What does JP3 do on a 500+ 8A1 motherboard?

Astral

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AmiBayer
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I want to modify my 500+ that currently has trapdoor 1 mb chip expansion fitted to be able to have 3 modes: 512k chip only / 1mb chip only / 2mb of chip.

JP2 allows for 512k or 1mb of chip on the motherboard. JP7A enables or disables the trapdoor expansion. But I don't know what JP3 is for, exactly what it does, and how to configure. So, what exactly does it do, and how? And why is it necessary if the other two jumpers take care of 512k/1mb motherboard switching and trapdoor enabling/disabling?

Or alternatively, how exactly do I setup this 3 mode mod?

I have searched for all kinds of references and ideas but none the wiser.
 

Pinback

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afair with JP3 you set the location of your second 512kb, either trapdoor or on the pcb. can be wrong.
 
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