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Thread: ROM binaries for a PET 2001 clone

  1. #31
    Join Date
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    Quote Originally Posted by KC9UDX View Post
    Yes.

    But my original prognosis may be correct (I don't recall how I arrived at it): that there's corrupt data in the PETVet that I have. Only BASIC 4 worked, which was disappointing; I had hoped to be able to run the earlier versions.
    You are talking about a 2001(N) series computer, i.e. one without a CRTC, correct?

  2. #32

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    Quote Originally Posted by GK2001 View Post
    Incidentally, the video system vertical blanking signal fed to CB1 ( pin 18 ) of PIA#1 is the systems frame-rate interrupt line and is also fed to PB5 of the VIA chip to serve as the jiffy clock signal.

    Hmm, I read that somewhere (can't remember where), but some experimenting has revealed that the 1/60s "jiffy" counter and RTC clock (BASIC: TI, TIME$) is derived from PIA#1 and not the VIA.

    Living in a 50Hz/PAL-timing land I designed the video circuitry of my clone to operate at a 50Hz frame rate. AFAIK the PET 2001 motherboard was never implemented with 50Hz video circuitry? The PIA-based interrupt syncs the display update to the vertical blanking period in certain modes to give "snow"-free screen writing. This is only a little bit annoying as I was hoping that the jiffy clock was VIA based, as that would have allowed me to feed a separate 60Hz clock to the VIA without modifying the PIA interrupt.

    However now I'm just going to have provide an option (switch-selectable) of clocking the PIA/VIA from either the vertical blanking signal as standard, or from 60Hz clock source. In the latter option the jiffy clock and RTC will keep accurate time, but the snow-free "slow" mode(s) of video RAM writing will loose their lack of snow was the CPU interrupt wont be in sync with the video frame rate.

    What purpose the vertical blanking feed to the VIA PB5 input serves I currently don't know. Anyone out there have an idea?

    I've discovered another quirk with BASIC#3 - my clone has a "turbo" mode where the system/CPU master clock can be toggled (synchronously) at will whilst the computer is running between 1MHz (the original PET clock rate) and 4MHz. BASIC#3 reliably boots every from initial power-up time in turbo mode.

  3. #33

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    Quote Originally Posted by GK2001 View Post
    Incidentally, the video system vertical blanking signal fed to CB1 ( pin 18 ) of PIA#1 is the systems frame-rate interrupt line and is also fed to PB5 of the VIA chip to serve as the jiffy clock signal.

    Hmm, I read that somewhere (can't remember where), but some experimenting has revealed that the 1/60s "jiffy" counter and RTC clock (BASIC: TI, TIME$) is actually derived from PIA#1 and not the VIA. TI and TIME$ continue to operate when the VIA PB5 input is disconnected.

    Living in a 50Hz/PAL-timing land I designed the video circuitry of my clone to operate at a 50Hz frame rate. AFAIK the PET 2001 motherboard was never implemented with 50Hz video circuitry? The PIA-based interrupt syncs the display update to the vertical blanking period in certain modes to give "snow"-free screen writing. This is only a little bit annoying as I was hoping that the jiffy clock was VIA based, as that would have allowed me to feed a separate 60Hz clock to the VIA without modifying the PIA interrupt.

    However now I'm just going to have provide an option (switch-selectable) of clocking the PIA/VIA from either the vertical blanking signal as standard, or from 60Hz clock source. In the latter option the jiffy clock and RTC will keep accurate time, but the snow-free "slow" mode(s) of video RAM writing will loose their lack of snow was the CPU interrupt wont be in sync with the video frame rate.

    What purpose the vertical blanking feed to the VIA PB5 input serves I currently don't know. Anyone out there have an idea?

    I've discovered another quirk with BASIC#3 - my clone has a "turbo" mode where the system/CPU master clock can be toggled (synchronously) at will whilst the computer is running between 1MHz (the original PET clock rate) and 4MHz. BASIC#3 reliably boots every from initial power-up time in turbo mode.

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