ZX Spectrum internals (Was: ZX81 kits)

From: Iggy Drougge <optimus_at_canit.se>
Date: Mon Oct 1 17:51:41 2001

Tony Duell skrev:

>> Tony Duell skrev:
>> >As I've said a couple of times before, I can make a video circuit with
>> >somewhat better capabilities than the ZX81's built-in video (40*25 text,
>> >block grapghics, colour) in 2 or 3 chips. Add another chip or so to
>> >encode the outputs to PAL (although I'd probably prefer to feed the RGB
>> >signals into a TV's SCART socket these days...)
>> What chips would you use? I assume that the Speccy uses one chip for its
>> entire video generation, but where would you split the functions?

>I should have said _standard_ chips. Once you start allowing ULAs (as the
>ZX81, Spectrum, Oric, etc all used) then it's easy to do video in a
>couple of chips.

Of course I don't expect you to be making your own ULAs. =)
I just wondered if you'd take some off-the-shelf VDU controller or something a
bit more open-ended.

>Didn't we go through this a couple of weeks back? The 2 chips I would
>start with are an SAA5243 Teletext display chip (or an SAA5245 for the
>States) and an 8K*8 static RAM (6264). You need to get a 6MHz clock from
>somewhere (maybe another chip for that, if you can't also use it for the
>CPU clock in some way) and maybe you want to buffer the video output lines.

Teletext? That's a funny idea.

>> (update a few minutes later, aren't off-line readers a wonderful thing?)
>> I went and opened up my own seemingly-dead Spectrum... Wait, that's no ZX=
>> 81.
>> I've been thinking about SZ Spectrums all the time.
>> Oh well, I might just ask anyway, since I've opened it up...
>> What is that chip next to the Z80? It's labelled PCF1306P.

>If it's another 40 pin chip, then that's the custom ULA chip that
>includes most of the glue logic.

Is that like a lot of 74138s or something?

>> And why is there a switch glued on top of the Z80 with wires going to the
>> character ROM? Since it's a localised character ROM piggybacked on top of=
>> the
>> one soldered to the motherboard, could it be for switching charsets?

>Does the Spectrum really have a separate character generator ROM. Doesn't
>sound right to me...

I wouldn't know, but this is the chip which somehow enables Swedish
characters. Is it a patched BASIC ROM?

En ligne avec Thor 2.6a.
The most useful program will be continually improved until it is useless.
Received on Mon Oct 01 2001 - 17:51:41 BST

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