Image Map Image Map
Page 4 of 11 FirstFirst 12345678 ... LastLast
Results 31 to 40 of 110

Thread: Commodore PET restoration help needed

  1. #31
    Join Date
    Apr 2020
    Location
    Basel-Stadt, Switzerland
    Posts
    41

    Default

    Quote Originally Posted by Hutch View Post
    Great. That's to be expected with C7 removed. IRQ is normally high

    How about with C7 connected?
    Normal would be toggling. Both high and low should light up.
    IIRC video generates an interrupt at vertical refresh.
    If video isn't working, it might not be toggling rapidly, but you should be able to generate an interrupt by pressing a key on the keyboard.
    Yes, ok, with a 6520 in C7, IRQ on the 6502 goes to "Low" and stays there. No toggling or going to high.
    swapping the two 6520's creates the same result. The logic probe shows "low" and nothing more.

  2. #32

    Default

    So my guess would be that some input to the 6520 is telling it to hold the interrupt.

    There is a video signal and you said that pin 18 (CB1) is toggling so I doubt it's the video signal causing it.
    Even if the cassette sense line was shorted or a keyboard line was shorted, those should only generate a momentary interrupt.

    Do the pull-up resistors R18 thru R25 look OK?
    You should see high on pins 10-17

    Do you still have the # symbol on the screen?
    No flashing cursor right? Toggling on the h-sync and v-sync pins on the video connector?
    Last edited by Hutch; June 2nd, 2020 at 09:37 AM.

  3. #33
    Join Date
    Apr 2020
    Location
    Basel-Stadt, Switzerland
    Posts
    41

    Default

    Quote Originally Posted by Hutch View Post
    So my guess would be that some input to the 6520 is telling it to hold the interrupt.

    There is a video signal and you said that ping 18 (CB1) is toggling so I doubt it's the video signal causing it.
    Even if the cassette sense line was shorted or a keyboard line was shorted, those should only generate a momentary interrupt.

    Do the pull-up resistors R18 thru R25 look OK?

    Do you still have the # symbol on the screen?
    No flashing cursor right? Toggling on the h-sync and v-sync pins on the video connector?
    The resistors look ok and all register a resistance of 9.8K

    the # is not consistent anymore, but there is more often than not a single random character on screen.

    no flashing curser

    About the H-Sync and V-Sync. im trying to find that on my schematics but all im finding is VERT DRIVE on pin 3 of J7. I assume that is v-sync? The Horizontal Drive i have too but i do not see how it connects in the J7, at least from what i can gather from the schematics. But i am pretty new to this.

  4. #34
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    7,349

    Default

    Quote Originally Posted by SkyCaptain View Post
    About the H-Sync and V-Sync. im trying to find that on my schematics but all im finding is VERT DRIVE on pin 3 of J7. I assume that is v-sync? The Horizontal Drive i have too but i do not see how it connects in the J7, at least from what i can gather from the schematics. But i am pretty new to this.
    Both on the same page,
    2001N/320349-6.gif

    J7:
    1 - Video (p.8, top right)
    3 - Vertical (p.6, mid right)
    5 - Horizontal (p.6, bottom right)
    2, 4, 7 - GND

  5. #35

    Default

    It is also possible that there is a socket broken and the 6520 didn't get initialized properly. The interrupt output has to be setup at the beginning of the code.
    Make sure all the data lines, address and selects go to the right place, from the 6520 chip in the socket.
    A NOP test adapter can be handy here.
    Dwight

  6. #36
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,852

    Default

    I have seen issues like this before if I remember correctly.

    Things are OK until the PIA is initialised and then the interrupt line goes low for no apparent reason. In this case a replacement PIA was the solution. However, as you have swapped the PIAs over, I suspect there is something more going on here than a faulty PIA.

    Looking at the BASIC disassembly; CA1 (C7 pin 40) and CB1 (C7 pin 18) can generate interrupts as part of the software initialisation.

    CA1 is the cassette read data whilst CB1 is the video interrupt.

    The default 'power up' configuration is with the cassette read data interrupt disabled but the video interrupt enabled. So I would concentrate on the video interrupt pin (C7 pin 18).

    In the first instance, check these two pins of C7 with your logic probe to check (a) for activity (good) and (b) for whatever permanent level is on the pin and report back.

    You may have already done this before if I read back far enough.

    In the second instance I might be inclined to bend pin 18 of a PIA out before inserting it into the C7 socket. I will need to work out whether you need to pull this pin high or low for correct operation (no interrupt) though. You could try both scenarios (via a 100R resistor) and see what the effect is on the CPU /IRQ pin and the operation of the PET. It might give us another data point...

    Dave
    Last edited by daver2; June 3rd, 2020 at 01:01 AM.

  7. #37
    Join Date
    Apr 2020
    Location
    Basel-Stadt, Switzerland
    Posts
    41

    Default

    Alright, im trying to reply to everyone in a single comment.

    First, i understand now about the pins on J7. Im still struggling when a single component is split up in many parts of the schematics

    In response to Dwight Elvey
    "Make sure all the data lines, address and selects go to the right place, from the 6520 chip in the socket"
    How do I see what is a data line? or basically, "check all pins and where they go"? which is fair,

    I have checked Pin 1, 3 and 5 of J7 with following results.
    Pin 1: High
    Pin 3: Pulse greater than 50Hz (dont know if the rate is relevant. I my Logic probe just has a cut off, pulses lower than 50Hz make the LED blink, pulses higher than 50Hz make the LED's be constant)
    Pin 5: Pulse greater than 50Hz

    To daver2

    So i think we might be getting somewhere
    On socket C7 (without a 6520 plugged in) pin 18 is showing 3.9V or "High", and activity (pulse higher than 50Hz)
    But on Pin 40, i have 0V and get no reading for High, Low or if there is a pulse on it. The machine has no cassette deck, it is a full keyboard 2001n-16

    I dont think i have a 100R resistor, i have a few other ones lying around but id need to order 100R. wouldnt be a problem though. Should I?

  8. #38
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,926

    Default

    If you order some parts, make sure you get a few 40 pin DIP sockets with good machined pin sockets. Since both PIA chips do not work in that socket, the socket may be bad as Dwight as mentioned.

  9. #39
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,852

    Default

    That's OK for C7 pin 40. Without a cassette player, pin 40 will be floating - so the logic probe will indicate 'open circuit'. If you are reading 0V on a multimeter, it just means 'no volts' as opposed to 0V. Try putting the positive multimeter lead on +5V and the negative multimeter lead on C7 pin 40. You will probably still read 0V on the multimeter (indicating that the pin is actually floating).

    Suggestion. Power off, plug the two PIAs back in and measure the resistance between C7 pin 18 and C5 pin 15 with your multimeter. Read the resistance on the IC pin itself and not on the bottom of the socket. These two pins should be connected together with a bit of copper track, so we should have a low resistance between them. This would ensure that C7 pin 18 is in good contact (via the socket) with the copper track.

    J7 pin 1 being high would indicate a pure black screen. Is that what you are seeing?

    Dave

  10. #40
    Join Date
    Apr 2020
    Location
    Basel-Stadt, Switzerland
    Posts
    41

    Default

    With all the chips back in place and power OFF, I flipped the board and checked the resistance between the two pins with my multimeter set to 20K and measured 2.86kOhm. I measured the same on top of the board at the pins of the chips just to make sure im actually reading the right pins as flipping was a little disorienting at first ^^

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
  •