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How does device ID numbering work with WIDE vs NARROW devices?

April 26, 2017device Devices Id narrow numbering vs wide

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How does device ID numbering work with WIDE vs NARROW devices?

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Narrow SCSI devices can only use IDs 0 through 7. WIDE SCSI devices on a SCSI-3 system with 68 pin P cables, can use IDs 0 through 15. It is generally wise to reserve 0-7 for narrow devices though. SCSI-2 only specified the use of IDs 0-7 even for WIDE devices, but SCSI-3 allows 0-15 for WIDE devices. All devices on one bus must have unique IDs of course. The arbitration priorities are as follows: highest ID 7 … ID 0 ID 15 … ID 8 ID 23 … ID 16 ID 31 … ID 24 lowest (I doubt you’ll ever see a system using WIDE 32 which is required for use of IDs 16 thru 24) A WIDE device that is set to ID 10 knows not to respond to selection for ID 2 because the parity bit P1 (for bits 8-15) will not be set by the initiator. During a selection of ID 10, the P parity bit (for bits 0-7) will not be set by the initiator, but the P1 bit will be. To use both WIDE and narrow devices on the same bus, the host adapter must be set to ID 7 (or less) so that the narrow devices can talk to it.