TITLE
    Hard Drive SCSI Termination Power
Article ID:
Created:
Modified: 		5472
5/15/90
6/29/90
TOPIC



    I'm experiencing a startup problem with Mass Microsystem's DataPak attached to
    my Macintosh. Sometimes I could start up fine with or without the DataPak
    turned on. Other times, the system hung until the DataPak was turned on.

    What's going on?

DISCUSSION

    The SCSI bus requires termination power on the bus to provide clean, quality
    SCSI signals. Mass Microsystems' DataPak supplies its own termination power to
    the SCSI bus. (This characteristic is also common to early SuperMac
    DataFrames.) Other SCSI devices that we checked use the termination power
    provided by the Macintosh.

    When a SCSI device provides its own termination power, the SCSI bus signals may
    be corrupted when that device is powered down because it leaves the SCSI bus
    unterminated. An unterminated SCSI bus can cause startup problems because of
    noise on the bus.

    Sometimes when a SCSI device that supplies its own termination power is powered
    down, the Macintosh will still start up because the SCSI bus signal is clean
    enough for the startup process to take place. However, there is no assurance
    that the bus signals will stay clean and uncorrupted since the SCSI bus is not
    properly terminated in this situation.

    SCSI devices that use the termination power provided by the Macintosh do not
    have to be powered on for the SCSI bus to be properly terminated. The
    termination power from the Macintosh provides the correct termination to the
    SCSI bus.



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