Image Map Image Map
Page 1 of 3 123 LastLast
Results 1 to 10 of 21

Thread: MK14, 2716 Eprom Adapter, help...

  1. #1

    Thumbs up MK14, 2716 Eprom Adapter, help...

    35 year after I have now the time to revive my first "computer", Science of Cambridge MK14.

    After some repairs ( 2111 Ram) , and some long evenings it has wake up again. and it works well.

    Now, I want to replace the Prom by a 2716 Eprom, loaded with a copy the supermonitor.

    So I have designed a piggy board that is plugged into the 8154 Ram/IO socket. (see attached picture)

    Unfortunatly, it does not work until now. No display after reset

    Attached is the drawing I have designed.

    May one of you could help me by checking what I have done ??

    I am not fully sure that the address scheme is correct ?

    I also search the description of the 1,5k Ram extension ( schematic )

    If one of you have a copy ...

    Thanks

    Jean-Pierre
    (France)
    Attached Images Attached Images

  2. #2
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,524

    Default

    Pin 18 of EPROM needs to be low when chip is to output data. Ground it or tie it to pin 20.

  3. #3
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,466

    Default

    Hi again Jean-Pierre,

    Great to see that you posted your question on the forum rather than PM me. As I said, this way you get the full support of the whole community rather than one person's view!

    Dave_m nailed the floating pin 18. You should not have any floating inputs at all in your design. I would personally connect pin 18 to pin 20. If you look at the data sheets for the EPROMS you may find that some (all?) EPROMS exhibit a lower power consumption when pin 18 and 20 are both high (not selected). This will be good news for your power supply...

    The same thing is true of the unused inputs to IC3 (7404). I would tie these low to prevent them from floating. Floating inputs can sometimes cause chips to draw excessively high currents as they are no longer a digital signal but an analogue antenna! You may also benefit from installing 0.1 uF ceramic disk decoupling capacitors close to the VCC and GROUND pins of the 7404 and the EPROM.

    Resistors R1 and R2 are not strictly necessary - but there is no harm if they are there as they would prevent shorting out of the power supply if someone linked the stake pins incorrectly.

    Now - as to the decode using the inverted A11 line as an EPROM select. I need to have a little think about this one - but I beleive that your MK14 will stop working if you install your EPROM adapter. I don't think this is the intended behaviour! If I undestand what you are trying to achieve is addressing the EPROM when A11 is high - is this correct? If so, the EPROM would be addressed from 8xx to Fxx. Unfortunately, the keyboard is at 9xx and the RAM and expansion RAM at Fxx and Bxx respectively so the EPROM will interfere with the correct operation of the DISPLAY/KEYBOARD and the RAM.

    Where do you actually want the EPROM contents to appear in the MK14 memory map and I should be able to help with the address decoding?

    Dave

  4. #4

    Default

    Hello Jean-Pierre,
    the original PROMS are selected when A11 is low (Enables the PROM from 000H on).
    Your 2716 is selected when A11 is high. Remove the inverter and tie A11 to pin 20 of the 2716 direct.
    As mentioned before, connect pin 18 of the 2716 to GND.

    In the Address table the ranges 200-3FF and 400-5FF are interchanged.

    Frank
    Last edited by Frank S; August 13th, 2014 at 07:19 AM.

  5. #5
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,466

    Default

    I partially agree with Frank. This modification (enable the external EPROM when A11 is low) will cause all memory address from 0000-0000-0000 to 0111-1111-1111 (0x000 to 0x7FF) to be mapped into your external EPROM (replacing the on-board PROM) - assuming you remove these devices or physically disable their enable signal. There is then no requirement for IC3 (7404).

    I am not too sure about removing A7 and A8 from the EPROM though - this would only decode 128 bytes of EPROM (assuming I am reading the post correctly). A0 through A8 are required to correctly decode 512 bytes.

    Unfortunately, you will still only have 512 bytes of ROM shaddowed four times in the MK14. Sure, the switches will permit you to select which 'bank' of memory within the 2716 is physically selected.

    Question - why do you not attach your EPROM board to the edge connector. You should then be able to use all 2K of the EPROM directly? Just a thought.

    Dave

  6. #6

    Default

    Quote Originally Posted by daver2 View Post
    ...I am not too sure about removing A7 and A8 from the EPROM though ...
    Yes, you are right. I deleted the sentence in my last post
    Leave A7 and A8 as you have them connected.
    Then you have the four times mirrored 512 Byte of the Monitor.
    Frank

  7. #7
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,466

    Default

    You were also asking about the 1.5K RAM expansion.

    The basic MK14 came which a whopping 256 bytes RAM. This could be expanded by a further 256 bytes by adding the two additional 2111 RAM chips. Adding the I/O RAM chip added a further 128 bytes - making a total of 640 bytes.

