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.