SCSI Communication Test Bus

The SCSI connection sustains the transfer of data at a greater rate.

Author: Lyndon B. Johnson Space Center, Houston, Texas
Source: NASA Tech Briefs, Dec 1990 (page 27 physical)

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.

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.

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