Small Computer System Interface (SCSI)

Background

Modern bus interfaces such as USB and Thunderbolt use serial communications
serial links can achieve Mbps and Gbps speeds through the use of improved signaling and encoding methods
a serial interface essentially transfers 1 bit at a time
a parallel interface transfers 8 bits (1 byte) or more
- requires more wires in the cable and more pins in the connectors
- bulkier

Small computer system interface (SCSI) is one example of a legacy parallel bus.

One SCSI host bus adapter (HBA) can control multiple devices attached by internal ribbon cables or external SCSI cables
SCSI standard also defines a command language that allows the host adapter to identify which devices are connected to the bus and how they are accessed
could be used for both internal devices and external peripherals
- but now unlikely to find it used for any purpose other than the connection of internal hard disk drives
support data rates up to 320 MBps
numerous versions of SCSI with many different physical connectors
- only likely to come across:
  - high density (HD) 68-pin connectors
    - HD-68 is used with Molex power connectors
  - or single connector attachment (SCA) 80-pin connectors
    - SCA incorporates a power connector
Each device on a wide SCSI bus must be configured with a unique ID, from 0 to 15
- host adapter is usually set to 7 or 15
- bootable hard disk is usually allocated ID 0
first and last devices on a SCSI bus must be terminated
Termination may either be enabled internally on the device by setting a switch or by physically connecting a terminator pack to a device or the host adapter
while parallel SCSI as a physical interface has almost completely disappeared, the software interface and command set are used in many other storage technologies, including serial attached SCSI (SAS)
SAS is a dominant interface for enterprise-class storage devices in the PC workstation and server market