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").