Feature Desc riptions
6- 10 Maxt or Atl as 10K III
The DIFF SENS signal in the SCSI bus defines the current operating mode of the
SCS I bus. A level below +.5 volts defines a single-ended bus. A level between +.7
and +1.9 volts defines an LVD bus. A level above +2.4 volts defines a High Voltage
Differential (HVD) bus. Before LVD was introduced, HVD w as simply called
Differential SCSI .
All sing l e- ende d-onl y de vic e s co nnect t he DIF FSENS s i gnal t o g r ound. On an LVD
bus, the terminators drive the DIFFSENS signal to +1.3 volts unless some device is
holding the signal at ground. HVD devices pull DIFFSE NS up to +5 volts via a
resistor.
The Maxtor Atlas 10K III disk drive is designed with a circuit that monitors the level
of DIFFSENS at power-on. The drive then enables the appropriate circuits for LVD
or single-ended operation, or disables its interface completely if it detects an HVD
level on DIFFS ENS.
The two normal operating environments for the Maxtor Atlas 10K III disk drive are
an all-LVD or (LVD/MSE) bus, in which case it operates in its LVD mode, or a bus
that c ontain s at lea st one s ingle- ended d evice, i n which case t he Maxtor Atlas 10K III
disk drive operates in single-en ded mode.
HVD mode is not supported by th e Maxtor Atlas 10K III disk drive. S i gnal level s on
an HVD bus can reach as high as +15 volts due to allowed common mode transients.
After establishing its initial opera ting mode, the Maxtor Atlas 10K III disk drive
conti nue s to m o nito r the DIFFSE NS sign a l . If a ch a n ge in l e v e l is d e te cted , s uch as
might occur if a device is added to or removed fr om the bus, the DIFFSENS circuit
detects the change and presents a mode change interrupt to the drive firmware. The
operating mode of the interface switches to the new mode 100 msec after the change
is detected, providing the new level has remained constant. This delay is required to
prevent spurious mode switches due to noise on the DIFFSENS line.