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SCSI
Communication Test Bus
The SCSI connection
sustains the transfer of data at a greater rate.
Lyndon B.
Johnson
Space Center,
Houston, Texas
NASA Tech Briefs, Dec 1990, page 25
The Small Computer System Interface
(SCSI) communication
test bus provides a high-data-rate, standard
interconnection that enables
communication among an International Business Machines
(IBM) Personal System/2
Micro Channel, other devices connected to the Micro
Channel, test equipment,
and a host computer. Previously, slower and
less-intelligent interface buses have
been used for testing: these include the IEEE-488
[general-purpose interface
bus (GPIB)], the RS-232, and the RS-422. The SCSI
communication test bus is
implemented by use of the SCSI bus (see figure),
which, heretofore, has been
used primarily commercially as a communication channel
between a host computer
and a direct access storage device.
Figure
The SCSI Communication Test Bus
serves primarily as a
non-intrusive input/output attachment to the PS/2
Micro Channel bus, providing
rapid communication for the debugger
One requirement that motivated this
application of the SCSI
is the need for a "debugger" that can monitor
activities on the Micro
Channel non-intrusively. The debugger must also enable
test equipment to
communicate with an 80386 microprocessor on the Micro
Channel. The debugger
operates in two modes: nonintrusive and interfacing.
In the non intrusive mode,
the debugger behaves as a passive monitoring device
that does not affect any
activities on the Micro Channel. In the interfacing
mode, it enables test
equipment to communicate with the 80386 microprocessor
through the Micro
Channel. The debugger also enables the host computer
to read from, and write to,
the memory or input/output ports of the Micro Channel.
This interfacing mode is
also known as the "bus master" mode of the Micro
Channel.
The data-transfer rate of the SCSI
- 4 MB/s - meets the high-speed-communication
requirement for the debugger, and upgrades to higher
speeds are under
consideration. The maximum attainable speeds of
previous test buses (e.g., 1 MB/s
in the IEEE-488) were insufficient for sending data
back to host computers
while continuously acquiring more data. Usually, to
hold large traces of
collected test data, previous debuggers included large
and costly high speed
memories. The collected data were transferred back to
the host computer only
after the traces had been stopped. The data could be
used for monitoring only;
real-time processing and other actions were not
possible.
With the high speed of the SCSI bus
and the intelligence of
the SCSI controller, data from the debugger can be
sent back to the controlling
host computer continuously while the debugger collects
more data from the Micro
Channel. The number of data that can be collected by
the debugger is limited
only by the storage capacity of the controlling host
computer. This opens up
the possibility of using the debugger in real-time
applications that were
impossible with older test buses.
This work was done by Chanh V. Hua,
John J D 'Ambrose, Richard C. Jaworski,
Elaine M. Halula, David N. Thornton, Robert
L Heligman, and Michael R. Turner of International
Business Machines Corp. for
Johnson Space Center. For further information,
Circle 17 on the TSP Request Card.
MSC-21704
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