The MK14

MK14 Photo


Not an Acorn product but an important stepping stone to the creation of Acorn Computers.  Chris Curry worked with Clive Sinclair on the development of the MK14 as well as other Science of Cambridge/Sinclair products but eventually went his own way setting up Acorn Computers, the Acorn System 1 of course being the first Acorn product.

(MK standing for Microprocessor Kit not sure why 14 ?)

The MK14 was made by Science of Cambridge (later becoming Sinclair Computers and finally Sinclair Research), it was based on the National Semiconductors SC/MP (INS8060) processor and was sold as as a kit:

MK14 Kit Photo

As well as the basic machine a number of add-on boards became available as shown in this advert from Wireless World January 1980: 

Wireless World MK14 Advert January 1980

National Semiconductors Introkit

The starting point for the MK14 appears to have come from an earlier National Semiconductors product The Introkit:

National Semiconductors Introkit Photo

This was a much simpler computer that only came as a kit, the PCB also providing prototyping space for adding your own peripheral circuits.

The first versions required a Teletype for the 'user interface' which was not an option for the hobbyist so National Semiconductor produced an add-on kit which included a modified National Semiconductor's Novus calculator, you then just had to wire up the circuit on the breadboard area ! (as in the photo).

I have found a manual for the keyboard interface, again it has a resemblance to the MK14:

Here is a copy of the National Semiconductor SC/MP Kit Users Manual.

There were two versions of the OS depending on the 'user interface', the Teletype version was called KITBUG whilst the later keyboard version SCMPKB:

Interestingly the early version MK14 OS, SCIOS, still has SCMPKB as the title in the assembler listing in the manual !  I had access to an Introkit with SCMPKB fitted and can confirm this is exactly the same as the early version of MK14 SCIOS.

National Semiconductor produced quite a lot of other documentation for the SC/MP processor which will be of interest to MK14 owners:


There were a number of versions of the MK14 but details of the changes between versions are sketchy at best.  The 'early version' schematic is most likely the first version, the 'later version' schematic refers to a version 5 so I guess the 'early version' is version 4.  The 'later version' states that unused gates are used on version 5 so I'm guessing that version 5 includes the updated address decoding required for the VDU.

The modifications are described in this document, Address Decoder Mods (component references and pin numbers refer to the 'early version' schematic as they were changed in the 'later version').



There are two versions of the MK14 'monitor'.  The first, I have called it Version 1, was on the first release of the MK14 and is just National Semiconductors SCMPKB.  An upgrade, I have called Version 2, was produced that included the tape interface routines (previously these had to be typed in each time the machine was switched on !).

The manuals above include listings of the firmware 'Monitor program listing' but have errors, especially Version 1.  I have repaired the listing pages from the manuals.

There is a modified version of SCIOS Version 1 that stops blanking of the 9th display digit (just one byte difference at $0046 becomes $08 'NOP' was $CA 'ST'):

I have also created source files in SB-Assembler format: (see

The Version 2 listing in the 'later version' manual has a mistake at $01FC incorrectly shown as $42 when it should be $4B, this has been corrected in the listing and binary here.

Again I have created source files in SB-Assembler format:

Tape Interface

A simple add-on circuit enables the storing and retrieval of code to cassette, made easier with SCIOIS Version 2 which includes the necessary routines.

Version 1 PCB [10]

MK14 Tape Interface PCB Comp SideMK14 Tape Interface PCB Track Side

and here is the manual which includes the schematic:


It was possible to add a VDU to the MK14 though this required wiring of the data and address bus to the unused underside of the rear connector for its attachment.

MK14 Wiring Mods for VDU

Again this came as a kit [8]:

MK14 kit

PROM Blower

There was also an add-on 'PROM Blower' which allowed programming of the 74S571 PROMs used on the MK14 (and Acorn System 1).


MK14 PROM Blower PCB


and here is the manual which includes the schematic:

Its interesting to note the similarities between the circuits of the MK14 PROM Blower and the Acorn PROM Programmer.

Memory Map

Expansion Connector


I am compiling a library of interesting articles and documents related to the MK14 and SC/MP Processor which can be found here SC/MP Library.


I have made a replica MK14 details of which can be found here:

I am also working on an Enhanced version of the MK14, details will be here soon !


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