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  • daver2's Avatar
    Today, 01:54 AM
    daver2 replied to a thread Mek6800d1 in Other
    Hi, The RAM (and ROM) is definitely only partially decoded. The RAM will 'shadow' when A15 is low. U8 ROM = 111X XXXn nnnn nnnn (typical $E000)...
    36 replies | 945 view(s)
  • daver2's Avatar
    Yesterday, 09:58 PM
    daver2 replied to a thread Mek6800d1 in Other
    Not able to check at the moment, but the RAM may shadow itself through memory (thus giving the impression there is more RAM than there actually is)....
    36 replies | 945 view(s)
  • daver2's Avatar
    Yesterday, 12:44 PM
    daver2 replied to a thread Prolog for a PDP-11? in DEC
    Just done. Funnily enough, I have a business trip to Edinburgh for a few days starting tomorrow... Dave
    2 replies | 86 view(s)
  • daver2's Avatar
    Yesterday, 12:06 PM
    daver2 replied to a thread DecMate Terminal in DEC
    I thought so too. It has a 6120 PDP-8 inside... Dave
    20 replies | 386 view(s)
  • daver2's Avatar
    Yesterday, 11:19 AM
    daver2 started a thread Prolog for a PDP-11? in DEC
    OK, I've drawn a blank trying to track down either: 1) A copy of PROLOG that will run under RT11 or RSX11. 2) The sources for a PROLOG in...
    2 replies | 86 view(s)
  • daver2's Avatar
    Yesterday, 10:27 AM
    No problem. Dave
    7 replies | 128 view(s)
  • daver2's Avatar
    Yesterday, 10:24 AM
    daver2 replied to a thread Quest COSMAC ELF in Other
    It is only a changeover contact button, a couple of inverters acting as a debounce circuit and a diode - so I would chase it through with your logic...
    80 replies | 2682 view(s)
  • daver2's Avatar
    Yesterday, 07:39 AM
    daver2 replied to a thread Quest COSMAC ELF in Other
    Based on that I would say there is something 'strange' about the switch connected to /EF4 (the input switch). By default CPU pin 21 (/EF4) should...
    80 replies | 2682 view(s)
  • daver2's Avatar
    Yesterday, 04:02 AM
    Sure thing Matt, $FFF8/9 = Maskable Interrupt Vector (IRQ) $FFFA/B = Software Interrupt Vector (see SWI instruction) $FFFC/D = Non Maskable...
    7 replies | 128 view(s)
  • daver2's Avatar
    Yesterday, 01:30 AM
    I learned both the 6800 and 6502 when I was 'young'... You should be able to pick up the 6800 easily (it is a cut-down 6502 in my mind)... The...
    7 replies | 128 view(s)
  • daver2's Avatar
    October 15th, 2017, 10:38 PM
    daver2 replied to a thread Quest COSMAC ELF in Other
    Try the following programs (both starting from 0000): 7A 30 00 This should extinguish the 'Q' LED. 7B 30 00 This should illuminate the...
    80 replies | 2682 view(s)
  • daver2's Avatar
    October 15th, 2017, 09:58 AM
    daver2 replied to a thread A strange SWTPC MP-A card in Other
    It depends what the code in the EPROM is expecting. Although I would have thought myself that the 2128 would have been configured to accommodate the...
    3 replies | 120 view(s)
  • daver2's Avatar
    October 15th, 2017, 09:37 AM
    daver2 replied to a thread Quest COSMAC ELF in Other
    Sorry - we are not necessarily saying it is the CPU... It could be a short between data lines (or cross-talk between them) that would drive you...
    80 replies | 2682 view(s)
  • daver2's Avatar
    October 14th, 2017, 11:03 PM
    daver2 replied to a thread A strange SWTPC MP-A card in Other
    If you want a lot of RAM, then having a ROM starting at $A000 is a bit self-defeating. Re-assembling the ROM for $F800 and burning it into an EPROM...
    3 replies | 120 view(s)
  • daver2's Avatar
    October 14th, 2017, 10:54 PM
    daver2 replied to a thread Quest COSMAC ELF in Other
    Correct, the 4508 only drives the LEDs. This was an add-on board to replace the TIL311 displays. If the LED goes OFF at some point, then it is not...
    80 replies | 2682 view(s)
  • daver2's Avatar
    October 14th, 2017, 11:20 AM
    I was doing a little reading and PETTESTER may only check the lower 1K of RAM - which (if working) should give you at least a BASIC prompt. There...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 14th, 2017, 10:59 AM
    Now you come to mention it - you may be right (which is good news as you have a screenful of beautiful 'g's!) Dave
    41 replies | 810 view(s)
  • daver2's Avatar
    October 14th, 2017, 10:49 AM
    Looks good to me (although I haven't checked that all of the characters are there...). See at https://www.youtube.com/watch?v=MZYn0HAGLjk. You...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 14th, 2017, 09:49 AM
    If your PET contained 'garbage' characters on first power-up without the CPU working correctly (due to a duff ROM) and you now have a blank screen...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 14th, 2017, 12:54 AM
    Good going snuci! I agree with Frank regarding pin 40, and pin 7 looks good as well. At this point I would have told you to start looking at...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 12th, 2017, 11:05 PM
    +1 for Dave's suggestion to remove the PIA & VIA chips next (morning Dave). If that doesn't kick the beast into life, then either swap the 6502...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 11th, 2017, 10:15 AM
    Excellent... Pin 40 of the 6502 CPU should be high (/RESET). We don't want the CPU to be permanently held in a reset state... Pin 2 of the 6502...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 11th, 2017, 09:30 AM
    If you look at my posts on The PET threads you should find the information you require. If not, let me know and I will describe it for you. ...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 11th, 2017, 03:59 AM
    That makes sense now... I (personally) wouldn't bother with the video memory at this stage of the game. Extremely unlikely that this will be the...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 11th, 2017, 12:22 AM
    That's what I thought - but the main board PCB on the site that snuci linked to didn't appear to have the edge connector I was expecting to see -...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 10th, 2017, 10:51 PM
    daver2 replied to a thread Quest COSMAC ELF in Other
    The 4508 is a dual 4-bit latch really. It is used in this application as an 8-bit latch. Sorry for the confusion... Just out of interest, are you...
    80 replies | 2682 view(s)
  • daver2's Avatar
    October 10th, 2017, 10:39 PM
    The little board by the PET power supply unit looks like another dc supply including rectifier diodes and a smoothing capacitor. I would agree...
    41 replies | 810 view(s)
  • daver2's Avatar
    October 10th, 2017, 05:27 AM
    daver2 replied to a thread My other COSMAC ELF in Other
    A 2N3904 is only a general purpose NPN BJT - so anything will probably suffice from the 'bits box' (e.g. BC109C...) It would be worth playing with...
    8 replies | 285 view(s)
  • daver2's Avatar
    October 9th, 2017, 12:43 PM
    daver2 replied to a thread Mek6800d1 in Other
    User error :-)! Dave
    36 replies | 945 view(s)
  • daver2's Avatar
    October 9th, 2017, 11:48 AM
    daver2 replied to a thread My other COSMAC ELF in Other
    I would also agree. Any video circuit with just a handful of resistors as a 'mixer' is going to be susceptible to all sorts of strange behaviour! ...
    8 replies | 285 view(s)
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