The first, still very common, was parallel SCSI (now also called SPI), which uses a parallel bus design.

IEEE 1394 replaced parallel SCSI in many applications, because of lower implementation costs and a simplified, more adaptable cabling system.

It is a point-to-point serial protocol that replaces the parallel SCSI.

The speed is realized on each initiator-target connection, hence higher throughput whereas in parallel SCSI the speed is shared across the entire multidrop bus.

For comparison, in parallel SCSI, the SCSI ID is the port identifier and device name.

If one initiator is connected to one target, there is no opportunity for contention; with parallel SCSI, even this situation could cause contention.

SAS has no termination issues and does not require terminator packs like parallel SCSI.

Both SAS and parallel SCSI use the SCSI command-set.

Ultra320 was the highest level of parallel SCSI available, but SAS has since replaced it as the highest-performing SCSI technology.

The SCSI-3 standard then split the framework into separate layers so parallel SCSI is now just one of a number of available implementations.