SCSI uses a logical bus topology, which means that all SCSI devices on a single SCSI bus connect to and share that bus. The logical bus is implemented with a daisy-chain, whereby the first device connects to the second device, which connects to the third device, and so on. The physical cabling used to implement this daisy-chain varies with the type of SCSI device, as follows:
Many external SCSI devices and some older internal SCSI devices have two narrow SCSI connectors. To build the daisy-chain, you use a cable to connect the “out” SCSI connector on the first device to the “in” SCSI connector on the second device, the “out” SCSI connector on the second device to the “in” SCSI connector on the third device, and so on.
Some external SCSI devices and most recent internal SCSI devices have only one SCSI connector. These devices connect to a cable that contains multiple device connectors, similar to a standard IDE cable. You can connect as many devices to these cables are there are positions. In effect, the daisy-chaining is done within the cable itself.
Each SCSI device on a bus is identified by a unique SCSI ID. On a Narrow SCSI bus, the SCSI ID must be in the range of 0 through 7, inclusive. By convention, the SCSI host adapter is assigned SCSI ID7, the primary hard disk (if present) is assigned SCSI ID0, and the secondary hard disk (if present) SCSI ID1. A Wide SCSI bus doubles the number of supported devices from 8 to 16, using SCSI IDs 0 through 15, with ...