Storage Interfaces

Termination

An electrical signal traveling along the SCSI chain will eventually hit the physical end of the SCSI cable. Unless the signal is terminated at this point, it has the potential to reflect back up the SCSI chain and interfere with other signals being transmitted down the chain, causing inconsistent performance of the SCSI bus and possible data loss. To prevent this, a SCSI chain must have terminators at both ends of the cable.

Most modern SCSI controllers will terminate the end of the cable where the chain begins. Either a device with onboard termination or a stand-alone terminator will have to terminate the other end of the SCSI chain. There are several different types of termination:

Passive - Simple resistors are used for termination. This method is recommended only for low performance SCSI implementations, and should NOT be used on a SCSI bus attached to any high speed SCSI devices.
Active - Voltage regulators are added to the resistors found in passive terminators, adding reliability and increasing performance. Active termination is generally used for SE SCSI.
Forced Perfect - Diodes are used to force the termination to the correct voltage. Forced perfect termination was a standard that was never widely used and has been phased out.
Differential - HVD devices require differential termination. There is no active or passive variant; differential is the only option for HVD devices.
LVD - LVD terminators are required for LVD devices, and are backwards compatible with SE implementations. LVD is currently the prevalent method of termination due to the popularity of LVD SCSI devices.

Each SCSI device has a jumper that provides termination power. Most often, the devices provide power for their own terminators, but if they don't, the TERMPWR provided by the host adapter will be used.

SCSI Bus Width

The SCSI bus width refers to the amount of data the SCSI bus can accommodate. There are two commonly used widths: narrow and wide. Narrow SCSI uses data pathways that are 8 bits wide, while wide SCSI uses data pathways that are 16 bits wide.

Wide SCSI has become more popular than narrow SCSI for several reasons. It can have a maximum of 16 devices connected to the SCSI bus while narrow SCSI can only have 8 devices. The use of wide SCSI also allows for higher throughputs of up to 320MB/sec compared to narrow SCSI which is limited to a maximum of 40MB/sec.

It is possible to mix wide and narrow devices. The optimal way to mix narrow and wide devices would be to use a multichannel SCSI controller and place the wide devices on one channel and the narrow devices on the other. If narrow and wide devices must be placed on the same channel, several issues must be addressed.

Physical Connection

SCSI cables are selected to match the SCSI controller present in a system, and generally have 68-pin (wide) or 50-pin (narrow) connectors. Any device that uses a different pin type than the controller will need an adapter to physically attach to the SCSI chain. For example, a narrow CDROM drive that has a 50-pin connector will need a 68-pin to 50-pin adapter before it may physically be attached to a 68-pin wide SCSI cable.

Termination

When wide and narrow devices are connected on the same SCSI bus, there is increased possibility of signal data reflecting on the bus and causing instability and data corruption. The "extra" 8 bits of signal traffic that occurs when 16-bit to 8-bit communication takes place must be eliminated. This is called High Byte Termination and is performed by wide to narrow adapters.

Performance

Narrow controllers are limited to a maximum throughput of 40MB/sec. Any wide SCSI devices connected to a narrow controller will suffer reduced performance as a result of this limitation. Placing a narrow device on a wide controller does not allow the device to perform at greater speed than it would have on a narrow chain- narrow devices are limited to 8-bit transfers at all times.