Seagate Ultra 320 User Manual download pdf (Page 31)

Parallel SCSI Interface Product Manual, Rev. A ) 17

When information unit transfers are enabled, the P1 signal shall be continuously negated by the SCSI device

driving the DB(15-0) signals and shall be ignored by the SCSI device receiving the DB(15-0) signals during DT

DATA phases.

During DT DATA phases when information unit transfers and paced transfers are enabled the P1 signal shall

be sourced by the SCSI device driving the DATA BUS. The P1 signal is used to indicate the data valid or data

invalid state during paced transfers.

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. 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 connector) 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.

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Seagate Ultra 320 User Manual Page 31