20 July 2016

Sinclair MK14

As with so many Sinclair projects, 1979's MK14 was a miniature computer with an equally diminutive price tag: £39.95. Notable for bringing cheap computing into our homes, those of us used to current technology may well be disappointed by the MK14's specification. Rest assured that while little more than a circuit board with a keypad and calculator-style nine-digit display, the MK14 was cheap and groundbreaking, simply by virtue of allowing anyone - not just those computer scientist boffins - to get their hands on a real, live computer. Selling over 50,000 in its time, the MK14 has since become incredibly rare, and a working MK14 is highly-prized amongst Sinclair collectors.

This single board computer didn't even have a case - there simply wasn't the option available. A tall, narrow circuit board held the various chips, which varied depending on the skill and enthusiasm of the owner: the MK14 came in kit form, so while every MK14 featured the SC/MP 2 processor, the amount of memory on there varied from 256 bytes all the way up to 640. How did this work? 256 bytes came as standard, but another 256 could be soldered onto the board, giving a grand total of half a kilobyte. And if 512 bytes just weren't enough, the optional 8154 I/O controller chip provided a further 128 bytes. In addition to this overwhelming bounty of RAM, the MK14 was also equipped with 512 bytes of ROM. The MK14 also had two edge connectors, one for attaching a better keyboard to the machine, the other simply allowing the user access to the processor's signals: an expansion port, in other words; the fore-runner to the similar feature of the ZX series.

Sir Clive was already into membrane keyboards at this point, and the 20-key membrane has much in common with the dodgy input methods that plagued early users of the ZX80, ZX81 and ZX Spectrum computers. Just to the left of the keypad was a red push button, which would reset the machine. The twenty keys were shared out into the sixteen characters necesssary for hexadecimal input, while the remaining four keys were assigned as 'Go', 'Mem', 'Abort' and 'Term'. 'Go' began program execution at the current address, 'Mem' moved on to the next address and allowed the user to assign it a hexadecimal value. 'Abort' moved to address entry mode, where the user could quickly change to a different address, rather than cycling through, and 'Term' provided a data entry mode, where memory contents could be quickly changed using the keypad.

At the heart of the MK14 was National Semiconductor's SC/MP 2 - the Simple Cost-effective Micro-Processor, running at 4.4MHz. Intended for use in control applications, the SC/MP chip was cheap, and perfect for learning about programming. Interaction with the user took place through a nine-digit display, although the machine required some hardware modification to actually use the ninth digit.

Veterans of the early ZX computers will remember how the processor would halt the display routines whilst calculating. Although the MK14 could be expanded to use a CRT, it also suffered from this peculiar but necessary design feature. Nevertheless, a 32x16 text mode and 64x64 graphics mode, although monochrome, was impressive for the time, and undoubtedly paved the way for the ZX80.

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