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dfscsi.txt
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1993-04-23
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4KB
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69 lines
THE IMPORTANCE OF DIFFERENTIAL SCSI TO NETWARE
Given that SCSI (Small Computer System Interface) is the dominant
peripheral cabling scheme, almost all NetWare servers are
prevented from running cables to peripherals under the floor of
the raised floor computer room. This is because they are almost
all using single-ended SCSI. Unfortunately, single-ended SCSI
was designed for use within a cabinet or desktop unit and has a
total cable limitation (add all cables up on one bus) of 6
meters (almost 20 feet). Factor in fast SCSI devices (10 MB/sec
SCSI-2 standard) and the recommended limit is only 3 meters!
The solution is differential SCSI. This "flavor" allows a total
cable run of 25 meters. Be careful though, the hardware looks
similar. In fact the cables are the same. Be sure to identify all
other components as differential (drives, host adapters,
terminators).
For the technically oriented reader, single-ended SCSI uses TTL
logic levels while differential uses EIA RS-485 signals.
Differential requires roughly twice the pins, chips and SCSI
board area of single-ended, whether it is a separate host adapter
or integrated onto the motherboard. Thus, it costs a little more
but then so does the longer cable.
Novell has been certifying differential SCSI components since
1991. Included in the "Novell Labs Tested and Approved" database
are: disk drives, tape drives, disk arrays, MicroChannel and EISA
host adapters, and even a single-ended : differential translator
or converter which allows devices of one "flavor" to connect to a
bus of the other type.
Differential devices, in many cases, have been used for many
years on the mainframe and minis that are
getting replaced or at least having to share room with a NetWare
server. It is even possible that the old peripherals could
migrate to the new NetWare server.
If you think that a conversion to differential will be enough,
watch out! Many manufacturers are coming out with "wide" SCSI
which supports a 16 bit data path instead of 8 bits. This has
been implemented with 68 wire cables with 68 pin high density
connectors. In most cases, this is combined with fast controllers
for "fast and wide" performance of up to 20 MB/sec. (SCSI-2
allows more options for up to 40MB/sec which has been realized in
vendors labs.) Novell has been certifying fast/wide devices since
1991. One small drawback of wide SCSI between two 16 bit devices
is that it is not permissible to put "narrow" (8 bit) devices in
the middle of the bus because the end units may negotiate for
wide when no wide path exists between them.
There is a SCSI bus extender on the market for single-ended. It
would seem one would have to locate the device under the floor on
a long cable run. In fact, it may cost more than a differential
configuration. Such a device introduces signal delays on the bus
lines and may impact synchronous SCSI performance and
reliability. Test under load before investing. Similarly, one
could convert from single-ended to differential for a long cable
run and then convert back to single-ended. This again could
impact performance and/or reliability and should be thoroughly
evaluated.
With differential SCSI, NetWare can now realize the
environmental benefits of the MIS computer room (constant
temperature, clean electricity, fire control, security) as well
as the visibility through the windows (the proverbial "glass
house").