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TonyDuell · 29th Jun 2015, 2:48 pm

I've started this based on the last messages in the 'Favourite Microprocessor' thread so as not to get too confusing.

The MK14 was of course Sinclair's first microcomputer. Based on the SC/MP, with 256 bytes of RAM and 512 bytes of ROM containing a monitor program. I got one many years ago, I still have it somewhere, although I raided the CPU chip for a project and then wrecked it when the +5V line in that project went sky-high,

Anyway, back to the MK14. There were many things to dislike about it. The first was the keyboard, which was typically horrible. Early versions had a conducive rubber sheet over PCB tracks. There was an 'upgrade' to metal dome contacts and plastic buttons which was supposed to be better but which I found to be as unusable. The good thing was that the keyboard area on the PCB had some undocumented sets of holes in it. It turned out you could solder a keyboard switch that Maplin then sold into each group of 4 holes and get an actually useable keyboard. I remember labelling the keycaps with Letraset, eventually it all rubbed off but by then I knew where the keys were. There was a set of edge connector fingers at the front right of the PCB which carried the keyboard signals. For some unknown reason this was not documented anywhere, It wasn't hard to trace them out, though.

This leads me on to the other main dislike. The manual. In my opinion it was terrible. Remember that for many people this was the first time they had ever used a computer. A program listing like

0F12 C4 05
0F14 07
0F15 90 FE

may be obvious now to mean store C4 in location 0F12, 05 in location 0F13, etc, but it took me many months to work that out. I could not get any of the demo programs to run until I realised that. There should have been explicit instrucitons on how to type in one of the programs, exactly what to type.

One bit of the hardware sticks in my mind. The display output port used 74157s as the data latches. Now a 74157 is a multiplexer, but one of the data inputs was tied to the output to make a sort of transparent latch. In my experience it worked with some 74157s and not others. The story is that they intended to use 74175s (which are 4 bit D type registers) and there was a typo in the order. This led to a redesign to use the wrong part. And I am afraid that really sums the machine up to me.

Oh well.. The good points outweighed those, though. It was my first computer, I learnt a lot from figuring it all out and I have thus got to work on many more interesting machines since.

I really must try to find a spare SC/MP (I have plenty in other devices, but not one I want to raid at this point) and get it going again.

29th Jun 2015, 2:48 pm	#1
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	The Sinclair / Science of Cambridge MK14 I've started this based on the last messages in the 'Favourite Microprocessor' thread so as not to get too confusing. The MK14 was of course Sinclair's first microcomputer. Based on the SC/MP, with 256 bytes of RAM and 512 bytes of ROM containing a monitor program. I got one many years ago, I still have it somewhere, although I raided the CPU chip for a project and then wrecked it when the +5V line in that project went sky-high, Anyway, back to the MK14. There were many things to dislike about it. The first was the keyboard, which was typically horrible. Early versions had a conducive rubber sheet over PCB tracks. There was an 'upgrade' to metal dome contacts and plastic buttons which was supposed to be better but which I found to be as unusable. The good thing was that the keyboard area on the PCB had some undocumented sets of holes in it. It turned out you could solder a keyboard switch that Maplin then sold into each group of 4 holes and get an actually useable keyboard. I remember labelling the keycaps with Letraset, eventually it all rubbed off but by then I knew where the keys were. There was a set of edge connector fingers at the front right of the PCB which carried the keyboard signals. For some unknown reason this was not documented anywhere, It wasn't hard to trace them out, though. This leads me on to the other main dislike. The manual. In my opinion it was terrible. Remember that for many people this was the first time they had ever used a computer. A program listing like 0F12 C4 05 0F14 07 0F15 90 FE may be obvious now to mean store C4 in location 0F12, 05 in location 0F13, etc, but it took me many months to work that out. I could not get any of the demo programs to run until I realised that. There should have been explicit instrucitons on how to type in one of the programs, exactly what to type. One bit of the hardware sticks in my mind. The display output port used 74157s as the data latches. Now a 74157 is a multiplexer, but one of the data inputs was tied to the output to make a sort of transparent latch. In my experience it worked with some 74157s and not others. The story is that they intended to use 74175s (which are 4 bit D type registers) and there was a typo in the order. This led to a redesign to use the wrong part. And I am afraid that really sums the machine up to me. Oh well.. The good points outweighed those, though. It was my first computer, I learnt a lot from figuring it all out and I have thus got to work on many more interesting machines since. I really must try to find a spare SC/MP (I have plenty in other devices, but not one I want to raid at this point) and get it going again.