Section
7.4
List
of
B2000
Motherboard Jumpers
J101 Thisjumper determines the high-order address bit for Fat
3
2
1
Agnus. In
its
normal position shown the high-order bit is
A23: in
its
other position, this bit is A1
9.
The current Fat
Agnus chip requires the A23 signal for proper manage-
ment of the memory at
SC00000. Future Fat Agnus chips
may map things differently.
J200
This jumper is used to set the light-per, port number. In
the normal position shown, the light pen input will be the
FlRE input of mouse/joystick port
1,
as with the A500.
With the jumper in the other position, the light pen input
will be the
FlRE input of mouse/joystick port
0,
which is
the scheme used on the A1 000 machine.
J300
This jumper determines the time base used for the
501
1
2
3
60Hz CIA timer chip. In the normal position, the 5016OHz
W
TICK clock, 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 oper-
ate 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
X
X
box.
J500
This jumper is used to enable the 51 2K of
RAM
at
SC00000.
It
is normally closed; opening
it
will disable this
X-X
extra RAM.
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