External SCSI devices usually have a device number switch on the rear of the
case, equipped with small "+" and "-" buttons. Using a pin, press the buttons
to increase or decrease the device number.
Internal SCSI devices will require that you manipulate jumpers or DIP switches
to set the SCSI ID#. If you're lucky, a small chart will appear on the case
of your internally mounted SCSI device, illustrating how to set the jumpers
or DIP switch for each SCSI ID#.
If your internal SCSI device doesn't have such a chart, you can still figure
out how to set the SCSI ID#. Internally mounted SCSI devices usually have
three DIP switches or pairs of jumper pins in a row (four pairs of jumper pins
for "Wide" SCSI devices). Figuring out how to set the SCSI ID# for such
devices is a matter of simple addition.
Each switch (or pair of pins) has a numeric value associated with it. From
left to right, these values are 1, 2, and 4, (and 8 for SCSI-3 devices only).
If the switch is on (or the pair of pins has a jumper attached), its value is
added to the total. If a switch is off or there is no jumper on a pair of
pins, its value is not added (counts as zero). Add these numbers together,
and the result is the SCSI ID.
Most SCSI controllers support eight (really seven, as the controller itself
is the eighth) devices, each device with a corresponding SCSI ID# number of
0-6 (SCSI ID# 0 is device number one and the controller is SCSI ID# 7,
so 1+2+4=7).
Note: Each SCSI device must be assigned its own unique ID. For example,
it would not be possible to set two SCSI devices on the chain to ID 2.
These devices would conflict and not function. The only way for this
to work is if each device is attached to its own SCSI controller card.
EXAMPLE 1:
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To set the SCSI ID# to 0, leave all jumpers uncovered or set all DIP
switches to OFF (0 + 0 + 0 = SCSI ID# 0).
To set the SCSI ID# to 1, cover the first pair of pins with a jumper block
or set the first DIP switch to ON.
Leave the remaining jumpers or switches OFF:
__________________________________________________
o o o
DIP switches -> | 1 + 0 + 0 = ID 1
o o o
values -> 1 2 4
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Only the first switch is on. Because this switch is on, its value (1) is
added to the total. The second switch is off, so its value is not added.
This switch counts as 0. The third switch is also off, so its value counts
as 0. Added together, this is 1 + 0 + 0 = 1. The SCSI ID is therefore 1.
EXAMPLE 2:
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To set the SCSI ID# to 5:
__________________________________________________
o o o
DIP switches -> | | 1 + 0 + 4 = ID 5
o o o
values -> 1 2 4
__________________________________________________
The first switch is on. Because this switch is on, its value (1) is added
to the total.
The second switch is off, so its value is not added. This switch counts
as 0.
The third switch is on. Because this switch is on, its value (4) is added
to the total.
Added together, this is 1 + 0 + 4 = 5.
The SCSI ID is therefore 5.
For other SCSI IDs, just close the appropriate pins or switches to add up
the number you want.
While it is possible to set the unit to SCSI ID 7 by closing all three
jumpers, this is not recommended because the SCSI Controller itself will
almost always be factory preset to SCSI ID# 7. Setting a SCSI device to use
the same SCSI ID# as the SCSI controller would cause the entire SCSI chain
to malfunction.
For those of you who don't want to do the arithmetic, here is
a handy chart showing the settings for all SCSI IDs:
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ID 0 ID 1 ID 2 ID 3
o o o o o o o o o o o o
| | | |
o o o o o o o o o o o o
_______________________________________________________________
ID 4 ID 5 ID 6 ID 7
o o o o o o o o o o o o
| | | | | | | |
o o o o o o o o o o o o
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