Acer SmartRAID V User Manual Page 198

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Theory of Operation
The original SCSI specification synchronous transfer rates of up to 5MHz.
The SCSI-2 specification increased the maximum synchronous rate to
10MHz. Devices that support the faster 10MHz rate are called Fast SCSI
devices. The SCSI-3 specification defines transfer rates up to 20MHz (Fast-
20/Ultra SCSI).
The SCSI-3 SPI-2 specification defines synchronous data transmission
speeds of up to 40MHz. The 40MHz transfer speed provides data transfer
rates up to 40MB/sec on an 8-bit bus and 80MB/sec on a 16-bit SCSI bus.
Devices that support these rates are called Ultra2 SCSI devices. Ultra2
SCSI is also known as Fast-40 SCSI. A device must use Low Voltage
Differential signaling to achieve the maximum Ultra2 speed.
Fibre Channel Arbitrated Loop (FC-AL) controllers transfer data at a
maximum speed of 1GHz per loop (100 MB/sec).
Cabling Single-Ended SCSI
The SCSI cable is an electrical transmission line that has a characteristic
impedance. The value of the impedance depends on the type and
configuration of the cable. Differences in this impedance can cause signal
reflections. These impedance variations can be the result of extra
capacitance internal to SCSI devices, connectors, incorrect termination,
mixing of different cable types, cable stubs, and so on.
A properly configured Single-Ended SCSI bus can reliably transfer data at
up to 20MHz. However, the following guidelines should be used to ensure
success:
Use as short a cable length as possible. For 10MHz data transfers
the total SCSI bus length should not exceed 3 meters (9.8 feet). For
20MHz data transfers, total SCSI bus length should not exceed 3
meters with up to 4 devices on the bus, or 1.5 meters (4.9 feet) with
between 5 and 8 devices on the bus.
NOTE When an external SCSI storage cabinet is used, the cabinets
internal SCSI backplane will add additional length to the bus.
Avoid stub clustering. Avoid spacing SCSI devices on the cable closer
than 0.3 meters (11.8 in). When devices are clustered closely together
on the SCSI cable, their capacitance adds together to create an
impedance discontinuity and thus reflections. SCSI devices should be
spaced as evenly as possible.
Cable stub length should not exceed 10 cm (3.9 in). Some SCSI
devices may create stubs internal to the device which exceed this
value. This can result in excessive capacitive loading and signal
reflections. This factor is controlled by the SCSI device manufacturer.
The SCSI cabling itself should not include stubs.
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