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Thread: Composite picture from a CRTC PET

  1. #61
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    Quote Originally Posted by MikeS View Post
    Just a reminder that AFAIK every 8032 has jumpers to invert the sync pulses if necessary.
    Hi Mike,
    That might do for Giovi as his PET CRT is kaput, but in my case mine is still working so I will leave the signal polarities to the J7 Video connector alone, and use the User Port to make the mod circuit for composite video.
    -Dave

  2. #62
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    Quote Originally Posted by MikeS View Post
    Just a reminder that AFAIK every 8032 has jumpers to invert the sync pulses if necessary.

    Folks have been using various versions of the same simple circuit to generate composite video from a PET since the 80s; in the original small-screen pre-CRTC 2001/30xx/40xx models it was close enough to NTSC that it worked with most TVs and monitors.

    As described in the first couple of posts in this thread the only difference with the 8032 and FAT40 using a CRTC is that some polarity jumpers need to be moved and a modified 'E' edit ROM is needed to change their 20kHz video signals to whatever the display in question can handle (normally ~15 kHz NTSC).

    While talking about the CGA, note that there is a mod out there to display colour with an 8032 (albeit at only 40 columns)...
    Mike,

    good to know that, I didn't know...

    I took a look at schematics, I guess you're refering to jumpers 2 and 3 at the 8032029-10 (CRTC and Sync buffers) sheet....?

  3. #63
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    Quote Originally Posted by Hugo Holden View Post

    I'd suggest using the 74LS244 to buffer your H & V syncs and RGBI drive, just like IBM did, regardless of the rest of the circuit that generates the sync & video.
    Hugo,
    Will a circuit like this do? The LS244 buffers have a lot of drive capability with I out (low) of 24 mA sink and a I out (high) of 15 mA source. That will certainly drive the resistor network on the CGA. The PET signals can be obtained from the J7 video connector or the J2 User port.

    Note: The Steve Gray EDIT EPROM is required for the 4000/8000 PET which will change 6545 CRTC parameters to generate NTSC TV timing.
    -Dave

    PET to CGA Video.jpg
    Last edited by dave_m; May 2nd, 2020 at 08:45 AM.

  4. #64
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    Quote Originally Posted by MikeS View Post
    Just a reminder that AFAIK every 8032 has jumpers to invert the sync pulses if necessary.

    Folks have been using various versions of the same simple circuit to generate composite video from a PET since the 80s; in the original small-screen pre-CRTC 2001/30xx/40xx models it was close enough to NTSC that it worked with most TVs and monitors.

    As described in the first couple of posts in this thread the only difference with the 8032 and FAT40 using a CRTC is that some polarity jumpers need to be moved and a modified 'E' edit ROM is needed to change their 20kHz video signals to whatever the display in question can handle (normally ~15 kHz NTSC).

    While talking about the CGA, note that there is a mod out there to display colour with an 8032 (albeit at only 40 columns)...

    Out of curiosity: the schematics shows separate jumpers for the UC2 (7486), pins 2 and 5; pin 2 is connected to PULLUP 1, pin 5 to GND.

    In one of my boards (i can't say the other two), assy 8032030 - 80 column cpu, pin 2 and 5 are tied together on the board and connected both to PULLUP 1.
    I can (and will) put the two jumpers, but I need to cut a trace.

    Does anybody know if there's a reason to keep the vertical drive in a way and the horizontal drive in another, as the schematics suggested?

  5. #65

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    Quote Originally Posted by dave_m View Post
    Hugo,
    Will a circuit like this do? The LS244 buffers have a lot of drive capability with I out (low) of 24 mA sink and a I out (high) of 15 mA source. That will certainly drive the resistor network on the CGA. The PET signals can be obtained from the J7 video connector or the J2 User port.

    Note: The Steve Gray EDIT EPROM is required for the 4000/8000 PET which will change 6545 CRTC parameters to generate NTSC TV timing.
    -Dave

    PET to CGA Video.jpg
    Yes that looks good.

    Hugo.

  6. #66
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    Did some giant steps ahead!

    Resuming: the schematics (8032029 sheet 10) tell that J2 (UC2 7486 pin 5) is connected to GND and J3 (UC2 pin 2) to PULLUP. In my board both jumpers were connected to PULLUP directly by a trace on the board. So I cut the board and put the J2 and J3 jumpers.

    I found that my Samsung MA2671 (an MDA monitor) works fine with J2 to PULLUP, J3 to GND and the 8032 Edit ROM (901474-04) but not with the NTSC edit ROM. Pay attention: J2 and J3 are opposite to schematics!


    WhatsApp Image 2020-05-02 at 21.34.22.jpeg


    This is the solution to my problem: to find a way to replace the broken monitor. I will do some tests; theoretically, it should fit into the 8032 monitor case, that would be fine.


    There's only a little problem using the Dave's PETtester 2 V.4:

    WhatsApp Image 2020-05-02 at 21.32.59.jpeg

    maybe some different video initialization?

    -------------------------------------------

    The PET to VGA through the CGA -> VGA adapter still doesn't work, despite all the tests I did with jumper in every position and both edit ROM (standard and ntsc).

    -------------------------------------------

    I'm waiting for the composite video saga produce some results (I think I will wait for Dave to build the board, to debug it, and to post here the working circuit, LOL).

  7. #67
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    Good work.
    But you may be reading an old schematic. On the latest schematic (Universal 2), both jumpers are tied to separate pull up resistors. By grounding jumper 3, the HSYNC is inverted to positive going which is what the MDA wants. The MDA Horizontal frequency of 18.43 KHz is closest to the 20 KHZ of the standard 8032 EDIT ROM. So that is why it may be working with the standard ROM. But the MDA Vertical frequency is 50 Hz, so there may be some incompatibility.
    See the chart HERE for MDA parameters.
    Last edited by dave_m; May 2nd, 2020 at 05:58 PM.

  8. #68
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    Glad it's working! Yes, as Dave says the MDA is fairly close to the 8032's frequency so it's not surprising that it works with the original edit ROM; if you wanted to use a composite or CGA monitor you'd probably need an NTSC version 'E' ROM.

  9. #69

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    One thing, using the LS244 to drive a cable, rather than putting the small capacitors on the outputs of the '244 like IBM did, a series 15R resistor is a good idea. It doesn't affect the ability to drive the receiving gates high, or low, but it provides a degree of isolation from the cable capacitance to the direct IC output. This increases the stability and reduces any chance of ringing by lowering the Q.

  10. #70
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    Yes, it looks like if you are using a monitor other than the PET you may have to 'tune' the CRTC parameters a bit to the specific EDIT ROM you find that works.

    Dave

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