The Three Buses
There are three types of parallel copper SCSI buses: single-ended (SE), dif-ferential SCSI (now called high-voltage differential, or HVD) and low voltage differential (LVD). Each defines its own wire assignments, so they are completely incompatible with each other.
Single-Ended is the oldest and least expensive version. It is the most prone to "noise", or EM interference (EMI), and signal quality problems. Because the signal levels are so low, cable quality must be high and cable lengths carefully controlled. (Single-ended SCSI is often referred to as "Normal SCSI".) Most Ultra Wide and earlier SCSI devices are single ended. A single-ended bus generally cannot transfer data faster than 40 MB/sec using a 16-bit data path. Cable lengths for single-ended SCSI busses vary from 1.5 me-ters (Ultra SCSI) to a maximum of 3 meters (Fast SCSI-2). Single-ended devices are generally less expensive than differential SCSI devices, but they can provide adequate performance in servers that do not have large numbers of users or that do not require high-speed data transfer.
With each successive generation SCSI, performance has doubled. However, there has been a price to pay for the performance increases. While overall data transfer rates were increased, the total available bus length (SCSI con-troller+ all cables+ devices) has decreased. This limitation can be a problem for users who want to connect a large number of devices to a single SCSI bus or to locate an array enclosure at a distance from the host system.
Differential SCSI96 was developed to provide better noise immunity and sig-nal quality when using longer cables. The specification calls for a maximum cable length of 25 meters.
High Voltage Differential (HVD)97 was defined in the SCSI-298 standard be-cause servers needed a higher quality of signal level over a distance in order to locate storage systems in practical locations. Differential signaling ex-tends the SCSI bus by transmitting two signals using a pair of wires. Because of the signal differences, single-ended devices are not compatible with HVD signals, and single-ended devices cannot be attached to an HVD bus. Placing single-ended devices on an HVD bus may result in damage to the device. Since the adoption of the Ultra2 LVD specification, HVD devices are not generally available.
Low Voltage Differential (LVD)99 is a feature defined by the Ultra2 SCSI standard. LVD uses differential signaling, but does so with a lower voltage (5V HVD vs. 3.3V LVD). Differential signaling allows an LVD bus to be up to 12.5 meters in length. The bus can be up to 25 meters if only a single de-vice and controller are on the bus (point-to-point connection). Another advantage of LVD is that it uses a lower voltage and a separate logic circuit that can detect the type of device. Single-ended devices can be connected to an LVD bus without damage to their onboard SCSI circuits.
Currently, most of the new high-performance SCSI hard drives available use an LVD interface. Any disk drive that supports Ultra2 or Ultra160 SCSI is an LVD drive. However, when a non-LVD device is attached to an LVD SCSI bus, the LVD controller reverts to single-ended mode. This will reduce the clock rate to 20 MHz and the bus is limited to 1.5 meters with no more than 8 devices.
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