ZX-80/ZX-81/ZX-82 (spectrum)

ZX-80/ZX-81/ZX-82 (spectrum)

Post by garet » Sat, 22 Apr 2006 04:37:33

Hello all,

Very basic question...

what changed between the ZX80, ZX81 and ZX82 (spectrum).

There's a wonderful site that goes through the creation of a ZX80 from
easily available chips and I wondered if that could be built upon to
create a ZX81 and then the holy grail of the spectrum??



ZX-80/ZX-81/ZX-82 (spectrum)

Post by Mike Wynn » Sat, 22 Apr 2006 04:52:23


If you've studied the ZX80 circuit diagrams, you'll know how it works.

The ZX81 is really just a reimplementation of the ZX80 with all the logic
chips replaced by one custom chip (The ULA) plus a little circuit called an
NMI generator which helps it multitask between running code and displaying
the screen better. The Spectrum is an evolution of the integration that
went on in the ZX81. This time the custom ULA chip takes over display
generation from the Z80 completely, freeing it to spend all its time running
user programs.

You can turn a ZX80 into a ZX81 by adding the (fairly) simple NMI circuit
and a new ROM.

Turning a ZX81 into a spectrum would be virtually impossible without a
complete redesign.



ZX-80/ZX-81/ZX-82 (spectrum)

Post by spike » Sat, 22 Apr 2006 05:01:01

gareth < XXXX@XXXXX.COM > did eloquently scribble:

Put simply, the zx81 was very similar to the zx80, but all those chips were
compressed into about 2, the ROM was improved (8k instead of the zx80's 4k)
and the display was capable of constant display. (the zx80 only had "fast

The zx80 and 81 were so similar you could upgrade the zx80 to 81 with a ROM
which gave it everything but the SLOW mode display.

As for the spectrum... I'll leave that to mike, inventor of the speccybob.
(spectrum built from discrete logic in the same way the 80 was)

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ZX-80/ZX-81/ZX-82 (spectrum)

Post by zigg » Sat, 22 Apr 2006 08:45:53


is speccybob still around? I wonder how hard it would be to take that
and shove it into a cheap fpga... 'spectrum on a chip'

ZX-80/ZX-81/ZX-82 (spectrum)

Post by jrok » Sat, 22 Apr 2006 10:51:32

In article ziggy says...

For the simple approach you can fit the functionality of a spectrum ULA into a
cheap Xilinx xc95108, in the 84pin PLCC package. Then it's just a case of adding
a z80, some fast SRAM and a ROM, hey presto instant spectrum. It doesn't take a
lot of gates to do the job, so a relatively small FPGA that can fit a z80 core (
there is a freeware one available ) and the ULA. IMO it's probably worth keeping
the RAM and the ROM external to an FPGA design.

- James

JrokLand http://www.yqcomputer.com/

ZX-80/ZX-81/ZX-82 (spectrum)

Post by spike » Sat, 22 Apr 2006 17:24:04

jrok < XXXX@XXXXX.COM > did eloquently scribble:

If you have the gates to spare? why?
A spectrum on a single chip... In fact, could you also fit the ay on? and
the extra RAM of the 128? possibly the currah?
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ZX-80/ZX-81/ZX-82 (spectrum)

Post by Matthew We » Sat, 22 Apr 2006 17:39:53

Has the SP0256 speech chip even been emulated properly in software yet
(i.e. not just triggering pre-recorded samples), let alone hardware?

ZX-80/ZX-81/ZX-82 (spectrum)

Post by jrok » Sat, 22 Apr 2006 23:09:07

In article XXXX@XXXXX.COM says...

Well.. my take on keeping it external is that ROM's are easier to swap
externally, plus you still have to fit the ROM into the FPGA configuration
EEPROM, so it's a lose-lose situation. You need a bigger FPGA to fit in the ROM
and an bigger configuration EEPROM to store it, vs. an external ROM which takes
no resources and you can easily swap. Plus you don't get that much RAM on an
FPGA unless you want to use quite a large one, which can push up costs a lot (!)
If cost's no option go for a million gate device and fit everything on there ;)

- James

JrokLand http://www.yqcomputer.com/

ZX-80/ZX-81/ZX-82 (spectrum)

Post by zigg » Sun, 23 Apr 2006 06:54:29

In article < XXXX@XXXXX.COM >, jrok < XXXX@XXXXX.COM >

1m gates isnt too expensive. you can get a dev board with one for under

ZX-80/ZX-81/ZX-82 (spectrum)

Post by KenLe » Thu, 27 Apr 2006 02:43:16

>>Has the SP0256 speech chip even been emulated properly in software yet

Joe Zbiciak and Frank Palazzolo reverse engineered the SP0256. See
http://www.yqcomputer.com/ ~im14u2c/intv/tech/sp0256_instr_set.html

My free Chiptalk application concatonates samples but doesn't really do
justice to the SP0256.


ZX-80/ZX-81/ZX-82 (spectrum)

Post by Gary Pearm » Fri, 28 Apr 2006 17:29:43

Yeah, but you could get a mint condition 48k for 5 ;-)