SCSI adapters and SCSI controllers (SCSI cards) are computer interface cards that are installed in an expansion slot. They are used to connect the SCSI system to several devices and peripherals using a daisy chain method. Small Computer Systems Interface (SCSI) is an intelligent I/O parallel peripheral bus with a standard, device-independent protocol that allows many peripheral devices to be connected to the SCSI port. A single SCSI bus can drive up to eight devices or units: the host adapter or controller, and seven other devices. Each device is assigned a different SCSI ID, ranging from 0 to 7.  Bus or interface type, SCSI standards, computer platform, termination and other specifications are important to consider when selecting SCSI adapters and SCIS controllers.

Bus and Interface Types

Choices for bus or interface type for SCSI adapters and SCSI controllers include ISA or EISA, MCA, PCI, PC/104 (PC/104-Plus, EBX, ETX), parallel interface, PCMCI Type I card, PCMCIA Type II card, PCMCIA Type III card, and PCMCIA CardBus.  Common SCSI standards include SCSI-1, Wide SCSI, Fast SCSI, Wide Fast SCSI, Ultra SCSI, Wide Ultra SCSI, SCSI-2, Ultra2 SCSI, Wide Ultra2 SCSI, SCSI-3, Ultra3 SCSI (Ultra160), Ultra320 SCSI, Ultra640 SCSI, Fibre Channel, and serial SCSI (Firewire®). Narrow SCSI is the conventional SCSI specified in the original SCSI. The bus width is 8 bits.  Wide SCSI uses a data bus that is 16 bits wide. This doubling of the bus width respect to the Narrow SCSI comes at a higher cost, and it requires additional cabling and connectors.  SCSI-1, the original SCSI, was standardized by ANSI in 1986.  Allows asynchronous data transfer rates of 1.5 MB/sec and synchronous transfer rates to a maximum of 5 MB/sec. It has an 8-bit SCSI port that uses Single-ended open-collector drivers. Uses a 50-pin connector. SCSI-1 is an obsolete standard after the development of SCSI-2. SCSI-2 has significant improvements over SCSI-1 including faster data transfer rates, better connectors, wider bus path, better reliability via synchronous negotiation and parity checking. SCSI-2 has a data transfer rate from 5 MB/sec to 10 Mb/sec. This speed change is called Fast SCSI-2. Also, SCSI-2 provides the option to double the bus from 8 bits to 16 bits. This change is known as Wide SCSI. By combining the Fast SCSI-2 with the Wide SCSI bus a maximum data rate of 20 MB/sec can be achieved. The SCSI-3 specification allows for faster transfer rate, more devices on a single chain (a maximum of 32), and also it incorporates serial connections in addition to the traditional parallel interconnect of SCSI-2. The serial interconnect of SCSI-3 incorporates three technologies: Serial Storage Architecture (SSA), Fibre Channel, and IEEE P1394. The serial transfer mode allows faster data rates, more devices per bus, simple connectors, and longer cables.  Regular is the default speed of the original specifications of SCSI-1. The regular bus speed is 5 MHz. Fast SCSI increases the bus speed to 10 MHz. This doubling of the bus speed was defined in SCSI-2.  Ultra is also known as Fast-20, it is specified in SCSI-3. The bus speed of Ultra SCSI is 20 MHz. Ultra SCSI buses have a maximum transfer rate of 20 MB/sec for Narrow SCSI, and 40 MB/sec for Wide SCSI.

Computer Platform

Choices for computer platform for SCSI adapters and SCSI controllers include PC, Mac, Alpha, and SPARC®.  Termination parameters to consider include terminator class and terminator type.  The termination class is either internal or external.  Choices for type include passive, active, active negation, high voltage differential, low voltage differential, multimode, forced perfect terminator, and high byte terminator.  Some devices have auto-termination support.  Other specifications to consider when searching for SCSI adapters and SCSI controllers include data transfer rate and data transfer mode.