Maxtor ATLAS 10K III Manual download pdf (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.

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Maxtor ATLAS 10K III Manual Page 311