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If SCSI technology was stagnant, SE signaling would still be in use today; however, when the SCSI bus was reaching speeds of 40 MHz and higher, SE couldn't cope. Each time SCSI speeds doubled, the length of cable used with SE singling halved. Fast SCSI using SE, for example, had a cable length of 3 meters, and Ultra SCSI dropped the cable length to 1.5 meters. SE is not defined for SCSI standards beyond Ultra SCSI, which is a good thing as attaching an external tape drive with a 2-inch cable would be rather difficult!
For the faster SCSI standards, LVD signaling is used. In many ways, LVD is a compromise between its two predecessors, providing a cheaper alternative to HVD and at the same time offering longer cable distances than SE can accommodate. LVD defines cable lengths of 12 meters up to 25 meters (if only two SCSI devices are attached to the bus).
Like HVD, LVD uses two wires for each signal; however, the lower voltage requirements allows for a reduced cost and power requirements to be kept under control. As a bit of a bonus, this also allows SE devices and LVD devices to coexist on the same SCSI bus. HVD, on the other hand, is completely incompatible with the other two signaling methods. There is a significant caveat when talking about SE and LVD compatibility, though. If not all devices on the SCSI bus are LVD capable, the bus will operate within the confines of SE signaling. Not only will the speeds be limited, but the cable lengths must also conform to the SE specifications.
Today's modern implementations will almost certainly use LVD signaling. In fact, SE signaling is not defined for SCSI speeds beyond Ultra2 SCSI, and HVD is not defined for SCSI standards beyond Ultra2 SCSI.