The MK14 computer suffers from a number of shortcomings that make it increasingly difficult to build (as replicas) and then to upgrade:
Keypad the original keypad was a special (and poor quality)
Display the National Semiconductor displays are becoming increasingly hard to find and getting more expensive (even when recycled from a Texit calculator)
RAM the MM2111 RAM chips and alternatives are almost impossible to obtain
PROMs again the 74S571 PROMs are now almost impossible to obtain and the alternatives (Tesla) can only be programmed on a few specialist programmers (see my Tesla Programmer)
Memory Map the memory map is not fully decoded so there are multiple images and is limited to a total of 4K
So here is an enhanced design which overcomes those issues and also provides a much more flexible platform upon which future experimentation can be carried out whilst still being 100% compatible with the original SCIOS firmware, perhaps the next version of the MK14 that would have been if Sinclair had not then developed the ZX80 ?
I have tried to use components that are still available and in keeping with the original.
Here are the highlights:
similar sized PCB to the original (just slightly longer)
new keypad and display (same as used on my Acorn System 1 Trainer)
RAM and PROM replaced with 6116 and 2716/32 EPROM respectively
PCB edge connectors changed to two part plug/sockets, DIN41612 for the main bus compatible with the original MK14 VDU
two variants of backplane available
the address bus is fully decoded and 16-bits wide (if required)
dedicated Serial and Fast Loader interfaces
The main board also incorporates features that were left off the original - the Cassette Interface and Single Step function.
I've already mentioned that the expansion bus connector is a DIN41612, this can be connected to one of two variants of Backplane which then allows for the addition of further boards:
a VDU - either the original MK14 VDU (see my replica) or my enhanced MK14E VDU design which has had the impossible to find character generator replaced
MK14E Expansion Board which provides a further 4K of RAM and 16K of EPROM both with flexible memory mapping that allow NIBL or NIBL-E to be used, it also has a breadboarding area for prototyping
Follow these links for details of each of the boards:
And here is the memory map for the complete system