cgn@ast.COM (Chris Nieves) (03/08/91)
I'm hearing a lot of differing views on single-end vs differential
SCSI cables. Some say you can do 10MB on SE cable, others say you
can go 75 feet on SE cable, and some say you can go 10MB AND 75 feet
on SE cable. What's the real story here. Inquiring minds want to
know!
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Chris Nieves AST Research UUCP: ...uunet!legs!cgn
16215 Alton Parkway INTERNET: cgn@ast.com
Irvine, CA 92718 PHONE: (714) 727-8494
=======================================================================neese@adaptx1.UUCP (03/12/91)
>/* ---------- "Single-Ended vs Differential" ---------- */ >I'm hearing a lot of differing views on single-end vs differential >SCSI cables. Some say you can do 10MB on SE cable, others say you >can go 75 feet on SE cable, and some say you can go 10MB AND 75 feet >on SE cable. What's the real story here. Inquiring minds want to >know! Never mind what people say. The spec allows a single-ended SCSI bus to be up to 21 feet long. Going poast that is at your own risk. The initial speed ceilings and bus lengths were not chosen out of the air. They are and are based on known technological reasons and theory. To violate these specs put your data at risk. Now that the SCSI-2 spec is firm, there are a lot a folks who are anxiously waiting for those FAST SCSI-2 devices (10MBytes/sec). They are in for quite a few surprises and problems. I can see some user taking the adapter out putting in a FAST adapter and hooking up thier external FAST drives with the cables from the old setup. When problems start occurring and there is a good chance there will be problems, they will call either the drive company, or the adapter manufacturer and complian long and loud. Not realizing the true potential problem is the cable. Even if it worked fine on the slower SCSI-1 bus, it may not be good enough for the FAST SCSI-2 bus. The problem lies in many of the SCSI cable manufacturers. Cable design is a true science and some would say an art as well. I have seen external cables of all types of designs. Some do not use twisted pairs internally. Some do, but do not properly route the pairs internally. Some have the cable pairs just bundled together in an arbitrary manner inside. Some have a shield around the bundle, and it may or may not be properly grounded. To say it is a bag of worms is a slight understatement. Anyway, I digress, in the SCSI committee there is a proposal on the table for a new method for single-ended termination. One that would be compatible with the current spec, but would allow for higher noise immunity and better impedance. With this type of termination, one could feel comfortable putting a FAST SCSI device on the external bus. It costs more to implement this type of termination, but is still cheaper than differential and does not require the extensive board real estate that differential does. A happy medium as it were. I would not put a standard single-ended SCSI device on an external cable on a bet. When I say FAST in this context I mean 10MBytes/sec. FAST does not imply 10MBytes/sec. It does imply faster than 5MBYtes/sec. You will see many devices in the near future claiming to be FAST SCSi and they are right to do so as they will be running 6.xMBYtes/sec. This qualifies as FAST SCSI-2. Of course, none of it means diddly. The continous data rate is the real trick. Bursting data at high rates is one thing, but sustaining those rates are something else. Internally, you can get away with the current single-ended design. The cable is normally short (< 5 feet). It is normally a flat (50 wire) ribbon cable, which is a very good cable. Enough for now. Roy Neese Adaptec Senior SCSI Applications Engineer UUCP @ neese@adaptex
paul@actrix.gen.nz (Paul Gillingwater) (03/13/91)
In article <3246@legs.UUCP> cgn@ast.COM (Chris Nieves) writes: > I'm hearing a lot of differing views on single-end vs differential > SCSI cables. Some say you can do 10MB on SE cable, others say you > can go 75 feet on SE cable, and some say you can go 10MB AND 75 feet > on SE cable. What's the real story here. Inquiring minds want to > know! According to an HP Technical Highlight on SCSI, a single ended SCSI HBA limits cable lengths to under 6 metres. Differential SCSI can handle distances of up to 25 metres. Obviously, this refers to what the standard guarantees -- some vendors may wish to extend these lengths with a sufficient tail wind. They further claim that SCSI-2 Fast/Wide can go up to 40 Mb/sec, while SCSI-1 can handle 10 Mb/s with fast SCSI and up to 20 Mb/s with wide SCSI. Interestingly, HP also claim that "no major vendors have offered Fast/Wide SCSI." Anyone care to contradict that remark? -- Paul Gillingwater, paul@actrix.gen.nz
dave@elwood.coyote.trw.com (Dave Agabra) (03/16/91)
In section 4.1 of the SCSI-2 spec, it has a note that says that use of
single-ended drivers and receivers with the fast synchronous data transfer
option is not recommended. In section 4.8 it defines the fast data transfer
option as any data transfer having a transfer period of less than 200ns. My
question is: Are people really abiding by this restriction? It sounds like
some manufacturers are planning to go faster than 5MB/s on single-ended cables
even though it is not technically "legal". Anyone know?
---- Dave
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A person should always believe in something. dave@elwood.coyote.trw.com
I believe I'll have another beer. {cit-vax | trwrb}!wiley!dave
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