Sinclair ZX81 Hardware


Although physically smaller than the ZX80, the new machine weighs in at 130z, about 20z heavier than its predecessor. The system needs a UHF television, a cassette recorder and a power supply to make it usable.

Technical Specification

CPU:NEC Z80A, 3.5 MHz
Memory:1k RAM expandable to 16k
Keyboard:Plastic membrane under-surface printed
Screen:Domestic UHF television
Cassette:Domestic audio recorder
Firmware:8k ROM containing Basic and operating system

On my colour television the screen is a pleasant green and all characters are displayed in black. The machine offers no colour facilities and my guess is that you shouldn't hold your breath waiting for them. I suspect that a projection system based on three of Uncle Clive's miniature TV tubes might appear one day but, then again, I also suspect that he'd introduce another computer to take advantage of this. The display is 24 lines of 32 characters of which two lines are reserved for system messages and commands. Low resolution graphics are provided to give 64 by 44 plotting points. As with the ZX80, the display is very clear and rock-steady.

I'm pleased to see that the new power supply has its own flying lead for the attachment of a normal mains plug. (The ZX80 was awkwardly designed with an integral plug which often needed an additional socket or extension lead.) This power supply must give 600 mA at 9V but, since theZX81 draws close to this, the standard power supplies actually give 700 mA and I would recommend that readers using their own supplies go for the higher rating, too.

Once again, the keyboard is formed by an underprinted plastic membrane which is everything-proof (water, chemicals, Coca-Cola, cigarette ash, monkeys, editors, etc). The keyboard layout is different from the ZX80's so, if you're upgrading, prepare to make a few mistakes at first. At the same time it is an improvement, since each keyword is frequently placed at or near its initial letter. (All you have to do now is learn the qwerty layout!)

Here are a few ideas for Uncle Clive: a plug-in battery pack, a plug-in singleline LCD display and a remote (infra-red or ultrasonic) facility so that you can sit in your armchair beaming the display information at an aerial adaptor on the television.

Compared with the ZX80, the ZX81 looks very smart indeed - one could almost say tasteful. It has a nice shape and texture and the keyboard is made of a non-reflective material, a definite improvement.

The plug-in 16k RAM pack fits to the edge of the PCB where it protrudes from the rear of the casing. The cursor takes a while to appear at switch-on, because the system is checking to see how much memory is present in order to set certain system variables. If you're a machine code freak you can reset the RAMTOP variables in order to give you somewhere safe to tuck your precious program.

Five screws hold the ZX81 together; three of them are hidden under the pads on which the machine stands (footpads? - surely not). You know what I meanthose non-skid things. There's a substantial heat sink for the regulator under the rear of the keyboard - it's a good place to warm your hands on a chilly morning. The PCB is held into the casing by two screws. The keyboard is separate from the main PCB and is connected to it by a couple of flat printed cables. The main PCB is well designed and neatly made. Assembly of the ZX81 is done very professionally by the Timex Corpontion in Scotland (the same people that are making Sinclair's latest miniature television).

The basic ZX81 contains four chips - ROM, 3.5 MHz Z80A CPU, 1k memory and the Ferranti custom-made chip - plus a limited assortment of bits and pieces. It's very, very simple - I think even I could build it. A few spare positions on the board give the manufacturer a certain amount of flexibility to tweak the machine to the requireI ments of different television systems and to be prepared in case a memory chip famine occurs. The Ferranti chip, handles all the I/O and control signals between the various elements of the machine. Nosing around inside, I notice that it has a very cosmopolitan flavour, with memory from Malaysia, the CPU and ROM from Japan, a UHF modulator from the Philippines, a regulator from EI Salvador and the custom chip from Britain. The edge connector is not gold-plated (what do you expect for 70?); it's just the PCB printing taken out to the edge. The 16k RAM pack contains two boards connected at the edge. One board contains eight 4116s which are driven by the other board's assortment of seven chips which handle the memory addressing and refreshing.

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