• Specification
• Keyboard
• Inside the machine
• The "Outside World"

Specification

• CPU: NEC 780C-1 (copy of Z80) 3.25 MHz
• 1K static RAM, expandable to 16K
• Keyplate, under-surface printed, keyboard
• Use own television.
• Pixel graphics, 24 lines x 32 chars.
• Use domestic audio cassette recorder for magnetic storage
• Edge connector with 44 lines - 37 from CPU, 0V, 5V, 9V, Clock, External memory indicator and two earths
• 4K ROM containing BASIC, Editor and Operating System

Keyboard

The keyboard is most interesting; it's one of those waterproof, chemical proof, completely sealed units and it's stuck on the main PCB. Made of a special tough plastic, the under-surface is printed with the key symbols so as to eliminate any rubbing off. Between this keyplate and the PCB containing the metal contact strips (about five per key) is a piece ofsticky plastic containing forty holes which line up with the "keys". This material is about .006" thick and is just sufficient to keep the metal underside of the keyplate away from the contacts, except when touched of course.

Typing gives a sensation of drumming your fingers rather than of doing anything useful. This is a totally mistaken inpression because it really works rather well. For those who are interested, I found that a "wiping" action was more successful than the tapping movement useually associated with typing. Typists may be pleased to hear that the keys are in standard QWERTY layout although somewhat compressed compared to, say, the office IBM.

Inside the machine

Looking inside the machine, I find that it's controlled by a NEC 780-1 processor chip...a copy of the well known and very successfull Z80. This CPU, running at 3.5 MHz, does all the work for the ZX80, including driving the TV and the cassette recorder. You'll notice that if any work is taking place, be it calculation, accepting input from the keyboard or driving the cassette recorder, then the TV picture disappears - only to return when the activity is complete. This can be irritating to observers (at a demonstration for example) but I found it positively beneficial when keying in programs because it gave me positive feedback whenever a key made successful contact.

The BASIC interpreter, operating system, character set and editor are all held in a 4K byte ROM. If you are feeling adventerous ther's no reason why you should't pop your own ROM (TMS 2532) in its place.

Memory in the basic system comprises 1K statis RAM; you can add to it via the expansion port, giving a maximum potential of 16K. The memory expands with the aid of plug in modules, each designed to carry up to 3k in 1k increments. This five modules would be required to give the 16K maximum. At switch-on the machine does a memory check which also tells the system how much memory is online. Should you reconfigure the memory, then the command NEW will execute the memory check cycle again.

The "Outside World"

Moving on to the "outside world" connections, there's a cassette interface, TV socket and a hefty edge connector. The cassette interface comprises two 3.5mm jack plug sockets, securely mounted on the main (an only) PCB. One connects to the "ear" socket on the cassette recorder and the other to the "mic" socket. There is no facility for remote control of the cassette motor. Anyonewith DIN sockets will have to buy a jack plug to DIN connector lead.

Although I encountered one or two problems at first, once working, the cassette interfae proved trouble free. My particular recorder had a nasty habit of recording noises when the CPU was "sending out" silence. This caused the system to get its knickers in a twist when reading from the cassette because it expected silence just before the file header record. After a couple of hours (what a confession) the culprit was found - the "ear" lead, which acts as a monitor while recording, was setting up some sort of oscillation. Answer - simple - disconnect the "ear" jack when recording. Another tip which ensures trouble free loading is to move the tape into the silent section before issuing your LOAD instruction. Rumour has it that the cassette operates at around 250 baud - I believe it, although it doesn't seem terribly important when you're only loading the 1K that I was.

The television connector is simplicity itself. Plug one end of the cable (supplied) into the ZX80 and the other into the television aerial socket, tune to channel 36 and you're in business. The display is magic; rock steady and very clear although reversed characters(white on black) are not so good.

I have already mentioned the business of the display switching off every time the processor needs to do something else. If this drives you mad then you'll have to forfeit some of the undoubted pleasures that this machine has to offer. The screen is not memory mapped; it's treated like a serial file - like a printer in fact - which means that fast moving graphics are out of the question. No doubt some clever Dick out there will take up the challenge and fudge the system, just to prove me wrong. More about the reason for this is the Software section, but anyone who is hooked on white characters on a bloack background can suitably modify the PCB, though why they should want to I'll never know. It's a matter of cutting one track and making a small bridge to another.

Amrican Television (525 line) users are catered for as well; all they need to do is solder in one diode and the system is converted from 50 to 60Hz standards. Do you take your computer camping with you? You'll be pleased to hear it can run from a car battery, provided that the leadregulates the supply. I believe you can buy a cigarette lighter plug with the built in regulator...couple that with a portable TV and a battery powered cassette recorder you'll be the envy of the campsite.

Now let's look at the hefty edge connector. This is where the memory expansion modules fit in, each one being "piggy backed" on the one previous. Thus there are always 44 contacts available for outside use. There are 37 lines drawn from the CPU ple 3 power lines (at 0, 5V and pV); the other lines comprise two eartchs, a "clock" signal and an "external memory in use" indicator.

All in all, the Sinclair ZX80 is a well designed, well produced personal computer. Memory addition comas a bit expensive at �300 for the full expansion, but Clive Sinclair tells me bigger RAMs are on the way - that means cheaper expansion when they appear. If you like soldering and are good at it, it'll probably take you an evening to assemble the kit; if you're at all aprehensive then I suggest that you stump up the extra cost and save yourself twenty pounds worth of heartache aby purchasing the ready built ZX80.

Please, come this way and move into the software section......

ZX80 LINKS & WEBSITES

Planet Sinclair Microsite giving background information, pictures and buying info. [ visit ] dmoz - Open Directory Project List of sites in the Open Directory Project, the largest, most comprehensive human-edited directory of the Web. [ visit ] Obsolete Technology Website The zx80 page gives some general information about the zx80. Elsewhere on the site gives informaiton on many old computers from the 1970's and 1980's. [ visit ] ZX80 Hardware page How to build your own ZX80/ZX81. This page details the construction of your own ZX80. [ visit ]

A Course in BASIC Programming ZX80 Operating manual and a Aourse in BASIC programming. A work in progress, but pretty good. [ visit ] ZX80 Emulator XTender and XTender2 are ZX81 emulators for MS/DOS (Windows compatible). [ visit ] Wikipedia on the Sinclair ZX80 From Wikipedia, the free encyclopedia, a one pager on the ZX80 that you can update if you have more information. [ visit ] History of Home and Game Computers One page history of the ZX80 with background of Sinclair Research and Sir Clive. [ visit ]