Asus PCI-DA2200, User Manual

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ASUS GUI RAID Manager: Array Management
4.6 Setting Channel Configuration
4-15
assigned as the default value for host channels, and 7 for
drive channels.
♦
Default Sync Clock (MHz), Default Xfer Width. These
parameters set the data transfer clock rate for synchronous
communication over the SCSI bus, and enable or disable
wide transfer, respectively. For a detailed description of
these parameters, see the discussion below.
Data that is transferred across the SCSI bus in synchronous
transfer mode is clocked using a synchronous transfer
clock signal. The frequency of this signal determines the
rate at which data is transferred. If, for example, the
synchronous transfer clock is 10 MHz, data transfer rate
will be 10 million bytes per second (assuming narrow
transfer).
Data transfer across the SCSI bus can be either 8 bits or 16
bits at a time. The former is referred to as narrow”
transfer, while the latter is referred to as wide” transfer.
At the same synchronous transfer clock rate , data transfer
rate using wide transfer will be double that using narrow.
With synchronous transfer clock of 10 MHz, data transfer
rate will be 10 Mbytes/second under narrow transfer, and
20Mbytes/second for wide.
Occasionally, under conditions in which SCSI signal
quality is poor, such as long cables, poor connections, or
bad termination, it may be necessary to reduce the
synchronous transfer clock to allow the SCSI channel to
function normally. Under worst conditions, it may be
necessary to switch to asynchronous communication
mode.
Furthermore, certain older SCSI devices may only support
narrow transfer and behave abnormally when an attempt to
negotiate wide transfer is made. Under such conditions,