Sinclair ZX80 questions

Tony Duell ard at
Tue Aug 22 17:08:57 CDT 2006

> > 
> > Well, the keypad was terrible (I ended up soldering 20 switches from 
> > Maplin onto the MK14 PCB, the holes and traces were there for them...). 
> > There were some very marginal bits of logic design (hint : a '157 is a 
> > multiplexer, a '175 is a latch, but the MK14 uses the former as a the 
> > display output register. It works for _some_ makes of '157...). The CPU 
> > buses are not broungt off-board, so expansion was a pain.
> Hmm, that is really surprising about the (lack of) expansion ability - I would 
> have thought it's one of the primary reasons in choosing a machine of that 
> class. I can't imagine doing something like in the ZX series (solder-pad edge 
> connector) would exactly have put the manufacturing cost up by much!

Oh, the rear edge of the PCB is an edge connector. But it's not the CPU 
buses. IIRC, it's power input (the 5V line is _not_ brought back out 
again), the CPU I/O lines (flags, sense, etc), and the port lines from 
the optional 8154 RAM I/O chip. 

There's another edge conenctor on the right of the PCB at the front which 
carries the keyboard lines, and is clearly there to allow you to connect 
a better keypad. But IIRC, the connections for that are not given 
anywhere in the manual.

> > And so on. I 
> > wish I'd saved a little more money and bought an Acorn System 1.
> True - at least there was the ability there to re-use the CPU card and add a 
> eurocard lack later along with all the later System cards. Mind you, from 
> memory of Acorn ads they didn't really play up any future expansion ability - 
> I don't think Acorn saw it as a real selling point back then.

Well, even if you didn't want to go that far, the ability to hang a 
couple mroe I/O chips off the machine was very useful. With the MK14 you 
were limited to the 16 lines from the single 8154 on the PCB. I had to 
add a bit of external logic to latch them in <n> '374w or something so as 
to get more outputs. With the Acorn I'd have just added a 6821 or something.

> > Releated to this was a horrible bit of Torch design. As you may know, 
> > Torch sold an upgrade for the BBC micro which added an internal PCB 
> > containing a Z80 running CP/N (no, that's not one of my typos...). The 
> > Beeb power supply couldn't really supply that as well, so what you did 
> > was remove the Beeb PSU altogether, connected a cable to the power 
> > connections on the Beeb's mainboard, and run the whole thing off a PSU 
> > in the (Torch-supplied) disk drive unit.
> You know, my prototype Torch Z80 disk unit has power out at the back; I can't 
> remember whether the production ones were like that or not. It had never 
> occurred to me that someone would want to power the machine from it!

I've seen Torch documentation which specifically tells you to remove the 
Beeb's PSU and fit the cable to plug into the back of the disk drive unit.

But I don't know if it was 'production' or not. Most the Beeb stuff I saw 
was at Cambridge University where there was a fair number of prototypes 

> In a way I'm surprised that the beeb PSU couldn't cope though - the current 
> draw can't be a lot worse than other various internal add-ons for the beeb. 
> I've certainly seen a lot of beebs running the Torch copro without any PSU mods.

YEs, but that wasn't 'offiical' either. IIRC Acorn specifically fobade 
you to run anything off the Beeb's PSU. I think Torch tried to do things 


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