Maxtor Atlas SCSI Drive 15K II, Product Manual

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Feature Descriptions
Maxtor Atlas 15K 6-9
6.16 SCSI CONFIGURED AUTOMATICALLY (SCAM)
SCAM is a process of automatic ID assignment that eliminates the need for user
involvement in the setting of unique SCSI IDs. The drives support and conform to
the SCAM-2 automatic ID assignment protocol contained in the SCSI-3 draft
standards for SCAM. The drives are also fully compatible with SCSI devices that do
not comply with SCAM requirements.
6.17 THE ULTRA160 and ULTRA320 LOW VOLTAGE DIFFERENTIAL
(LVD) SCSI INTERFACE
SCSI-3 provides increased performance and versatility to SCSI disks. LVD uses
lower level voltage swings, differential signaling, and double transition (DT)
clocking to allow a maximum bus speed of 160 or 320 MB/second in wide SCSI
configurations.
The Maxtor Atlas 15K disk drive supports the LVD/MSE (Multi-Mode LVD and
Single-Ended) interface standard. When installed on a bus with only LVD devices
the Maxtor Atlas 15K disk drive operates in LVD mode at transfer rates up to 160 or
320 MB/second. It is fully compatible with Ultra2 LVD devices operating at slower
transfer rates. If an Maxtor Atlas 15K disk drive is installed on a bus with one or more
single-ended (SE) devices, it automatically switches modes to operate in SE mode at
the slower, legacy data rates.
Differential SCSI operation has the advantage of higher reliability of data transfer
through increased immunity to electrical noise. Differential signaling uses a two-wire
active system in which current from each wire flows in opposite directions and
reverses direction for a signal transition. The direction of current flow determines
whether the voltage levels on the two wires have a high/low or low/high relationship
to each other. The common mode, or reference, level for the two LVD signals is
+1.25 volts. High and low is defined relative to this level. LVD permits a wide range
of signal amplitudes. The Maxtor Atlas 15K interface drivers are designed to provide
a high to low range on each signal of approximately 400 millivolts in a nominal
configuration, resulting in a differential signal of approximately 800 millivolts.
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.