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TARGET ROLE ROUTINES
6-2
6
particular device on the SCSI bus. The indexing is accomplished by simply
splitting the 64 entries into 8 entries of 8 peripheral devices. Each block of
eight devices corresponded to each SCSI device. The MVME147 is one of these
SCSI devices. Seven other SCSI devices may be added to the SCSI bus to
perform some application. As long as there is another SCSI device that can
play the initiator role on the bus, then the MVME147 may also play the
TARGET role on the bus. The eight entries under the SCSI device entry deal
with TARGET support of eight peripheral devices. In other words, the SCSI
firmware TARGET role routines allow support of eight peripheral devices
associated with the MVME147.
The method of addressing these peripheral devices is through the concept of
logical units. Because the MVME147 supports eight logical units on the SCSI
bus, LUN 0 may be a printer, LUN 1 may be an RS-232C port, and LUNs 2
through 7 may be some I/O devices on the VMEbus (as an example). Each
logical unit is independent of the other as far as the SCSI firmware is
concerned. To keep this independent feature, each service module for each
logical unit should provide a different return vector to the SCSI firmware.
SCSI Versus SASI Rules
As far as the SCSI firmware is concerned, a SCSI system is one that supports
arbitration, reselection, and the message-out phase. Typically, a SASI system
contains only one initiator and at least one TARGET. A SASI system with only
one initiator and no reselection clearly does not require bus arbitration because
only one SCSI device ever tries to acquire the bus. With these rules in place, it
is clear that for the MVME147 to operate both as an initiator and as a TARGET
on the SCSI bus, you must have a SCSI system; one that supports arbitration,
reselection, and the message-out phase. (The message-out phase is required
for identification of reselectability after the selection phase and for
identification of the peripheral logical unit immediately following reselection.)
If the MVME147 is to operate only as a TARGET on the SCSI bus with only one
initiator on the same bus, then you may use a SASI system with the MVME147
as a TARGET only.
MVME147 SCSI Firmware TARGET Role Structure
Requirements
The following sections describe the data structures required by the SCSI
firmware for the TARGET role custom sequence packets. Refer to Chapter 2.
Custom Sequence Packet
Custom Sequence packets are detailed in Chapter 2.