synchronous transfers frequency in MHz

configname: CONFIG_SCSI_NCR53C8XX_SYNC

Linux Kernel Configuration
└─>Device Drivers
└─>SCSI device support
└─>SCSI low-level drivers
└─>synchronous transfers frequency in MHz
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
The SCSI Parallel Interface-2 Standard defines 5 classes of transfer
rates: FAST-5, FAST-10, FAST-20, FAST-40 and FAST-80. The numbers
are respectively the maximum data transfer rates in mega-transfers
per second for each class. For example, a FAST-20 Wide 16 device is
able to transfer data at 20 million 16 bit packets per second for a
total rate of 40 MB/s.

You may specify 0 if you want to only use asynchronous data
transfers. This is the safest and slowest option. Otherwise, specify
a value between 5 and 80, depending on the capability of your SCSI
controller. The higher the number, the faster the data transfer.
Note that 80 should normally be ok since the driver decreases the
value automatically according to the controller's capabilities.

Your answer to this question is ignored for controllers with NVRAM,
since the driver will get this information from the user set-up. It
also can be overridden using a boot setup option, as follows
(example): 'ncr53c8xx=sync:12' will allow the driver to negotiate
for FAST-20 synchronous data transfer (20 mega-transfers per
second).

The normal answer therefore is not to go with the default but to
select the maximum value 80 allowing the driver to use the maximum
value supported by each controller. If this causes problems with
your SCSI devices, you should come back and decrease the value.

There is no safe option other than using good cabling, right
terminations and SCSI conformant devices.