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Choosing
a SCSI Host Adapter
When
choosing a SCSI host adapter for your optical storage system,
consider the following:
Calculating
Total SCSI Cable Length and
the Number of SCSI Devices
Single-ended
SCSI adapters
support a maximum electrical cable length of 6 meters
(approximately 19' 6"). Each SCSI device requires
an additional 0.6 meters (2') of electrical cable length in
addition to the cable length required to connect all devices.
Be sure to consider this when calculating total cable length.
The SCSI addressing specification allows you to address several
daisy-chained SCSI devices but you should limit the amount
to a maximum of 4 to 5 devices per bus when using a single-ended
SCSI adapter. It may be possible to operate more devices on
one single-ended SCSI bus, however this conservative method
will help ensure superior reliability.
If your configuration requires more devices or more cable
length, you have three choices:
-
Choose
a differential SCSI host adapter and differential
SCSI interfaces for all devices on this bus (best choice
for large jukeboxes with six optical drives or more).
-
Use
multiple single-ended SCSI host adapters and distribute
the load (best choice for multiple jukeboxes or drives
or combinations of these two).
-
Add
a SCSI repeater to the circuit.
Differential SCSI adapters support a maximum electrical
cable length of 25 meters (approximately 81' 6")
but MUST be connected to differential SCSI devices
ONLY! Connecting a single-ended SCSI device to a differential
SCSI host adapter can result in the destruction of the
single-ended device's SCSI interface (and in some cases
smoke!). Additionally, differential SCSI requires
differential SCSI cables, terminators and devices.
Single-ended SCSI devices cannot be mixed or used
upon a differential SCSI bus.
Some SCSI jukebox manufacturers include differential SCSI
bus capabilities within their devices, and have single-ended
SCSI drives residing within the jukebox. These drives
are connected to a SCSI differential to single-ended converter.
If your device has this option, and you have five or more
SCSI drives, choose a differential SCSI host adapter.
If the differential SCSI bus is not standard equipment
within your jukebox, and you have five or more SCSI drives,
order all devices with differential SCSI interfaces.
I/O
Bus Performance
Some
people feel that there is no reason to spend money on a fast
SCSI host adapter when it is connected to traditionally slower
SCSI devices (like optical disk drives). Although optical
disk drive speeds are increasing, they are still slower when
compared to a multi-head, multi-platter hard drive. Given
this, some users believe that a slower device will not benefit
from a faster I/O bus performance, so why spend the money?
Things to Consider:
-
ISA
bus adapter cards are typically clocked at approximately
7.15 MHz which is very slow by today's standards
-
ISA
bus adapter cards support 16 bit I/O
-
ISA
bus adapter cards typically use dated SCSI chip sets
-
EISA
bus adapter cards are typically clocked at 10 MHz
-
EISA
bus adapter cards support 32 bit I/O
-
VESA/VLB
adapter cards are typically clocked at approximately 33
MHz
-
VESA/VLB
adapter cards support 32 bit I/O
-
PCI
bus adapter cards are typically clocked at 33 MHz
-
PCI
bus adapter cards support 32 bit I/O
-
PCI
bus adapter cards use advanced SCSI chip sets
Summary
A
PCI bus SCSI host adapter will provide substantially higher
transfer rates and overall superior performance over an ISA
bus adapter. Even though the optical drive may not be able
to read or write anywhere near the 10 MB/second performance
provided by an ISA adapter, the drive will still operate substantially
faster when connected to the faster PCI bus card. This is
especially true when the SCSI adapter is used with a multitasking,
multithreaded operating system like the InveStore and
the Pegasus-OFS for OS/2.
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