Small Computer System Interface Bus Concurrent Maintenance Device

Small Computer System Interface Bus Concurrent Maintenance Device

      In a daisy-chained Small Computer System Interface (SCSI) Bus
environment (multiple Targets/Initiators), when it is necessary to
perform maintenance to a Target/Initiator, it is preferable to do so
without powering down or quiescing all outstanding operations.  One
of the problems inherent in adding or removing Targets/Initiators
from the Bus with power on is the noise produced in the process.  The
noise causes corruption of outstanding operations on the SCSI Bus.
This disclosure allows maintenance to be performed on an "active"
SCSI Bus without powering down or quiescing outstanding operations.

A typical daisy-chained SCSI bus is outlined in the figure.  Included
in the daisy-chain is a Bus Maintenance Device used to perform
maintenance on an "active" SCSI Bus.  This device contains a LED, a
momentary switch and a function to gain control of the SCSI Bus.  In
addition, a mechanically interlocked power switch is located on each
device.  This switch is used to control the regulator for its
respective device.

To perform maintenance on an "active" SCSI Bus without powering down
or quiescing operations, the following procedure can be used.

1.  STEP  1 - Push the Bus Maintenance Momentary Switch to the "ON"
    position (normally "OFF") to acquire the bus.  In accordance with
    SCSI architecture, the following steps will occur:

    a.  Wait for BUS FREE phase to occur on SCSI Bus.
    b.  Arbitrate for the SCSI Bus.
    c.  After winning arbitration, the Maintenance Device selects
        itself by maintaining BUSY...