6.14.1DIFFSENS

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 Maxtor Atlas 10K V 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 Maxtor Atlas 10K V 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 Maxtor Atlas 10K V disk drive operates in single-ended mode.

HVD mode is not supported by the Maxtor Atlas 10K V 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.

6.14.2DIFFSENS SWITCHING

After establishing its initial operating mode, the Maxtor Atlas 10K V disk drive continues to monitor the DIFFSENS signal. If a change in level is detected, such as might occur if a device is added to or removed from 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.

Maxtor Atlas 10K V 6-9