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16
DB(7–0) (8-bit data bus)—Each data bit that forms the 8-bit data bus. Bit significance and priority during arbi-
tration are shown in Table 1.
DB(15–0) (16-bit data bus)—16 data bit signals that form the 16-bit Data Bus. Bit significance and priority dur-
ing arbitration are shown in Table 1.
Greater detail on each of the SCSI bus signals is found in the following sections.
2.1.1
Drive select
For SCSI ID selection, install drive select jumpers as shown in configuration selection figure in the individual
drive's Product Manual, Volume 1. Refer to the "Physical interface" section of the individual drive's Product
Manual for the location of the drive select header. Drives using the 8-bit data interface can have one of eight ID
bits selected by installing 0 through 2 (3) jumpers in a binary coded configuration on the drive select header.
Drives using the 16-bit data interface can have one of 16 ID bits selected by installing 0 through 3 (4) jumpers
in a binary coded configuration on the drive select header. "LC" model drives (80-pin direct connect I/O con-
nector) can be assigned their bus ID over the SCSI interface.
2.1.2
Signal values
Signals may assume true or false values. There are two methods of driving these signals. In both cases, the
signal shall be actively driven true, or asserted. In the case of OR-tied drivers, the driver does not drive the sig-
nal to the false state, rather the bias circuitry of the bus terminators pulls the signal false whenever it is
released by the drivers at every SCSI device. If any driver is asserted, then the signal is true. In the case of
non-OR-tied drivers, the signal may be negated. Negated means that the signal may be actively driven false, or
may be simply released (in which case the bias circuitry pulls it false), at the option of the implementor.
2.2
Signal states
2.2.1
SE signals
Signals may be in a true (asserted) or false (negated) state. Signals that are asserted are actively driven to the
true state. Signals that are negated may either be actively driven to the false state or released to the false state.
A signal that is released goes to the false state because the bias of the terminator pulls the signal false. OR-
tied signals shall not be actively driven false.
Note.
The advantage of actively negating signals false during information transfer is that the noise margin is
higher than if the signal is simply released. This facilitates reliable data transfer at high transfer rates.
Bits of the data bus are defined as one when the signal is true and defined as zero when the signal is false.
2.2.2
LVD signals
Figure 3 defines the voltage and current definitions. A signal that is released goes to the false state because
the bias of the terminator pulls the signal false.
SCSI Interface Product Manual, Rev. B
– SIGNAL
V s
I +
+ SIGNAL
V +
– TRANSCEIVER
I –
V –
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