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Thread: PET 8032 Video to MDA Monitor

  1. #1
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    Default PET 8032 Video to MDA Monitor

    For my Commodore PET main board Tester, I'm looking at hooking up the 8032 PET video (80 column X 25 lines) to an external monitor. I tried composite video to an old Sony Watchman TV, but the Sony could not sync the the PET. I think the older 40 column PETS are closer to the TV bandwidth, but the the 8032 has a higher Horizontal rate. I was wondering if anyone has had success using an old PC Monochrome monitor. The PET has TTL level Horizontal, Vertical and video signals. The frequencies are 20 KHz and 60 Hz vs the MDA spec of 23 KHz and 50 Hz. Is this close enough?

  2. #2

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    It should be.

    I'm actually kind of surprised the TV wouldn't work. Was the image horizontally stable but the wrong size? Or was the horizontal not syncing at all?

    In my experience mismatching monitors, getting sync polarity correct is the hard part. Even that's not the end of the world. Years ago when I couldn't get my hands on TTL inverters I'd build inverting amplifiers out of ye olde 2N2222.

    I'll soon have an 80 column PET and was planning on using a TV- standard "video" monitor. Now I'm anxious to see what I'll run into

    I've done some pretty bizarre monitor mismatches in the distant past. When everyone else was using 14 and 15" VGA monitors, I was finding discarded 17-21" workstation monitors with every different frequency and sync under the Sun. I always managed to mate them to my Amigas. When I got the Cybervision 64/3D, that was a godsend for me. It would output any video configuration you wanted. Then I was adjusting both ends and had really crisp displays at oddball sizes like 1532x1106 or something.

  3. #3

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    When I say "it should be" I mean you should be able to adjust the horizontal and vertical hold and size controls far enough to get it to work, assuming the sync polarity is correct.

    Also, TV signals in this region are 15.575kc by 60c. That's closer to 20kc by 60c than 23kc by 50c. The TV should work better. Unless there's something unusual about the Watchman which there probably is.
    Last edited by KC9UDX; June 7th, 2015 at 05:59 AM.

  4. #4

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    There's a Tandy VM-5 on eBay for $5 plus shipping that might fit the bill too.

  5. #5
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    Quote Originally Posted by KC9UDX View Post
    When I say "it should be" I mean you should be able to adjust the horizontal and vertical hold and size controls far enough to get it to work, assuming the sync polarity is correct.
    Matt,
    That may be my problem, there are no controls for the Watchman TV. The PET Sync signals are negative going. I'll look for monitors with lots of controls. Typically do MDA monitors have such controls?

    I looked at some newer multi sync monitors, but they seem to start at 30 KHz and go to 100 KHz, not like the old NEC Multisync that started below 20 KHz.
    -Dave

  6. #6

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    Dave,

    I don't recall much about the real MDA monitors. I'm pretty sure there were no user controls for that on the IBM ones, don't recall what was inside. Most monitors have all four necessary adjustments inside. I think I have an Amdek monitor that had MDA and composite inputs. That would have to have all the controls outside.

    I do know there were a lot of general purpose monochrome composite monitors which had all the controls external. But I don't know if you want to combine the sync, or keep it separate for diagnostic purposes. I'm trying to think of a real advantage to that but I can't.

    It's not terribly difficult to modify a monitor without controls to sync to something just outside its normal range, but I know I probably wouldn't bother anymore.
    -Matt

  7. #7
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    Quote Originally Posted by dave_m View Post
    For my Commodore PET main board Tester, I'm looking at hooking up the 8032 PET video (80 column X 25 lines) to an external monitor. I tried composite video to an old Sony Watchman TV, but the Sony could not sync the the PET. I think the older 40 column PETS are closer to the TV bandwidth, but the the 8032 has a higher Horizontal rate. I was wondering if anyone has had success using an old PC Monochrome monitor. The PET has TTL level Horizontal, Vertical and video signals. The frequencies are 20 KHz and 60 Hz vs the MDA spec of 23 KHz and 50 Hz. Is this close enough?
    The original 2001 machines (non-CRTC based) had a 15kHz scan rate that matched standard TV frequencies. The "Fat40" and 8000 series with 12" monitor have 20kHz rate. I believe the video signal is also inverted. I have one of those monitor adapters you can get on ebay for $25 (which is really just a single chip and a few components worth about $2) and it would not work on my 8032 as-is. However the CRTC-based PETs can be easily re-programmed to modify the output to match normal NTSC TV frequencies. I was able to get the output to work. Here are the values you need:

    Code:
    ; PET/CBM EDIT ROM - CRTC Setup Tables
    ; ================   80 Column x 25 Lines, 60 Hz, 15.75 kHz (NTSC) for external monitor/TV
    ;
    ; These tables are used to program the CRTC chip to set the screen to the proper rows, columns, and timing.
    ; Note: These seem to be the same as the 40 column values. Must verify!
    
    ;--------- Text Mode - 80 Column, 60 Hz, NTSC
    
    CRT_CONFIG_TEXT
        		!byte $3f,$28,$32,$12,$1e,$06,$19,$1C
        		!byte $00,$07,$00,$00,$10,$00,$00,$00
        		!byte $00,$00 
    
    ;--------- Graphics Mode - 80 Column, 60 Hz, NTSC
    
    CRT_CONFIG_GRAPHICS
        		!byte $3f,$28,$32,$12,$1e,$06,$19,$1C
        		!byte $00,$07,$00,$00,$10,$00,$00,$00
        		!byte $00,$00
    This is from my CBM Editor ROM project:
    http://www.6502.org/users/sjgray/pro...rom/index.html
    And on GitHub:
    https://github.com/sjgray/cbm-edit-rom

    You need to poke the values into the CRTC register with a basic program like this:

    Code:
    10 rem pet crtc setup program for NTSC video output
    20 rem ---------------------------------------------------
    30 for a=0 to 17
    40 read n
    50 poke 59520,a:poke 59521,n
    60 next a
    100 data 63,40,50,18,30,6,25,28
    110 data 0,7,0,0,16,0,0,0,0,0
    If you want your machine to be permanently set for NTSC you can build a new editor rom from my source, or you can patch an existing ROM.
    Sorry, I've never tried to connect an MDA monitor so I don't know if the CRTC chip in the pet can output sync that high. I did write a small utility that allows you to play with the CRTC registers in real time. You might be able to use it to find the correct values to give a stable output for your MDA monitor.

    Steve
    WANTED: CBM-II hardware or software, PET software

  8. #8

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    That would work well, but not in a test-bench scenario.

  9. #9
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    Well ntsc lcds with composite are plentiful and a simple editor rom replacement would be pretty easy for quick testing. At least you could verify proper operation. The pet composite adapter would be easy to build and the signals are available on the user port for quick attachment.

    You might still be able to find some vintage nec projectors that can go down to 15khz. I picked up an mt-1000 for under $100. I also found a NEC 1550m lcd monitor that can handle 15khz as well. These will probably accept 20khz signals.

    Steve
    WANTED: CBM-II hardware or software, PET software

  10. #10
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    Hey, according to:
    http://en.wikipedia.org/wiki/IBM_Mon...isplay_Adapter

    MDA is 18.432 khz Horizontal and 50 Hz Vertical.
    I would think that would be close enough to the PET output, although the PET vertical is probably 60 Hz.

    Where is 23 kHz Horizontal coming from that was quoted in the first post?

    Steve
    WANTED: CBM-II hardware or software, PET software

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