Affordable home computing

[ken...@cix.compulink.co.uk]

The most important factor in the development of cheap computers was the
Gate array discuss. At least when Sinclair developed the ZX81 from the
ZX80 one gate array replaced 24 separate chips.

Ken Young

[Michael Black]

But the ZX81 was extremely limited, and other factors (like the keyboard
that didn't have real keys, and the method of putting the characters on
the screen, even the fact that the Z80 had built in hardware to refresh
RAM might have been a factor, I can't remember if it used static or
dynamic RAM) cut the cost of the unit so it could be sold
cheaply.

I paid something like $500 for my OSI Superboard II in the fall of 1981.
8K of RAM, a full keyboard, a video interface, a serial port and cassette
interface, BASIC in ROM. It used nothing but TTL for holding the main
components together. It was fairly big, which does impact on cost.

There were a bunch of iterations going on in those early days. Initially
a lot of things had to be done with TTL logic, lots of space, though not
necessarily expensive. Then larger scale ICs came along once the
companies could see a trend in what was needed. An early floppy disc
controller was pretty complicated, then made simpler when Western Digital
started making ICs (but still needed too many external components), then
not much more than the IC and buffers when Western Digital came out with a
revised controller.

In the same time span, RAM became more dense. The Altair 8800 only came
with 256 bytes of RAM, but not much later 1K x 1 RAM were relatively
cheap, then density kept going higher. Again, if you had to have a lot of
ICs for a specific amount of RAM, that used up space which ran up cost,
and increased current consumption, which meant a larger power supply. In
parallel with static RAM becoming more dense, dynamic RAM became more
dense, but also either became easier to interface, or designers became
better at interfacing them. Since dynamic RAM wsa always denser, it really
improved things, so by 1977, the Apple II could come with enough sockets
for a full 48K of RAM (the rest of the address space was kind of used up
by I/O).

Designers got better. The Apple II used the same circuitry to refresh the
dynamic RAM and to control the RAM for the video output. The computer wsa
pretty complicated in terms of circuitry, all that TTL, but it was simpler
than some of what came before.

Or take the VIC-20 and the C64, that came out about the same timein the
early eighties. The former used static RAM, which meant it didn't have
much, but by then it was a single 24pin IC, which beat the 16 16pin RAM
ICs in my OSI Superboard. The C64 went to dynamic RAM, allowing for 64K
of RAM with 8 ICs, but density of ICs had increased so the circuitry
wasn't particularly complicated.

The Apple II went through a few iterations, using custom ICs to lessen the
component count, but to a large extent it wasn't new design, just changing
the ttl ICs to one or two large pin custom ICs.

Some of what happened was there was a demand (or perhaps market created)
for cheap computers. So in order to address that market segment, costs
were cut, some of it was in less function (that 8K VIC-20) but some of it
was better manufacturing technique, including reducing IC count. Once you
can foresee selling a lot of an item, it becomes a different world when
you can only sell a relative few

Demand can drive things pretty fast, which may mean that nothing
particularly mattered in bringing cheap computers to the masses except the
idea, at which point everything was done to reach that market segment.
And once people were buying, that helped drive costs, and prices, further
down.

Michael

[Ahem A Rivet's Shot]

--
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[Andy Leighton]

On Tue, 1 May 2012 21:57:05 -0400, Michael Black <et...@ncf.ca> wrote:
> On Tue, 1 May 2012, ken...@cix.compulink.co.uk wrote:
>
>> The most important factor in the development of cheap computers was the
>> Gate array discuss. At least when Sinclair developed the ZX81 from the
>> ZX80 one gate array replaced 24 separate chips.
>>
> But the ZX81 was extremely limited, and other factors (like the keyboard
> that didn't have real keys,

Although it wasn't unique in that. It is true however designed down to a
price. Under Ł50 was very cheap, and just about affordable to a lot of
families who wouldn't otherwise have bought a computer.

> and the method of putting the characters on
> the screen, even the fact that the Z80 had built in hardware to refresh
> RAM might have been a factor, I can't remember if it used static or
> dynamic RAM) cut the cost of the unit so it could be sold
> cheaply.

Static. The original design was for two 2114s. Later issues had a
single 4118. The ram was soldered on, I guess sockets would have been
too expensive.

> I paid something like $500 for my OSI Superboard II in the fall of 1981.
> 8K of RAM, a full keyboard, a video interface, a serial port and cassette
> interface, BASIC in ROM. It used nothing but TTL for holding the main
> components together. It was fairly big, which does impact on cost.

