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Thread: 8375 AGNUS for A500 rev6a mod

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    Default 8375 AGNUS for A500 rev6a mod

    Ongoing attempts to revive some of my amigas, I am looking for a 8375 AGNUS chip (318069-16).

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    8 Bit Dreams DjCook's Avatar
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    it is pin uncompatible to 8372, hope you know it
    Retro computer hardware & repair in Germany

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    Quote Originally Posted by unbrain View Post
    Ongoing attempts to revive some of my amigas, I am looking for a 8375 AGNUS chip (318069-16).
    HI!
    I have some used Fat Agnus (318069-16), if you are interested. (Not tested, sorry)

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    Thanks for your reply, I was under the impression that the 318069-16 model number of the 8375 should be pin compatible with the 8372A that was in there from the factory?

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    Quote Originally Posted by unbrain View Post
    Thanks for your reply, I was under the impression that the 318069-16 model number of the 8375 should be pin compatible with the 8372A that was in there from the factory?
    According this page:
    http://www.pmsoft.nl/amiga/amiga-parts-chips.html

    Agnus (2MB) 8375 (390544-xx) is not pin compatible with 8375/8372AB/8372B (318069-xx) !!

    But I just read here:
    http://theamigamuseum.com/the-hardwa...chipset/agnus/

    ++ 318069-16 (PAL) or -17 (NTSC). Vbb replacement to the 8372A. 1 MB support, pin compatible to the 8370/71/72A family.
    Typically found on very late Amiga 2000-B systems (some require a small fix for stable operation with these chips – same when using them as a replacement to an existing 8370/71/72A).

    Them, depends of your A500 revision.
    Last edited by Minter; 17th October 2017 at 14:50. Reason: fix paragraphs

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    Math inside Amibayer! BLTCON0's Avatar
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    @Minter
    Yep, I wrote that entry for theamigamuseum.
    The rework necessary is documented in the amigawiki webpage.

    What puzzles me is the OP mentions a "mod", I suppose he means the 1 MB chip RAM mod, in which case the usual request is for the quite common 8372A (318069-29) and not for the 8375 substitute (318069-16 or -17), let alone that applies to the very few rev6A's which shipped with the earlier 8371 Agnus. The rest already have the 8372A.

    Maybe it's the 2 MB chip RAM mod for the rev6A (which uses either the quite rare 318069-03 Agnus (8372B) or the Vbb substitutes 318069-18 or -19 from the 8375 series) and the OP got the -16 (1 MB version) and -18 (2 MB version) mixed up.

    (*) -16 PAL, -17 NTSC
    (**) -18 PAL, -19 NTSC
    Last edited by BLTCON0; 17th October 2017 at 15:24.

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    Quote Originally Posted by BLTCON0 View Post

    Maybe it's the 2 MB chip RAM mod for the rev6A (which uses either the quite rare 318069-03 Agnus (8372B) or the Vbb substitutes 318069-18 or -19 from the 8375 series) and the OP got the -16 (1 MB version) and -18 (2 MB version) mixed up.

    (*) -16 PAL, -17 NTSC
    (**) -18 PAL, -19 NTSC
    if i got You right: 318069-18 and also 8372B/AB can be placed instread of standard a 8372 giving 2Mb of chipRAM? If so - i am looking for one of these too
    Last edited by DjCook; 17th October 2017 at 17:26.
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    Quote Originally Posted by BLTCON0 View Post
    @Minter
    Yep, I wrote that entry for theamigamuseum.
    The rework necessary is documented in the amigawiki webpage.

    What puzzles me is the OP mentions a "mod", I suppose he means the 1 MB chip RAM mod, in which case the usual request is for the quite common 8372A (318069-29) and not for the 8375 substitute (318069-16 or -17), let alone that applies to the very few rev6A's which shipped with the earlier 8371 Agnus. The rest already have the 8372A.

    Maybe it's the 2 MB chip RAM mod for the rev6A (which uses either the quite rare 318069-03 Agnus (8372B) or the Vbb substitutes 318069-18 or -19 from the 8375 series) and the OP got the -16 (1 MB version) and -18 (2 MB version) mixed up.

    (*) -16 PAL, -17 NTSC
    (**) -18 PAL, -19 NTSC
    Ah, my bad, yes, I'm after doing the 2MB chip conversion for my A500, got the wrong part number, sorry!

