Domain Validation is part of the SCSI standard. It describes how to negotiate the best possible transfer agreement between two devices. There are three different messages that can be sent:

• SDTR (Synchronous Data Transfer Request)
• WDTR (Wide Data Transfer Request)
• PPR (Parallel Protocol Request)

WDTR is used only to negotiate width (8 or 16 bit).

SDTR can be used to negotiate up to Ultra-2 speeds.

PPR subsumes the capabilities of both WDTR and SDTR and can negotiate speeds of up to Ultra-640. It can also set various parameters, such as Information Units, Quick Arbitration Select and Double Transition clocking