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Thread: Quest COSMAC ELF

  1. #21
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    Well, U1 pin 3 HAS to be wired up somewhere!

    Can I suggest chasing the track back from U1 pin 3 to see where it ends up? If nowhere, then this is probably 'original sin' and this card never actually worked as intended...

    I am on a business trip for a couple of days, so will pick up on this when I get back (if you have not solved it by then that is).

    Dave

  2. #22
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    Quote Originally Posted by daver2 View Post
    Well, U1 pin 3 HAS to be wired up somewhere!

    Can I suggest chasing the track back from U1 pin 3 to see where it ends up? If nowhere, then this is probably 'original sin' and this card never actually worked as intended...

    I am on a business trip for a couple of days, so will pick up on this when I get back (if you have not solved it by then that is).

    Dave
    Nope.. it connects to nothing. I've followed from pin 3 all the way along to pin 6 of U8. Nothing tapped into it at all. Nothing looks like it was ever connected there. It just ends there with no apparent connection to anything. Unless he has something going on under the socket for U1 I don't know about.. I'm scratching my head a little here. I've followed U8 pin 7 up to the LED breadboard, and that ultimately lands at pin 6 on one of the two 4049s up there. If I understand the 4049 pinout correctly, this should go out pin 7 of the 4049 and you would think ultimately back to pin 3 of U1. But it doesn't appear to do that. I need to get a better light on it.. really hard to see but pin 7 of that 4049 is hooked up to something somewhere else on that breadboard. Just have to find it. Wirewrap really melts the brain when you're trying to follow it and it's all packed in there.
    Last edited by falter; September 11th, 2017 at 08:30 AM.

  3. #23
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    Did some more wire chasing.

    So U8 pin 7 essentially leads to the lower of the two CD4049s on this LED board, pin 6. From pin 6 is some sort of gate (?) to pin 7, according to my CD4049 pinout. Pin 7 is wired to pin 15 of that same CD4049. Pin 15 is wired to pin 14. Pin 14 seems to be connected to three places: the same pin on the header that comes up from U8 pin 7 (so pin 6 and pin 14 go to that same place), pin 14 is also connected to pin 6, and also to pin 3 on the mc14508.

    It's very hard to see with those tiny little wires. But I've verified continuity between these points with the DMM. But yeah, at no point does it go back to U8 pin 6. So I have no idea how this works.

    Here's a visual representation of what I'm trying to describe in case it's confusing:

    elfledboard.jpg
    Last edited by falter; September 12th, 2017 at 09:00 PM.

  4. #24
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    Hi Falter,

    Your visual representation is excellent.

    Basically, the arrangement of gates in the 4049 agrees very well with that published in Fig. 1 (9) on page 18 of http://www.exemark.com/Microcontroll...arElecwebc.pdf (as I keep banging on about...) with the exception of the 'purple' wire from the 4049 pin 14 to pin 3 of the MC14508B. According to the schematic, pin 3 of the MC14508B should be connected to 0V (but we'll worry about that one later). It may affect how the LEDs work.

    By my assessment, if U8 is not present on your board, I would jumber U8 pin 7 to U8 pin 6. This will connected the (debounced) RUN switch to the 1802 CPU (U1).

    This should (hopefully) give the CPU cause to RUN/HALT when we desire!

    Dave

  5. #25
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    Thanks Dave!

    I connected pins 6 and 7 and tried operating in PROM mode.. woohoo! It seems to behave like you'd expect. Here I put it in PROM mode, protected memory and then just cycled through memory contents:

    https://www.youtube.com/watch?v=FbsG3OGFmgs

    Also seems to work with PROM switched off. Yay! Now just have to figure out if it's really working or partly. I want to enter a short test program but I'm still not clear on how I set the data switches to, say, 50, or whatever called for in the guide. Have to learn that now.

  6. #26
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    Well done that man!

    I suspect this is the first time this card has ever worked then...

    You may find this resource helpful http://www.sunrise-ev.com//Membershi...eatsheet-i.pdf.

    My suspicions are that the LED's may work 'backwards' - i.e. they are ON when they should be OFF. This stems from reading up on the original modification to replace the TIL311 displays with discrete LEDs.

    My suggestions are not to bother with a 'program' at this point in time but to checkout the operation of the switches, RAM and discrete LED indicators.

    Switch the ROM out of circuit first and then deposit data values of 00, 01,02,04,08,10,20,40,80 and FF into consecutive memory locations starting at address 00 (a RESET condition).

    As you write each value in - you should see which LED's light up in relation to the switch that is ON (or whether all of the LEDs light up except the one that is on...).

    You should then read back (verify) what you wrote and ensure that the LEDs light up the same as when you wrote the values in.

    A simple program to then test out would be:

    00: 7B ; SEQ - Set the Q bit and turn ON the Q LED.
    01: 7A ; REQ - Clear the Q bit and turn OFF the Q LED.
    02: 30 ; BRA - Branch to .. (repeat for infinity).
    03: 00 ; Address to branch to.

    Enter the above 4-byte program starting at address 00 and run it from address 00. Look at the Q output with an oscilloscope - it should be oscillating!

    Dave
    Last edited by daver2; September 14th, 2017 at 02:56 AM.

  7. #27
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    Good work Guys for troubleshooting the board. I assembled mine after 36 years of having the bare pcb. I finally source every parts and built mine.
    Falter: If you are interested to get a copy of the PROM chip already programmed, try to contact Josh Bensadon from the Yahoo group. He was kind to have me buy one from him as he indicated that he still have some PROM chips left and he can program them.
    I also built a EPROM adapter (got the idea from old (scarce) pinball PROM chips being replaced), programmed it with Elf program. To demo it, I just flick the switch to PROM and start RUN...however, I have the TIL311 displays.

  8. #28
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    Nice!!

    I think the PROM on mine might be okay.. I have to get the time to do the checkout procedure fully. And learn how to read hex fron led displays and enter it.. lol But I appreciate the info, in case the PROM is bad.

  9. #29
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    Maybe restore it to the original design with some TIL311 displays instead of the discrete LED bodge it has now?

  10. #30
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    Quote Originally Posted by gslick View Post
    Maybe restore it to the original design with some TIL311 displays instead of the discrete LED bodge it has now?
    I was thinking about that but I wondered about the whole 'messing with originality' thing.

    Plus, I bought another ELF: http://www.ebay.ca/itm/cosmac-elf-wi...72.m2749.l2648

    I'm not sure if it's true vintage - the seller says his brother built it years ago. But it has the hex displays. So I have sort of examples of both display types anyway.

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