Re: Somewhat OT: Narrow and wide on the same SCSI bus - ID question
- From: "UZnal" <unalz-at-mail333-dot-com>
- Date: Fri, 18 Aug 2006 21:14:54 +0200
I found that even with a narrow ID bit "shared" between the narrow DFHS
and a wide device (with its wide ID bit set so there was no ID
conflict) that things still worked fine.
Narrow or wide, the SCSI-2 bus supports up to eight devices, the eighth
device is the adapter. The adapter has certainly an internal table to map
LUNs to SCSI IDs, hence, a driver from the outside sees only logical device
numbers.
If a narrow SCSI device cannot "see" this added "wide ID" bit, what
keeps it from responding to the bits it can "see"?
Re ANSI SCSI-II Specs:
4.5. SCSI Bus
Communication on the SCSI bus is allowed between only two SCSI devices at
any given time. There is a maximum of eight SCSI devices. Each SCSI device
has an SCSI ID bit assigned as shown in Figure 4-12.
When two SCSI devices communicate on the SCSI bus, one acts as an
initiator
and the other acts as a target. The initiator originates an operation and
the
target performs the operation. An SCSI device usually has a fixed role as
an
initiator or target, but some devices may be able to assume either role.
An initiator may address up to eight peripheral devices that are connected
to a target. The target may be physically housed within the peripheral
device
in which case the peripheral device is referred to as an embedded SCSI
device.
DB(7) DB(6) DB(5) DB(4) DB(3) DB(2) DB(1) DB(0) <-- DATA BUS
| | | | | | | |
| | | | | | | SCSI ID = 0
| | | | | | |
| | | | | | SCSI ID = 1
| | | | | |
| | | | | SCSI ID = 2
| | | | |
| | | | SCSI ID = 3
| | | |
| | | SCSI ID = 4
| | |
| | SCSI ID = 5
| |
| SCSI ID = 6
|
SCSI ID = 7
Up to eight SCSI devices can be supported on the SCSI bus. They can be
any
combination of initiators and targets provided there is at least one of
each.
Certain SCSI bus functions are assigned to the initiator and certain SCSI
bus functions are assigned to the target. The initiator may arbitrate for
the
SCSI bus and select a particular target. The target may request the
transfer
of COMMAND, DATA, STATUS, or other information on the DATA BUS, and in some
cases it may arbitrate for the SCSI bus and reselect an initiator for the
purpose of continuing an operation.
Information transfers on the DATA BUS are asynchronous and follow a
defined
REQ/ACK handshake protocol. One byte of information may be transferred with
each handshake on the A cable and, if the wide data transfer option is
implemented, one or three bytes of information may be transferred with each
handshake on the B cable. An option is defined for synchronous data
transfer.
.
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