How SCSI Works, Types Of SCSI Bus Systems

How SCSI Works

To understand SCSI hard drive technology, think of it as a SCSI bus system, a closed bus system that can include several devices, as well as the host adapter that acts as the bridge to the system bus. The figure below illustrates the SCSI as a bus concept:

On the left side, the CPU communicates with the hard drive controller that is contained in the hard drive case through the system bus. On the right side, the CPU communicates over the system bus to the SCSI host adapter, which communicates over the SCSI bus to the SCSI adapter in the hard drive case.

The SCSI adapter in the hard drive case communicates to the hard drive controller, which, in turn, communicates with the hard drive. Some SCSI devices, including hard drives, have the SCSI host adapter built directly into the device.

SCSI hard drives require a simple adapter card to communicate with the system bus, much like regular IDE drives do. These devices are called embedded SCSI devices and, because the host adapter technology resides on the drive logic board, can only have the one device on the SCSI bus.

Other SCSI devices on the computer are altogether separate from this SCSI bus system. Embedded SCSI devices often don’t conform to standard SCSI specifications, because they don’t accommodate any other SCSI device. Embedded SCSI devices are the only devices on a SCSI system.

Types Of SCSI Bus Systems

SCSI standards have improved over the years and use different names. The two general categories of all SCSI standards used on PCs have to do with the number of bits that travel on the SCSI bus, either 8 bits (narrow SCSI) or 16 bits (wide SCSI). In almost every case, if the SCSI standard is 16 bits, then the word “wide” is in the name for the standard. In most cases, the word “narrow” is not mentioned in names for 8-bit standards. Narrow SCSI uses a 50-pin cable, and wide SCSI uses a 68-pin cable.

A SCSI cable can be built in two different ways, depending on the method by which the electrical signal travels on the cable: single-ended and differential. A single-ended cable is less expensive than a differential cable, but the maximum cable length cannot be as long because data integrity is not as great. Cables for both narrow SCSI and wide SCSI can be either single-ended or differential. Single-ended cables and differential cables look the same, so you must make sure that you are using the correct cable. Single-ended cable is more popular than differential because it is less expensive.

The table below summarizes the different SCSI standards, including the three major standards: SCSI-1, SCSI-2 and SCSI-3 that are more commonly known as Regular SCSI, Fast SCSI, and Ultra SCSI, respectively. Other names used in the industry for these standards are also listed in the table. Both Fast SCSI and Ultra SCSI have narrow and wide versions.

Since there are many variations of the SCSI bus, make sure that a new SCSI device is compatible with an existing SCSI bus.

If the SCSI device is not compatible, you will have to buy a new host adapter card for the device. More than one SCSI bus system with its individual host adapters can be on the same computer, but this is more costly since you have to buy a host adapter for each SCSI system.

The wide SCSI specification allows for a data path of 32 bits, although this has not been implemented in PCs. When you see a SCSI device referred to as wide, you can assume 16 bits.