Quantum 10K II - Manual (Page 311)

Feature Descriptions
6-10 Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives
Single-ended SCSI, in contrast, uses one-wire active signaling with the signal return
wire connected to ground. The active signal range is switched between a high in the
range of +2.4 – 3.0 volts and a low in the range of 0 - +.5 volts. LVD/MSE SCSI
devices in single-ended mode do not have the signal return wires connected directly
to ground, as these same pins must be driven to LVD levels when the device is
operating in LVD mode. Instead, when in single-ended mode, the device turns on a
driver transistor for each return line that acts as a switch to connect the signal
return to ground.
6.17.1DIFFSENS
The DIFFSENS signal in the SCSI bus defines the current operating mode of the
SCSI 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 was simply called
Differential SCSI.
All single-ended-only devices connect the DIFFSENS signal to ground. 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 DIFFSENS up to +5 volts via a
resistor.
The Atlas 10K 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 DIFFSENS.
The two normal operating environments for the Atlas 10K disk drive are an all-LVD
or (LVD/MSE) bus, in which case it operates in its LVD mode, or a bus that
contains at least one single-ended device, in which case the Atlas 10K disk drive
operates in single-ended mode.
HVD mode is not supported by the Atlas 10K disk drive. Signal levels on an HVD
bus can reach as high as +15 volts due to allowed common mode transients.
CAUTION
The LVD and HVD alternatives are mutually exclusive and damage may occur
to SCSI devices if an LVD device is plugged into an operating HVD bus.