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    Math inside Amibayer! BLTCON0's Avatar
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    Quote Originally Posted by DjCook View Post
    if i got You right: 318069-18 and also 8372B/AB can be placed instread of standard a 8372 giving 2Mb of chipRAM? If so - i am looking for one of these too

    8372AB is probably no different from the 8372B silicon - all AB chips seem to have the B printed as an afterthought, which implies it was a batch of "B" versions fitted in "A" packaging and later the designation was "corrected" with the additional "B".


    As to the question, the 8372(A)B and its Vbb 8375 equivalent actually are not drop-in replacements for the 8372A.


    If you only have a single bank of RAM (that is, the default 512KB) then it'll still work, but if you have 1 MB of RAM (onboard + trapdoor) then the extra 512KB will disappear. This new Agnus only has a single RAS line, so it doesn't support two RAM banks.


    So if you wish to install this Agnus on a rev6A A500, it should best be done at the same time with upgrading the RAM to 2 MB. The process is 4-step:


    1. replace Agnus
    2. change jumper blocks
    3. remove the four 256Kx4 chips from existing RAM bank
    4. install four 1Mx4 chips in the other (previously empty) RAM bank


    It must be pointed out again that the whole 2 MB forms a single RAM bank now, so the trapdoor is rendered useless for RAM purposes. You can remove the RAM chips from a standard 512 KB trapdoor expansion however and have it work as an RTC-only card.




    Finally, while technically the requirement is a rev6A board, as it has the necessary provisions (suitable jumper blocks and trace support for 1Mx4 chips), there's a way out with using a rev5 board, but you must get an A501 rev6C trapdoor card, replace the chips with 1Mx4 chips there, change some jumpers on the A501 and a couple on the rev5. That way the onboard 512 KB will be bypassed and the Amiga will use the (now) 2 MB on the A501, naturally as chip RAM.
    Last edited by BLTCON0; 17th October 2017 at 23:21.

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    Quote Originally Posted by BLTCON0 View Post
    Quote Originally Posted by DjCook View Post
    if i got You right: 318069-18 and also 8372B/AB can be placed instread of standard a 8372 giving 2Mb of chipRAM? If so - i am looking for one of these too

    8372AB is probably no different from the 8372B silicon - all AB chips seem to have the B printed as an afterthought, which implies it was a batch of "B" versions fitted in "A" packaging and later the designation was "corrected" with the additional "B".


    As to the question, the 8372(A)B and its Vbb 8375 equivalent actually are not drop-in replacements for the 8372A.


    If you only have a single bank of RAM (that is, the default 512KB) then it'll still work, but if you have 1 MB of RAM (onboard + trapdoor) then the extra 512KB will disappear. This new Agnus only has a single RAS line, so it doesn't support two RAM banks.


    So if you wish to install this Agnus on a rev6A A500, it should best be done at the same time with upgrading the RAM to 2 MB. The process is 4-step:


    1. replace Agnus
    2. change jumper blocks
    3. remove the four 256x4 chips from existing RAM bank
    4. install four 1Mx4 chips in the other (previously empty) RAM bank


    It must be pointed out again that the whole 2 MB forms a single RAM bank now, so the trapdoor is rendered useless for RAM purposes. You can remove the RAM chips from a standard 512 KB trapdoor expansion however and have it work as an RTC-only card.




    Finally, while technically the requirement is a rev6A board, as it has the necessary provisions (suitable jumper blocks and trace support for 1Mx4 chips), there's a way out with using a rev5 board, but you must get an A501 rev6C trapdoor card, replace the chips with 1Mx4 chips there, change some jumpers on the A501 and a couple on the rev5. That way the onboard 512 KB will be bypassed and the Amiga will use the (now) 2 MB on the A501, naturally as chip RAM.
    seems You have a lot experience in it.. Respect!
    i have two amigas here, first is a standard A500 rev.6A and other is A500 with rev.8A board, 8375 Agnus, and (initially) 512kb of RAM. I have added TTL logic and 512kb onBoard, modded the jumpers JP2 & JP7A to add expansion to chipRAM, and now i have 1.5Mb, hope will get 1Mb expansion soon and then it will be 2Mb. My question is same as the topicstarter has:
    how can i reach 1.5 - 2Mb on a standard rev.6A board without purchasing expensive MiniMega? propably, the best way is just to lookin for rev.8A board, and do same upgrades i've done on my machine, cause the price for 8375 Agnus (or miniMega) is pretty high, and You'll end up with same price You A500 rev.8 can get!
    Retro computer hardware & repair in Germany

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