Hardware components of the PC
Computer Buses
SCSI ID numbers

Devices that interconnect via the SCSI standard have two special requirements, a SCSI ID number (SCSI ID) and bus termination. Each device on the SCSI bus needs a unique SCSI ID. The SCSI-1 specification sets the priority of the SCSI ID, with the highest being 7 to 0 the lowest. The SCSI-3 specification adds 15 to 8 as lower priority than 0. The SCSI controller is also considered as a device and always has the highest priority SCSI ID 7, and if a SCSI hard drive is to be the default boot device, it will have SCSI ID 0 to be compatible with BIOS parameters, then SCSI ID 1 through to SCSI ID 15 are available for allocation to the remaining devices. The SCSI ID is used to distinguish between devices so that messages are sent to the right device.

In April 1994 a specification was released which allowed for the SCSI configuration to be Plug-and-Play (PnP), which included:

• A single cable connector configuration.
• Automatic termination of the SCSI bus -allows for circuitry to be incorporated that will apply termination if the device is the last on the cable.
• SCSI configured automatically (SCAM) -automatically assigns IDs.
• Full backward compatibility -allows PnP devices to be mixed with older SCSI systems.