UNIX System Administration Handbook - Evi Nemeth [79]
SCSI-3 is actually a family of standards. It includes specifications for various physical media, including the traditional parallel buses and high-speed serial media such as Fibre Channel and IEEE 1394 (“FireWire”). It also defines the SCSI command sets and introduces enhancements to support device autoconfiguration, multimedia applications, and new types of devices.
Although the SCSI-3 specification has not yet been finalized, many of its features have already made their way to the marketplace, often under the name “Ultra SCSI.” SCSI-3 encompasses SCSI-2, so a certain degree of backward compatibility is built in. Keep in mind, however, that putting an older device on a newer bus can slow down the entire bus. It will also affect the maximum cable length.
Another feature that is sometimes used in SCSI systems is “differential SCSI.” In normal (“single-ended”) SCSI, every other pin is grounded to help reduce crosstalk among signals.2 This construction limits the cable length to 6 meters for SCSI-1 and 3 meters for SCSI-2. Ultra SCSI reduces the total bus length even further, to 1.5 meters. Differential signalling puts an inverted signal next to each pin instead of a ground, making the net voltage zero and reducing noise significantly.
Differential signalling increases the cable length limit to 25 meters for SCSI-2 and to 12 meters for Ultra SCSI. The extra length is a big win when all of your disks (or tape libraries) are in a distant external cabinet. However, since differential signalling is completely incompatible with nondifferential devices, you must make sure you have a differential controller, disk, cable, and terminator. Label these well to ensure that you do not accidentally mix them up with your single-ended devices, or things will not work at all.
Table 8.1 summarizes the different SCSI versions and their associated bus bandwidths and cable lengths.
Table 8.1 The evolution of SCSI
a. Varies; see the comments in the text below.
b. Wide Ultra SCSI and wide Ultra2 SCSI are sometimes called Fast-20 wide SCSI and Fast-40 wide SCSI, respectively.
c. These versions of SCSI use only differential signalling.
d. HVD is High Voltage Differential and LVD is Low Voltage Differential. HVD is used for the earlier SCSI versions and is not defined above Ultra2 SCSI.
e. Wide Ultra3 SCSI is sometimes called Ultra-160.
The maximum cable length for single-ended Ultra and wide Ultra SCSI depends on the number of devices in use. For 8 devices, 1.5 meters is the maximum; if only 4 devices are used, the bus can be extended to 3 meters. Wide Ultra SCSI only supports all 16 devices in differential mode.
Many types of connectors are used for SCSI devices. They vary, depending on the version of SCSI in use and type of connection: internal or external. Narrow SCSI devices have 50 pins, and wide SCSI devices have 68 pins. Internal devices typically accept a 50-pin header or a 68-pin male mini-micro connector attached to a ribbon cable. External drives usually connect to the computer with a high density 50 or 68-pin mini-micro connector. Apple reduced the 50 pins to 25 by tying all of the ground lines together and shoehorning the bus onto a DB-25 connector.
An interesting variant that’s especially useful for hot-swappable drive arrays is the Single Connector Attachment (SCA) plug. It’s an 80-pin connector that includes the bus connections, power, and SCSI configuration, allowing a single connector to provide all of the drives’ needs.
Exhibit A shows pictures of the most common connectors. Each connector is shown from the front, as if you were about to plug it into your forehead.
Exhibit A Common SCSI connectors (front view, male except where noted)
The connectors on SCSI devices are almost always female, and the ends of SCSI cables