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Thread: Amiga 2000 - Board Rev and Jumper Settings

  1. #1
    BigRed's Avatar
    Join Date
    Jan 2019

    Default Amiga 2000 - Board Rev and Jumper Settings

    Hey Folks,

    I got my hand on a Amiga 2000, which is a nice addition to my collection since I "only" own A500s & an A1200 so far.
    Had it running for a couple of minutes to test, went fine so far and showing 1Mb of Chipmem
    I stripped the system today and brushed of the dust and had a quick look around.

    The previous owner gave no Information beside a photo that it's a Rev. 6 Board. I looked up the "Ladies" Versions:
    Agnus: 8372A
    Paula: 8364 R7 - 252127-02
    Denise: 8373R3 - 390433-01

    which seems to me that I got an ECS Machine, and thus possibly a Rev 6.5 Board.

    J101 is closed on 2-3, which I gather is correct for ECS/1MB Chipmem?
    J500 is open in my board, but according to BBoAH this is normally closed. Or does this refer to an earlier Rev?
    J200 is closed on 2-3, which does "whatever" to the tick signal. Should I change this to 1-2?
    J300 is closed on 1-2
    J301 is open since I have two floppy drives

    Can anyone shed more light on the jumpers for me? And maybe on the ECS/Rev thing?
    Would be much appreciated


  2. #2
    Math inside Amibayer! BLTCON0's Avatar
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    May 2011
    Hellas (Greece)
    Chania, Crete
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    Rev 6.5 is probably the Vbb-ready revision to directly support the Vbb 1 MB Agnus (318069-16 PAL , 318069-17 NTSC, these belong to the 8375 series despite being 1 MB units and almost pin compatible to the 8372A). Rev 6.4 and older ones required some toying around. Some 6.4s didn't have the necessary adjustments yet had the Vbb Agnus.
    ECS A2000s also normally came with a 2.04 ROM IIRC.

    J101 and J500 work in tandem to determine the chip/slow RAM thing for the upper 512 KB

    J101 decides where the upper 512 KB is mapped.
    1-2 maps it to $C00000 (ranger RAM address space)
    2-3 maps it to $080000 (chip RAM address space, provided a 1 MB capable Agnus is installed)

    J500 controls the _EXRAM signal for Gary
    closed = _EXRAM grounded (active) so Gary knows ranger RAM exists at $C00000. Therefore this setting only makes sense with J101 set at 1-2
    open = _EXRAM pulled high (inactive) so Gary considers nothing happens at $C00000. This therefore is suitable for 1 MB chip RAM and J101 at 2-3

    The J101 2-3 / J500 open was the default for 1 MB chip RAM configured A2000s, this was probably the "full ECS" A2000s, i.e. only the very latest ones.
    Some 6.4 boards came with the Vbb Agnus and required a small mod to work correctly with it.

    J200 is not for the tick signal, it's for Light Pen triggering (BBOAH has it wrong).
    The Amiga 1000 took the signal from the Joy0 (mouse) port.
    On the A500 it was taken from the Joy1 (joystick) port. So the mouse didn't have to be removed.
    On the A2000 (at least US version) it was made selectable via J200 (also on later A500s via JP
    1-2 = A1000 way, trigger _LPEN via firebutton on mouse port
    2-3 = A500 way, trigger _LPEN via firebutton on joystick port (default for A2000)

    J300 is for the TICK timing signal for the CIA chips. TICK should count at 50 or 60 Hz, depending on whether it's a PAL or NTSC oriented Amiga.
    Since 50 or 60 Hz is also typically the power line frequency in PAL or NTSC areas respectively, it made sense to take the frequency directly from the powerline (via a suitable PSU) instead of _VSYNC (which was derived from the oscillator) to avoid some interference. This was especially true in the noisy A1000 which only used that option as a TICK signal source.
    On the A500 the PSU didn't provide any TICK signal, it was necessarily taken from Agnus's _VSYNC.
    On the A2000 the source can be either the PSU (the original A2000 PSU still provides a TICK signal) or _VSYNC from Agnus, selectable via J300
    1-2 = A1000 way, use TICK from PSU (default)
    2-3 = A500 way, use _VSYNC from Agnus as TICK

    So, say for example you are in the US and have a US A2000 but have a PAL-configured board in order to play PAL games etc. J300 gives you the flexibility to bypass the PSU TICK (which still counts at 60 Hz, incorrect for PAL) and feed a 50 Hz timer to the CIA.
    Or you can replace the original A2000 PSU with a standard one which doesn't provide a TICK signal, just set J300 to 2-3 and you're set.

    J301 is not a 2nd-floppy enable/disable jumper. Its logic is somewhat different due to the way the Amiga handles disk drives.
    Closed = the onboard circuitry on the A2000 will provide a standard drive ID on behalf of the 2nd drive (even if no 2nd drive is installed!). This allows a standard 3.5" DD drive to be used.
    Open = the onboard 3.5" DD drive ID circuitry is disconnected. This should be used when no 2nd drive is connected *OR* when a special 2nd drive which can provide an ID on its own is connected, such as the Amiga-specific 3.5" HD drives that rotate at 150 rpm in HD mode.

    J102 is not for PAL/NTSC selection as is commonly stated, rather for PAL/NTSC configuration with the appropriate 8372A Agnus (or the 8372B/AB Agnus in cased of the onboard 2 MB chip RAM hack).
    closed = "NTSC" = the 8372A Agnus assumes an NTSC oscillator is installed and adjusts the way it counts to provide NTSC-accurate timings.
    open = "PAL" = the 8372A Agnus assumes a PAL oscillator is installed and adjusts the way it counts to provide PAL-accurate timings.

    In mix-and-match configurations timings will be 'nearly PAL' or 'nearly NTSC' according to what J102 says, still close enough to regard J102 as a PAL/NTSC selection jumper.
    Last edited by BLTCON0; 31st January 2019 at 00:29.

  3. #3
    BigRed's Avatar
    Join Date
    Jan 2019


    Thank you very much for the (very, very, very) detailed explanation!

    I left all the jumpers as they were, and the machine works fine


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