This could be your answer :idea:
The Hardware database has this about the 2000 jumpers :rtfm:
Jumpers
These setting come out of the manual for an Amiga 2000 Revision 4.x and is compliant with Rev 6.x Amiga 2000 boards as well.
J101 This jumper determines the high order address bit for Fat Agnus. In it's normal position, the high-order bit is A23; in it's other position, this bit is A19. The current Fat Agnus chip requires the A23 signal for proper management of the memory at $C00000. Future Fat Agnus chips may do things differently; this keeps the current board compatible with simple enhancements.
J102 This jumper if available configures the A2000 for NTSC, or PAL operation. Agnus 8370, 8372A, 8372AB for example, support switching between NTSC and PAL. Rev 6.4 motherboards and possibly later revisions come with an 8375 Agnus by default which does not support switching. It was made in in separate versions for PAL and NTSC
J200 This jumper determines the time base used for the 50/60Hz CIA timer chip. In the normal posistion, the 50/60 TICK clock, based on AC line frequency, is used as a time base. In the alternate position, the verticle sync pulse from the video section is used. The system will not operate properly without one of these clocks.
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 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.
Dave G 8-)
