 SRAM device, which is cool as no refresh circuitry is necessary and it also lowers the overall current consumption.
The 32K SRAM is a std easily available cheap 62256 or 42256 device and in this current application has been fixed to 16K operation only.
With some extra decoding ( one 74LS chip ) it is possible to fully utilise the full 32K capacity of the chip by utilising Address line A14 and the ULA's display file request line. But of course this would not look as neat.
Two very minor mods to the board are required and a few wire runs to incorporate the Extended Address Bus to the SRAM....
I have also added a 10K pullup to the R/W line to give it a bit more oomph when using the Expansion Connector.
And of course it works fine...and runs cooler.
Anyone wishing to upgrade their ZX81's just drop us a line
Enjoy
TC
Another Amibay Exclusive for you...
Dedicated to Rob and Andy
Having been put to the task I have engineered a very nice neat and cheap solution to banish Ram pack wobbles for ever and provide a nice upgrade path for std System owners unable to source a fully working reliable 16K Ram Pack for Sir Clive's original ZX81...
The modification involves the removal of the on board 2x2114L Ram chips which provide the ZX81's std 1K of RAM and populating the existing 28 Pin DIL socket layout that the boards already have provisioned with a 32Kx8 SRAM device, which is cool as no refresh circuitry is necessary and it also lowers the overall current consumption.
The 32K SRAM is a std easily available cheap 62256 or 42256 device and in this current application has been fixed to 16K operation only.
With some extra decoding ( one 74LS chip ) it is possible to fully utilise the full 32K capacity of the chip by utilising Address line A14 and the ULA's display file request line. But of course this would not look as neat.
Two very minor mods to the board are required and a few wire runs to incorporate the Extended Address Bus to the SRAM....
I have also added a 10K pullup to the R/W line to give it a bit more oomph when using the Expansion Connector.
And of course it works fine...and runs cooler.
Anyone wishing to upgrade their ZX81's just drop us a line
Enjoy
TC
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