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A Physical Unit Number (PUN) is the device identification number assigned to a Small Computer System Interface (SCSI) device attached to a SCSI controller.
It enables distinguishing multiple devices that are simultaneously connected to a SCSCI controller.
A physical unit number is also known as an SCSI device ID and an SCSI address.
A PUN is primarily used when more than one device is connected to a SCSI controller.
The PUN assists the SCSI controller in identifying and managing communication. The SCSI controller can only communicate with one SCSI device at a time; therefore it prioritizes devices using their PUNs.
It also restricts multiple SCSI devices from sending and receiving data at the same time. Typically, PUN is set using priority IDs ranging from 0-15, where devices with the highest priorities are assigned numbers from 0-7 and devices with the lowest priorities are assigned 8-15.
In some scenarios, 7 is reserved for the SCSI controller itself.