High Voltage Differential: the logic signalling system originally defined in the SCSI-2 standard. HVD has a maximum logic voltage of 5V and uses a paired plus and minus signal level to reduce the effects of noise on the SCSI bus. It was functionally replaced by LVD (Low Voltage Differential) in the SCSI-3 variant of the standard. HVD and LVD SCSI are not directly compatible but can be interconnected by the use of a special adapter.
