A2000 jumpers question

[melott]

I bought an A2000, rev 4.5 MoBo.
This computer has a 2 position toggle switch wired
between jumpers J302 and J500. The J302 is a 3 post
jumper and J500 is a 2 post. There are 3 wires going
to the J302 and 2 wires going to the J500.

Anyone have any idea what this switch is supposed to do ?

[Chain]

J301
This jumper is closed to add a second internal floppy drive, open to leave the second floppy out of the main unit box. The Amiga expects an ID bit stream from each floppy drive; this lets it determine the drive type. External floppies have this ID circuitry on board, but as it`s not an industry standard capability, it has to be implemented on the A2000 motherboard to save cost on internal drives. Leaving the jumper open prevents the Amiga from seeing the ID sequence.


J500
    This jumper is used to enable the 512k of (SLOW) RAM at $C00000 and the real-time clock. It is normally closed; opening it will disable both this extra RAM and the real-time clock.