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Thread: Replica Science of Cambridge MK14 available for pre-order.

  1. #31
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    Yes, thanks for the link (I see it is on Martin's website).

    I am sure I came across a copy of the V2 manual somewhere on the internet - but I am darned if I can remember where and I don't think I downloaded a copy of it (or did I dream all this?)...

    Martin's schematics identify the diode as a 1N4148 (so we know that one should work).

    Dave
    Last edited by daver2; May 21st, 2018 at 05:17 AM.

  2. #32

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    I find that I've never had to make a .PDF from multiple images before and don't seem to have the tools to do that, so, here are the four individual pages of the instructions for the revised MK14 operating system, as supplied by Science Of Cambridge along with the PROMs containing the OS. The original document is quite sun yellowed and dog-eared so I spent a little bit of time cleaning up each scanned page in an image editor.

    Contrary to my earlier memory this particular document does not say which PROM to plug into which socket: That must have been on another scrap of paper.

    If anyone does have the means to make these into a single file .PDF, please do.
    Attached Images Attached Images

  3. #33
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  4. #34

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    Thanks Chuck, I'm looking at various options.

    (My) original scans of the pages are uncompressed BMPs (it was easier to clean them up in that format without degrading the images in subsequent loads / saves)

    I'll try various methods including the one you have pointed to, but if anyone else wants to try I'll be happy to pack the BMPs into a .zip and post it here, if .ZIPs are allowed as attachments. The trick is to balance overall file size with readability, as always. The forum engine downscaled the resolution of the .jpg files I uploaded by quite a lot. (I realise that is normal).

  5. #35

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    Some questions about the tape interface:

    With the circuit as I have it now, a zero is no tone and a one is a 1 kHz tone (yes, it is also time encoded).
    In the manual I read that the 1Mohm resistor generates a midlevel signal to keep automatic volume controls happy during recording. But with no signal my cassetterecorder goes to maximum amplification (and noise) and crunches the level as soon as the data begins, resulting in distorted first bytes.

    My idea was to invert the level so the tone is continously generated and only switched off during the low level of the data output pin. The recorder would be set to the correct level before the data starts. By inverting the playback circuit also this would result as far as I can see to much more steady recordings. Does this make sense ?

  6. #36

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    Yes, that's a great idea. Let us know how it works.

    One of the weaknesses of the MK14 cassette interface is that it did not properly allow for recorders which had automatic record level control (ALC), which covers the vast majority of cheap cassette recorders. Right from the first moment, it would be sending out actual data while the recorder was still trying to adjust from straining to hear nothing one moment to suddenly very loud sound the next moment. This would often cause the initial part of the recording to be distorted, making the whole recording useless.

    For this reason, recorders which had manual record level adjustment and record level meters tended to work better. You would do a 'dummy' save and use that to set the record level, and then, having established the right level, mark that setting and use it from then on when saving code to cassette.

    If you want to use the cassette interface in its historically correct configuration try using the sound ports of a PC to record from / replay back to the cassette interface because then you usually have manual record level control via the windows audio mixer or similar controls within your audio software (Soundrecorder, Audacity, etc).

    The ALC problem was addressed in the later ZX computers by first sending a length of steady tone out, a 'lead tone', if you like, the purpose of which was to make the ALC circuit pre-adjust to the volume level of the real data which would follow.

    The cassette interface has other problems as well - if you had the original OS, the cassette interface was not supported and you had to type in a short piece of code to enable you to use the cassette interface. I know you have the revised OS which does include the cassette routines in firmware but even there, there is a size limitation of 256 bytes so if you write something which is longer you have to load / save it in two or even three parts, if your machine is fully expanded. And.. the save / load speed of the cassette interface is, I seem to remember, about four characters per second. I suppose that's not too bad when the maximum number of characters is 256.

    I 'enjoyed' all of that back in 1977 / 1978, but the MK14 was literally the only 'computer' of any sort that I had, so I just had to get on with it. It was just how it was.

    Nowadays I prefer to use an assembler to write code on some other device - a Raspberry Pi can be used - and download it to the MK14 using an interface connected to the external keypad connector. The interface receives the code serially as Intel Hex and then 'types' the code into the MK14 at high speed. It also does away with the 256 byte limit and, since Intel Hex contains addresses as well as data, the code is just automatically sent to the right place, no need to preload memory locations with start addresses as you have to when using the MK14 cassette interface.

    Martin Lukasek (who I think you got some of your parts from) has made an Arduino project which looks to the MK14 as though it is an MK14 cassette interface connected to a cassette recorder, it communicates with the MK14 using the same TTL level protocol which would go to / from the cassette interface and allows you to save and load code in the same way, and also to have multiple 'saves' stored in the device so you can choose and reload them at will.

  7. #37
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    After waiting a long time for the memory chips to turn up I finally got my mk14 replica working but with one strange thing.
    I get the 0000 and then there should be two spaces and then 00 but in the second space I get a reverse C.I've only messed around with the mem and term buttons to test inputting data and that works fine so it functions but that reverse C was bugging me.
    then I found that if I pressed the GO button the reverse C would disappear.Is this something to do with the revised os?


    2018-06-16 12.57.00.jpg

  8. #38
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    I am still waiting for my RAMS as well... Unfortunately, I have been away for a few weeks on business - so the MK14 has been sat in a partial state of construction on the dining room table... The VDU and Cassette interface are built though.

    As to your issue - I don't think it is the revised 'OS'.

    Some possibilities:

    Did you check the display out on arrival?
    Have you checked for inadvertent short circuits between the keys?
    Can you use the memory display feature of the monitor to view the ROM code? If so, compare it to the listing of the revised ROM monitor to make sure the ROMs are behaving correctly.

    Why pressing GO should affect the display I don't know at the moment...

    Nice construction job though .

    Are you looking for the INS8154? I can only locate 'silly priced' items...

    Dave

  9. #39
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    I've not bothered about the 8154 since they are so hard to find.
    I have played with my MK14 a bit now and it seems to be working fine.
    I typed in duck hunt from the manual and it seemed to work ok.
    I just wonder if it's a voltage problem as I'm running it on 5V and just watched a video saying that they were running one on 7V.Will have to have a play.

  10. #40
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    If you are running it from a 5V supply WITH THE 5V REGULATOR ON THE BOARD that is too low a voltage. 7V is more like it.

    Do NOT run the MK14 from anything higher than 5.25V if not using the on-board regulator.

    Dave

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