I think they were cheaper than that in 1981. At least for the bare board.
A nice fully cased one might be a bit more expensive. We didn't see much
Ohio Scientific stuff in the UK. But we did have a clone called the
Compukit UK-101.

In late '80 / early '81 I was lusting after the Acorn Atom. Ł120 for
the kit. It had a real keyboard, tape interface, 2K RAM (expandable
to 12K) and 8K ROM (expandable to 12K). But there were too expensive
for me (and my family) at that time. However because of the pace of
development the Atom seemed quite outdated in '82 - when the second
wave of Spectrums / C64s / Beebs were launched.

--
Andy Leighton => an...@azaal.plus.com
"The Lord is my shepherd, but we still lost the sheep dog trials"
- Robert Rankin, _They Came And Ate Us_

[Ahem A Rivet's Shot]

On Tue, 01 May 2012 15:03:42 -0500
ken...@cix.compulink.co.uk wrote:

> The most important factor in the development of cheap computers was the
> Gate array discuss.

s/The most/An/ and then it would be correct, the microprocessor is
probably the most important factor, or perhaps the DRAM chip, or ...

> At least when Sinclair developed the ZX81 from the
> ZX80 one gate array replaced 24 separate chips.

Indeed, although there was a rumour around town that the only
reason the ZX80 was released was because there was trouble getting the gate
arrays to work. This rumour was very plausible, everyone in Cambridge
working with gate array designs around the time had trouble with them,
usually producing designs that ran too hot (Newbury Labs never got working
ULAs for the Newbrain which is why Grundy got it and Acorn got the BBC
contract, the BBC Micro "pyjama stripes" problem was caused by the vidproc
ULA overheating and cured by fixing an aluminium "case support" to the top
of the chip).

[D.J.]

On Tue, 1 May 2012 21:57:05 -0400, Michael Black <et...@ncf.ca> wrote:

>On Tue, 1 May 2012, ken...@cix.compulink.co.uk wrote:
>
>> The most important factor in the development of cheap computers was the
>> Gate array discuss. At least when Sinclair developed the ZX81 from the
>> ZX80 one gate array replaced 24 separate chips.
>>
>But the ZX81 was extremely limited, and other factors (like the keyboard
>that didn't have real keys, and the method of putting the characters on
>the screen, even the fact that the Z80 had built in hardware to refresh
>RAM might have been a factor, I can't remember if it used static or
>dynamic RAM) cut the cost of the unit so it could be sold
>cheaply.
>
>I paid something like $500 for my OSI Superboard II in the fall of 1981.
>8K of RAM, a full keyboard, a video interface, a serial port and cassette
>interface, BASIC in ROM. It used nothing but TTL for holding the main
>components together. It was fairly big, which does impact on cost.

The Sincalir ZX-81 I bought cost $100.00, had 1 kilobyte of ram. Used
a cassette tape interface, loaded/saved the programs as sounds. Had an
8KB ROM with tokenized BASIC in it. i.e. easy BASIC command was one
byte or two bytes. The later was due to touching the shift key to get
to the second defined BASIC command.
.
JimP.
--
Brushing aside the thorns so I can see the stars.
http://www.linuxgazette.net/ Linux Gazette
http://www.drivein-jim.net/ Drive-In movie theaters
http://story.drivein-jim.net/ A story Feb, 2011

[Tim Shoppa]

In terms of the ZX80 and ZX81, I think the most important factor was
Don Lancaster's _Cheap Video Cookbook_. No doubt about it.

Tim.

[Michael Black]

I alluded to that:

But the ZX81 was extremely limited, and other factors (like the
keyboard that didn't have real keys, and the method of putting the
characters on the screen,

It was definitely the same method Don Lancaster spelled out, jam the
databus with NOPs so it kept advancing the address bus (which meant no
need for counters or multiplexers on the address bus), which allowed the
reading of the memory contents for the video interface.

I was never sure if the Sinclair got the idea from Don Lancaster, or
if they thought of it independently. There were a few articles after the
Altair came out about bootstrapping computers, and they all used the idea
of jamming NOPs into the databus of the CPU so the CPU would advance the
address on the address bus. Even later, the same concept was shown for
bootstrapping the 68000.

The Apple II made a leap forward by using the same circuitry to refresh
the dynamic RAM and deal with the video interface, but Don Lancaster's
scheme made things really simple, albeit at the cost of interrupting the
CPU so it could handle the video interface work.

Michael