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Old 04 September 2019, 20:24   #1
Sim085
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A500 (rev6a) 1MB chipRAM mod - avoid cutting JP7?

At the moment I am checking about mods to have an A500 rev6a motherboard see 1MB chipRAM instead of 512KB chipRAM and 512KB slowRAM. From what I understand one option is to cut JP2 pad 2 and 3 and solder pad 1 and 2 instead and cut JP7 pad 2 and 3.

However I have found this article which states there are ram cards out there which have jumpers which achieve same results as cutting JP7.

My question is; how does this card (and possibly other cards as well) using a jumper achieve the same result as cutting JP7 on an A500 rev6a motherboard?
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Old 04 September 2019, 21:25   #2
jbilander
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I believe it's like this:

To activate Expansion RAM (slow mem) the EXRAM line (pin 32 on A500 trapdoor connector) needs to be connected to ground since it is active low enable. With the jumper in place you ground the signal. When you remove the jumper or cut the trace (JP7A) the signal is no longer grounded and goes high, the 512k can then be used as chip mem provided JP2 is altered and you have a 1MB Fat Agnus chip installed (e.g. 8372A).

Check pin 32 here: https://old.pinouts.ru/Motherboard/a...r_pinout.shtml

The old A501 expansion boards have a JP1 solder jumper for enabling/disabling the EXRAM signal

The EXRAM jumper has no relevance on a Rev8A A500+ motherboard, since it is treating any expansion ram as chip ram.
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Old 06 September 2019, 07:43   #3
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Quote:
Originally Posted by Sim085 View Post
My question is; how does this card (and possibly other cards as well) using a jumper achieve the same result as cutting JP7 on an A500 rev6a motherboard?
Basically if you cut JP7A, then the trapdoor card cannot ground the EXRAM signal, as the connection for that between the trapdoor and GARY is broken. In the end it is the same thing if you flip the trapdoor card's switch or cut the trace, both methods stop the signal from getting pulled low by the trapdoor card.

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Originally Posted by jbilander View Post
The EXRAM jumper has no relevance on a Rev8A A500+ motherboard, since it is treating any expansion ram as chip ram.
This depends entirely on the configuration of the motherboard's other jumpers. It is possible to reconfigure the Rev 8A board to act as a 512/512 setup. You will need the EXRAM signal in that case for GARY to notify Agnus about the presence of c00000 RAM.
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Old 06 September 2019, 20:57   #4
Sim085
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My card does have a switch on/off. So regardless of what I do to JP7, if I make the JP2 modification then if I let the trapdoor card off it is recognized as chipRAM and if I turn it on it is recognized as slowRAM?

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Originally Posted by Jope View Post
Basically if you cut JP7A, then the trapdoor card cannot ground the EXRAM signal, as the connection for that between the trapdoor and GARY is broken. In the end it is the same thing if you flip the trapdoor card's switch or cut the trace, both methods stop the signal from getting pulled low by the trapdoor card.
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Old 07 September 2019, 13:16   #5
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No, cutting JP7A means the jumper on the expansion card has no function (it is the same line but further down), so don't cut JP7A if you want to control this with a jumper on the expansion card. If you want to be able to easily revert back to 512k Chip + 512k Slow RAM then you can do something like this to JP2 (click and check picture) http://bax.comlab.uni-rostock.de/en/hardware/amiga500/

If you want both 1MB chip ram and slow ram (1,5MB) at the same time then you have Chris' or Pete's expansion ram solutions. It includes getting a gary adaptor.

http://eab.abime.net/showthread.php?t=93130

http://eab.abime.net/showthread.php?t=85395
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