ZX81 Pinout help

[Orac]

Hi,

On the ZX81 there are three memory sockets on the Issue 3 board.

Two small memory chips in sockets IC4a and IC4b

One 24 or 28 pin chip in IC4

Does anyone have the pinout of IC4 when configured for 28 pin use? (LK2 connected)

What chip was meant for use in the 28 pin configuration?

[Paul]

4148
Sorry, meant 4816

[1024MAK]

Have you read through this topic?

Does anyone have the pinout of IC4 when configured for 28 pin use? (LK2 connected)

What chip was meant for use in the 28 pin configuration?

The reason for using a 28 pin socket was in order to allow for a number of possible different RAM chips to be used. At the time that the ZX81 was designed, DRAM (dynamic RAM) type chips were cheaper per bit compared to SRAM (static RAM) for amounts of 1k byte or 2k byte. So there existed chips that used DRAM technology but which worked like SRAM as far as the processor was concerned. In practice, I’m not aware that Sinclair actually ever used any of this type of chip.

In most ZX81 boards, a 24 pin 1024 bit SRAM chip is used (from various makes/brands/manufacturers). In most TS1000 boards, a 24 pin 2048 bit SRAM chip is used (from various makes/brands/manufacturers). For example a NEC µPD4016D-1.

LK1 / LK2 is used to route either address line A10 or +5V to a pin on the RAM chip. A 1024 bit chip may need +5V on this pin. Whereas a 2048 bit chip needs A10.

Mark

[Orac]

4148
Sorry, meant 4816

PERFECT! That was exactly what I needed to know, thank you so much!

I found the pin outs here:

https://www.applefritter.com/appleii-bo ... Matrix.htm