Small Computer System Interface (SCSI)

Definition of Small Computer System Interface (SCSI) in The Network Encyclopedia.

What is SCSI (Small Computer System Interface)?

A hardware bus specification for connecting peripherals to a computer using a parallel transmission interface. The Small Computer System Interface (SCSI) was developed by Apple and is widely used in the PC world for high-end storage solutions. Microsoft Cluster Server (MSCS) uses a shared SCSI bus to provide failover support for two computers on which Microsoft Windows NT Server, Enterprise Edition is installed.

How It Works

To implement SCSI on a system, you use a SCSI adapter to interface with the system bus, suitable SCSI devices such as SCSI hard drives, SCSI cables to daisy-chain the devices, and SCSI terminators for the ends of the bus. Each device on a SCSI bus must have a SCSI device ID number assigned to it, allowing SCSI to be used for daisy-chaining a number of devices together on a single parallel bus. You can change SCSI IDs by using dip switches or jumpers, or by using configuration software.

SCSI devices come in two basic types:

  • Single-ended devices:
    Use one data lead and one ground lead to establish single-ended signal transmission over the bus. This type of device is more prone to the effects of noise and is less forgiving of cable lengths beyond specifications.

     

  • Differential devices:
    Use two data leads, neither of which are at ground potential. These devices are generally more expensive but are resistant to the effects of noise and can often function over distances that exceed the SCSI specifications.

     

The SCSI interface comes in several varieties, including the following:

  • SCSI-1:
    The original 1986 SCSI standard that supports transmission rates of 5 Mbps over an 8-bit bus for up to seven daisy-chained devices. SCSI-1 cables typically use Centronics 50 or Telco 50 connectors. The chained bus length must not exceed 6 meters (20 feet).

     

  • SCSI-2:
    Sometimes referred to as Plain SCSI, which is a common SCSI standard that supports transmission rates of 5 Mbps over an 8-bit bus for up to seven daisy-chained devices. SCSI-2 cables typically use Micro DB50 connectors. Fast SCSI is a variant that supports 10-Mbps transmission rates, while Fast Wide SCSI uses a 16-bit bus and supports 20-Mbps rates. The chained bus length must not exceed 6 meters (20 feet) for regular SCSI-2, and 3 meters (10 feet) for Fast SCSI.

     

  • SCSI-3:
    Also called Ultra SCSI, which is a SCSI standard that supports transmission rates of 20 to 40 Mbps over an 8-bit or 16-bit bus for up to 15 daisy-chained devices. SCSI-3 cables typically use MicroD 68-pin or Mini 68 connectors.

     

  • SCSI-5:
    Also called Very High Density Connector Interface (VHDCI), which is a SCSI standard similar to SCSI-3 but uses a smaller 0.8 millimeter connector. SCSI-5 is designed for next-generation SCSI connections where high performance is a key requirement. Manufacturers, such as IBM® and Hewlett-Packard®, are integrating this new 0.8-mm connector design in their controller cards. It’s the connector of choice for advanced SCSI multiport applications, such as Ultra SCSI Fast-20 and the new Low-Voltage Differential Signal (LVDS) technology.

    Graphic S-15. Four varieties of SCSI interface.

NOTE

Eight-bit SCSI data paths are referred to as “narrow” paths, and 16-bit data paths are called “wide” paths.

TIP

SCSI cables must always be properly terminated in order for devices to be properly recognized; they should also use high-quality active terminators. Diagnostic terminators that help identify problems in signal quality are also available.

Always use the shortest cable possible for SCSI connections, because longer cables cause signals to weaken and are more affected by noise due to electromagnetic interference (EMI). When you calculate the total length of the SCSI bus, add the lengths of all the SCSI cable segments plus any internal SCSI cabling.

Be sure that all devices on a SCSI bus are configured for either single-ended or differential transmission - do not mix these methods on a single bus. You can connect single-ended devices to differential transmission devices only by using a signal converter. If you don’t use a signal converter, your SCSI devices might be damaged by unexpected voltages.

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