Clive Drive

Clive Sinclair's ZX Spectrum home computer

Many years ago I started Microlite by designing an external disk drive interface for the ZX Spectrum. The Spectrum was one of the first home gaming computers in the UK. The firm I was working for had a bunch of Quick-Disk drives gathering dust so hired me to design the interface.

Quick-Disk

The Quick-Disk comprises a 2.8" floppy platter with a single spiral track rather like a record. To access data the drive traverses the whole track (48K-bytes) and this takes 8 seconds. You can turn the disk over to avail of a second 48K-bytes worth. The drive included a read/write head amplifier but no other processing.

The interface hardware was novel in that it used simple logic (TTL gates and registers) and the processing power of the Spectrum's Z80 microprocessor to decode or encode the drive data on the fly.

And then there was the necessary software. It was necessary to reverse engineer the Spectrum's operating system in order to integrate the disk drive and provide the necessary new commands to operate it.  All this was done on a 80286 desktop running MSDOS - long before Windows or the internet. My best companion during this period of sheer hard graft was Dickens' Spectrum Advanced User Guide.

My best companion

Thus I wrote a disk operating system or DOS. Others have made millions by so doing - I can't remember what I charged but it certainly did not make me rich, but it did kick-start my business and so I am eternally grateful to Clive, and to John my client, for the opportunity they gave me.

The final Clive Drive was technically a success.  A mere 8 seconds to load a game was much better than many minutes with a cassette recorder and the interface supported hacking via the "Keymaster" button. But regrettably the product was not well advertised and was launched towards the end of the Spectrum's lifetime, so I doubt if my client ever recouped his costs.

Surprisingly the world still remembers the Clive Drive - so it wasn't all a dream after all!  I found this picture and a couple of links here and here on the internet. The red button is the "Keymaster" button.

The Clive Drive interface and disk drive

For those interested here are some photographs of circuit schematics and operating instructions which, I regret, I no longer have in digital format. Click on a picture to enlarge it.

First page of Clive Drive operating instructions

Schematic page 1

Schematic page 2 -
the disk drive connector is at the bottom
with RDDT being the raw serial data
from the read head, and WRDT the
serial data to the write head.

"Peek-Poker" used the Keymaster button
to freeze a game and offer various
debugging / hacking commands.

Clive Sinclair, slide rules and log tables

Clive Sinclair, entrepreneur par excellence

Long, long ago, when I was in the sixth form at PSS, calculators of the add / subtract / multiply / divide type were starting to become commonly available but were not allowed in school... Calculators, it seemed, were for the weak.

Instead we were taught long multiplication and division, and to use log tables in maths although in science we were allowed to use a slide rule. A slide rule works like log tables but is a whole lot easier to use.

If you know all about logs and slide rules, or if you are already bored out of your mind, then this is the place to stop reading.


If...

then, by definition,

Thus the exponent (aka index) x in the first equation is the logarithm to base n of y.  The clever bit comes as a result of the rules of indices, thus

Here I have changed the variable names (otherwise I would have run out at 'z'), and shown that when two numbers r and s are multiplied, their logarithms are added.

The same applies whatever the base n is.

So, back then, long, long ago, we had 4-figure log tables, or 5-figure if you were lucky (or had a lot of time on your hands). These were look-up tables for finding the logarithm of a number to 4 or 5 decimal places. The book would also contain tables for the inverse operation "anti-log" (which is one and the same as exponentiation) together with tables for finding the sine, tangent or cosine of an angle and their inverse operations, square and cube roots, and such-like.  A mathematician's ready reckoner. To multiply two numbers you first look up their logs, then add these together and look up the anti-log of the answer.

You show a book of log tables to today's youth and they won't believe that this is how we did multiplications and divisions.

The slide rule has logarithmic scales so that the logarithm of a number is translated into physical length - to multiple two number you simply move the sliding part (which also has a logarithmic scale for the second number) thus adding the distances and effectively adding the logarithms. The final value is read off using the first scale which thus takes the anti-log and gives the product.  Other scales are included for finding trigonometrical functions and squares or square roots.

You can get 3 figure accuracy from a slide rule if you are careful which was good enough for most undergraduate experimental work.

At college a friend purchased a Sinclair Scientific calculator - the first scientific calculator within reach of students. Although it only had 5 digit accuracy this machine was a wonder to behold. I bow to Clive Sinclair whose inventions have been truly prophetic even if they were not always wholly reliable.

Talking about Clive, the same friend owned a Micromatic - a matchbox sized radio - on which, during school lunch break, we used to listen to I'm sorry I'll read that again, a radio show which was a precursor to Monty Python.

When I started work at Kingswood Warren I was introduced to the HP35 which, apparently, was introduced in 1972 as the first ever scientific calculator but too expensive for the average student. It boasted a splendid bug in that it reckoned that exp(ln (2.02))=2.  This was corrected in the next model HP45.

These calculators had power-hungry red LED displays - liquid crystal had not been invented. Talking of LED's I remember a boy bringing an LED to school - it was the first I had ever seen and blew my mind: at last a source of light without (much) heat just like a glow-worm!

And talking about inventions - an interesting coincidence, for someone that works in electronics, is that the transistor was invented around the time or just before I was born.

So much has changed since then - the rest is history...