    The additional 1.5K came from rearranging the address decode logic for the ROM and filling what would have been shadowed ROM with a further 3 pairs of 2111 RAM chips. If you are interested in performing this modification/addition I would be happy to work out the address decoding changes that you would require. It may be possible to add the additional RAM on the same board as your new EPROM if we are smart enough...

    Dave

    PS: Glad to hear that you have got the "0000 00" display on your MK14 once again!

  8. #8
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,466

    Default

    I have just sketched out the address decoding for the extra 1.5K of RAM and I think it amounts to a single SN74LS139 device (a dual 2 to 4 decoder).

    The solution I have in mind should work for either 3 pairs of 2111 chips or by using something like a 2K*8 CMOS RAM (e.g. the 6116) and loosing 512 bytes of it where the ROM lives. The solution should also cater for adding more ROM of your 2716 (e.g. 1024 bytes) - the amount of RAM will decrease accordingly.

    Let me know if you want me to sketch out the circuit for you. It may also be that when I have a look at it tomorrow it may be completely rubbish of course!

    By the way - you refer in your PM to a monitor and a super monitor. Are these the standard monitor and revised monitor (the standard monitor displayed '---- --' when it powered up whereas the revised monitor displayed '0000 00' if I remember correctly) - or are they something else that I am not aware of?

    You can also extend the memory on the MK14 by adding a 4-bit latch on D0 through D3 (triggered from not NADS) - this produces A12 through A15 (i.e. you can increase the memory capacity of the MK14 from 4K to 64K!!! You have to then decode the MK14 memory map on page '0' of the new scheme.

    I have the assembler source code for NIBL BASIC and was slowly modifying it so that it would run on the MK14 - however it will require EPROM/RAM from 0x1000 upwards (and the use of the serial port - or some other sensible user interface - to communicate with the human rather than the seven segment display and hex keypad of course!). Have a look at http://www.dos4ever.com/SCMP/SCMP.html if you are interested.

    Dave

  9. #9

    Default

    YES !!! It works fine with the 2716 Eprom :+)) See attached picture with the piggy board.

    MK14 Eprom_redimensionner.jpg
    So I want to thank all of you who help me to go back to the rigth track.

    Resumé:
    1) Pin 18 on my drawing was shown open, but in fact on my printed board, it was already tied to GND.
    - I moved it to A11.
    - the small table with J1 and J2 jumper was wrong. It is now corrected

    2) A11 signal was inverted on my design because I mistinterpreted the MK14 schematic, which shows a circle on A11 input at IC2/IC3

    - I removed the inverter, and then MK14 was OK, displaying the "0000 00".


    3) My original goal was to remove the proms that I thougth defective: I have to tell you that some years ago I tried to turn the MK14 on, but I made a mistake and connect it directly to a 12 VDC power supply !! Hopefully the power supply went into limitation mode, so may be it protected the electronic ?

    I thougth it was the end of itslife, until recently when I wanted to resurrect it. As it did not start, I cheked all others components.

    I checked the 2 proms together with a ultra simple prom reader I made with an Arduino (not vintage !). the content of the proms seems to be modified from the original value in many locations.

    I found an image of the Supermonitor prom on the Web, so I decided to build the Eprom board and to remove the 74S571 proms., but without success until you helped me.

    In the meantime , I did other tries, I also replaced the Ram and tried again with the 74S571 prom and miracle, it worked.

    So now I can use the original prom or the 2716 Eprom, in which I am planning to put the standard monitor "---- --" and the Supermonitor "0000 00"


    4) I could not attach the prom board to the edge connector as you suggested it , because this connector does not provide D0-D6 nor any address line.

    5) Yes I have a a set of IC2-IC3 with the monitor delivered with my MK14 Issue iV and I also have a set of IC2-IC3 I purchased a little bit later with the Supermonitor(revised monitor) , including sofware for the cassette.

    I also have the cassette interface board and a some board I made for single step operation. My MK14 has 512 byte of ram + the RamIO INS8154

    Back in 1978, I used my private MK14 at work to show my boss how to test connections to high accuracy (8 1/2 digits) voltmeter which had only a" BCD" interface. I was the beginning of my career in test instruments (Fluke)

    6) Next now is to connect the cassette interface to the sound recorder on my Windows PC

    7) For Dave, I will be interested to see your schematic for a 1.5Kram extension.
    In fact , I think it will be interesting to use Eprom space from the 2716 to permanently save some user programs, without having to load them from the cassette.
    It means of course to get full access to the 2 Kbytes of eprom

    Thank again
    JP

  10. #10

    Default

    I have posted a long message yesterday evening , to inform you that my 2716 board was running well now, but when I posted it, I got a windows saying that my message needed to be checked by a moderator before to be visible. So wait and see...
    In the meantime thank again to everybody for your messages.
    JP

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •