Issue 1 schematics question

[Lardo Boffin]

Having recently bodged a 16K RAM mod and tried to sort it out I am a bit confused by the schematics!

As far as I can tell the pin 7 of IC4 (which is what I bodged) should go to pin 32 of IC3 as shown below: -

image1.JPG

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But looking at my unbuilt ZX81 kit PCB it does not appear to be the case? That looks like 36 to me.

image2.JPG

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Am I reading this all wrong? It is entirely probable...

[Andy Rea]

don't count those top four pins ( 2 each side ) on the pcb.... the outline is for a different chip.

Regards

[Lardo Boffin]

don't count those top four pins ( 2 each side ) on the pcb.... the outline is for a different chip.

Regards

Of course yes - the first two pins are not used...

IC4 pin 5 does indeed go there.

Thanks!

[sirmorris]

We all have these 'D'ohments'

That's exactly the kind of thing that would have got me too.

[Andy Rea]

to be fair i was only able to answer it quickly because i had very recently fitted some memory to an issue one board myself.... spider loves his new home

Andy

[1024MAK]

If you look carefully at the schematic, you will see TWO different connections to different pin 1's of the RAM chip. One without a circle and one in a circle... That along with the note is a big clue...

But you know now

Also a note of caution when working with schematics of computer systems RAM circuits. As far as the CPU is concerned, it makes no difference which data pin goes to which RAM data pin (with SRAM) or which of the group of DRAM chips (if DRAM). The system will still work fine. It's a similar story with the address lines.

So always treat schematics that detail RAM connections as a guide and not gospel. The PCB track routing can be designed to be as easy as possible rather than exactly what a schematic may show. Even more so with the Sinclair ZX Spectrum schematics, where some are known not to accurate.

Mark