List of B2000 Motherboard Jumpers

			by Dave Haynie


J100	This jumper is used to set the latching signal for the register
	bus to 68000 bus data latches.  It may not exist on the final
X-X X	PC board.


J101	This jumper determines the high-order address bit for Fat Agnus.
	In its normal position, the high-order bit is A23; in its other
 X	position, this bit is A19.  The current Fat Agnus chip requires
 |	the A23 signal for proper management of the memory at $C00000.
 X	Future Fat Agnus chips may map things differently; this keeps
 	the current board compatible with simple enhancements to the
 X	chip set.


J200	This jumper is used to set the light-pen port number.  In the
	position shown, the light pen input will be the FIRE input of
 X	mouse/joystick port 0, as with the A1000.  With the jumper in
 |	the other position, the light pen input will be the FIRE input
 X	of mouse/joystick port 1, which is the scheme used on the A500
 	machine.
 X


J300	This jumper determines the time base used for the 50/60Hz CIA
	timer chip.  In the normal position, the 50/60Hz TICK clock,
X-X X	based on AC line frequency, is used as a time base.  In the
	alternate position, the vertical 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
X X	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 its 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
 X-X	both this extra RAM and the real-time clock.