PC Hardware Help
SCSI Hard Drive Interface

SCSI (Small Computer System Interface) Interface

SCSI is a parallel interface standard for attaching peripheral devices to a PC. SCSI provides transfer rates up to 80 MB per second and depending on the standard the SCSI bus can contain up to 15 devices. Due to the lack of a single SCSI standard, some devices may not work with some SCSI boards.

The following are types of SCSI in use today:

  • SCSI-1

    Uses 8-bit bus, supports data rates up to 4 MB per second and uses a 25-pin connector.

  • SCSI-2

    Uses a 50-pin connector instead of a 25-pin connector and supports multiple devices. Most common SCSI type.

  • Wide SCSI

    Uses a wide cable to support 16-bit transfers.

  • Fast SCSI

    Doubles the clock rate to support data rates up to 10 MB per second.

  • Fast Wide SCSI

    Uses 16-bit bus and supports data rates up to 20 MB per second.

  • Ultra SCSI

    Uses 8-bit bus and supports data rates up to 20 MB per second.

  • SCSI-3 (Ultra Wide SCSI)

    Uses 16-bit bus and supports data rates up to 40 MB per second.

  • Ultra2 SCSI

    Uses 8-bit bus and supports data rates up to 40 MB per second.

  • Wide Ultra2 SCSI

    Uses 16-bit bus and supports data rates up to 80 MB per second.

SCSI Cables

Their are two types of SCSI cables. Differential and single-ended. Both use a pair of twisted wires to send a signal. In a differential cable, both wires carry voltage and the signal is the difference between these two voltages. In a single-ended cable one wire is used for voltage and the other for ground. A differential cable is more expensive than a single-ended cable, but can also be of longer lengths and has better data integrity. Differential also sends an extra verifying signal for each bit. This makes them more reliable and reduces the chances of corrupt data.