Acer SmartRAID V Bedienungsanleitung Seite 196

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Theory of Operation
Transfer Padding and Residue Reporting
In unusual cases, a SCSI peripheral device might read or write more data
than was requested by the host. If this occurs during a write operation,
SmartRAID V controllers automatically transfer pad data by adding
additional bytes to the end of the data. If this occurs during a read
operation, the controller automatically strips off additional bytes of data
read from the device. This prevents the data from overwriting valid data in
host memory.
It is also possible for a SCSI peripheral device to read or write less data
than was requested by the host. If this occurs, SmartRAID V controllers
will report the actual number of bytes transferred as less than the
requested number of bytes.
Single-Ended and Differential SCSI
The SCSI electrical specification has two transceiver specifications:
1. Single-Ended TTL transceivers which allow:
Asynchronous data transfer at a maximum cable length of 6 meters
(19.6 feet). No maximum transfer rate is specified but typical rates
range from 1.5MHz to 3MHz.
Synchronous data transfer up to 5MHz at a maximum cable length
of 6 meters.
10MHz (Fast) synchronous data transfer at a maximum cable
length of 3 meters (9.8 feet).
20MHz (Ultra) synchronous data transfer at a maximum cable
length of 3 meters with up to 4 devices and 1.5 meters (4.9 feet) with
up to eight devices.
2. Differential SCSI: Differential electrical signal protocol transmits
information through a current loop rather than by changes in
voltage, thereby reducing susceptibility to electrical interference.
Differential SCSI has two variants:
High Voltage Differential (HVD) SCSI uses RS-485 transceivers that
allow up to 20MHz data transfer with a maximum cable length of 25
meters (82 feet).
Ultra2 SCSI Low Voltage Differential (LVD) transmission. This
transmission method has the benefits of differential SCSI without
the need for external, high-voltage bus transceivers. Ultra2 SCSI
allows synchronous transfers up to 40MHz with a maximum cable
length of 12 meters. SmartRAID V products support multimode
Ultra2 SCSI, which includes LVD operation.
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