At 00:44 1/16/2000 , Mary Wolden wrote:
>I disagree with Bill on the total length of the cables for the
>following reasons:
>
> >From Adaptec's manual on the 2940 UW :
>"When one or more Fast SCSI Devices are connected to the host adapter,
>the total length of all cables (internal and external) must not exceed
>3 meters (9.8ft) to ensure reliable operation. If no fast SCSI devices
>are connected, the total length of all cables must not exceed 6 meters
>(19.7ft)."
>
> >From Iomega's SCSI insider manual:
>"For any SCSI chain, the combined chain length (the total of all
>cables in the SCSI chain, including internal cables) should not exceed
>6 meters (19.6 feet)."
>
>Unless your scanner is a fast SCSI device then the total length of
>your cables should not exceed 6 meters. (19.6 ft).  Not all SCSI
>scanners cost less then cables. I have an HP 4C scanner as the last
>device on my external SCSI bus connected to an external zip, connected
>to an external jaz, connected internally to a Plextor CD Rom and a
>Plextor CDR then to the host adapter. I have not used any of the
>cheaper scanners around so I can't say what kind of termination they
>use. Internally I have 3 UW SCSI drives.  The scanner has termination
>enabled on the scanner and the last drive on the UW chain has
>termination enabled. The total length of the cables on my system
>definitely exceeds 5 ft. and my system has been running with no
>problems for over 2 years...


Hi SCSI Users

You can exceed the SCSI specifications (break a rule so to speak)
and *maybe* things will still work...although you don't know if
your SCSI bus is taking multiple attempts to do things since it
can detect errors and do retries.

If you do too many things wrong, you are asking for big trouble.
Brad was experiencing flaky behavior. Something was wrong and he
was definitely breaking some of the rules. In my opinion the way
to troubleshoot problems is to return to the basics by adhering
to the letter of the SCSI specifications and seeing what happens.
Anyway, let me defend my statements concerning SCSI specifications.

I believe that the Iomega's devices are at best SCSI-1 (5 MB/sec
or less) and for for SCSI-1, their statement is correct. But it
is not correct if Fast or Ultra devices (SCSI-2) are connected
to the SCSI bus.

I agree with Adaptec's statement. But that's for Fast SCSI and
no mention is made of Ultra SCSI. The maximum cable length for
SCSI-1 (data transfers at 5 MHz) is 20 feet. The maximum cable
length for Fast SCSI (10 MHz transfers) is 10 feet. My statement
is that the maximum cable length for Ultra SCSI (20 MHz) is
5 feet. Brad's Ultra controller is capable of transferring
data at 20 MHz and if he wants to use his hardware at the speed
it was designed for, he is limited to a maximum cable length of
half that allowed for Fast SCSI. (Twice the transfer frequency,
half the max length.) If you use a longer cable, you risk sub
par performance...or no performance at all. (By the way, I have
an Adaptec manual that does mention maximum cable length for
Ultra SCSI. They say that for 3 or less Ultra devices, you
can get away with up to a 10 foot length. This statement is
very liberal.)

I also have a hard copy of the SCSI specifications. It states
that for 3 or more devices connected to a controller doing
20 MHz, the SCSI bus should be no longer than 1.5 meters. You
can get much of this information by ordering a free copy of
"Basics of SCSI, 4th edition" using the form at
<http://www.ancot.com/books/fcpubs/Free_Books/free_books.html>.

The USENET SCSI FAQ at
<http://www.scsifaq.org/scsifaq.html#_Hlk407004683> states:

Speed of FASTEST device      Max. single-ended bus length
   5 MHz (SCSI1 synch.)             6 meters
  10 MHz (SCSI2 FAST)               3 meters
  20 MHz (Ultra)                  1.5 meters

I have three HP 4C scanners. They cost about $100 a year ago at
auction. The Granite Digital external cable costs about $79.
(The cables that came with my HP scanners do not come close
in quality to the Granite Digital cables.) I agree that the HP 4C
was top of the line a few years ago, but these scanners have built
in passive termination. The SCSI specification for Fast SCSI (let
along Ultra) is for Active termination.

Here is some more information from the SCSI FAQ, this time from
<http://www.scsifaq.org/scsifaq.html#_Hlk413567563>:

"There are two basic types of terminators, active and passive:

"Passive terminators consist of pairs of resistors. A 220 Ohm pulling
each signal up to TERMPWR and a 330 Ohm pulling each signal down to
GROUND. Passive terminators were considered adequate in SCSI-1 when
the bus only ran at 5 MHz... Active terminators consist of 110 Ohm
resistors connected from each signal line to a common 2.85 Volt'
regulated power supply. Active terminators both terminate the bus
better (less reflection), and supply cleaner pullup current (due to
their Voltage regulation).  They were first defined in SCSI-2.

"Typical passive terminators (resistors) allow signals to fluctuate
directly in relation to the TERM Power Voltage. Usually terminating
resistors will suffice over short distances, like 2-3 feet, but for
longer distances active termination is a real advantage.

"Recommendations and requirements:
In SCSI-2 when the fastest defined speed was 10 MHz, passive
terminators were allowed, but active terminators were recommended.
In SCSI-3...the SPI-2 standard only allows active termination for
single-ended buses regardless of speed...If you run into problems,
switching to an active terminator might well solve them."

I wouldn't use the passive termination from even a good scanner
to terminate my SCSI bus if it included any hard drives.

By the way, here's a quote from the SCSI FAQ telling why it
can be bad for too many devices to provide Termpower:

"TERMPWR is the power source for the SCSI terminators. Terminators
(both active and passive) require power because in addition to
providing the correct impedance to prevent reflections on the
SCSI bus, they source pull-up current to the SCSI signals. The
SCSI spec., allows for multiple devices to supply power, but also
limits the maximum current that should be available. The 'rule' is
that 'initiators shall supply TERMPWR'. Hence a SCSI controller
(host adapter) should supply TERMPWR, and on longer buses it is
worth having a device near the end to also supply it. However, no
more than about four devices should supply it, because in the
event of a failure (shorted cable etc), there could be dangerous
currents available."
<http://www.scsifaq.org/scsifaq.html#Generic050>

Regards,
Bill

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