Compaq AA-RHGWB-TE manual Scsi Bus Device Identification Numbers, Data Path, Scsi Bus Speeds

connect devices that use different transmission methods, use a SCSI signal converter between the devices. The DS-BA35X-DA personality module is discussed in Section 9.1.2.2. See Section 9.1 for information about using the DWZZ* series of SCSI signal converters.

You cannot use a DWZZA or DWZZB signal converter at UltraSCSI speeds for TruCluster Server if there are any UltraSCSI disks on the bus, because the DWZZA or DWZZB will not operate correctly at UltraSCSI speed.

The DS-BA35X-DA personality module contains a signal converter for the UltraSCSI BA356. It is the interface between the shared differential UltraSCSI bus and the UltraSCSI BA356 internal single-ended SCSI bus.

RAID array controller subsystems provide the function of a signal converter, accepting the differential input and driving the single-ended device buses.

3.2.3 Data Path

There are two data paths for SCSI devices:

•Narrow — Implies an 8-bit data path for SCSI-2. The performance of this mode is limited.

•Wide — Implies a 16-bit data path for SCSI-2 or UltraSCSI. This mode increases the amount of data that is transferred in parallel on the bus.

3.2.4Bus Speed

Bus speeds vary depending upon the bus clocking rate and bus width, as shown in Table 3–1.

Table 3–1: SCSI Bus Speeds

SCSI Bus	Transfer Rate (MHz) Bus Width in Bytes		Bus Bandwidth
			(Speed) MB/sec
SCSI	5	1	5
Fast SCSI	10	1	10
Fast-Wide	10	2	20
UltraSCSI	20	2	40

3.3 SCSI Bus Device Identification Numbers

On a shared SCSI bus, each SCSI device uses a device address and must have a unique SCSI ID (from 0 to 15). For example, each SCSI bus adapter and each disk in a single-ended storage shelf uses a device address.

Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware 3–5