Date: Mon, 04 Nov 96 10:49:46 PST From: BRETT_GLASS@infoworld.com To: "Jordan K. Hubbard" <jkh@time.cdrom.com>, hardware@freebsd.org Subject: OK, what's the deal with 2940W controllers and internal conn Message-ID: <9610048471.AA847132867@ccgate.infoworld.com>
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>From an EE standpoint, the key things to understand are as follows:
* Each line on the bus must be a continuous transmission line; no "Y"
configuration is allowed (though very short "spurs" into devices are).
* Each line must be terminated at both ends. If some lines are longer than
others (as when you mix narrow and wide devices), only SOME lines of the
bus may be terminated at a given device (or at the host adapter) while the
rest continue on.
For example, suppose you want to attach both narrow and wide devices to a
SCSI card like your Adaptec. The "narrow" section of the bus is a proper
subset of the lines comprising the full bus, and will be longer because it
goes to all of the wide devices as well as the narrow ones. So, in order to
obey the above rules, your Adaptec card has a terminator that can terminate
the "extra" lines that are ONLY part of the "wide" section while passing
the rest through unterminated. The result looks like this:
+-------+
|Host |
+-------------+ |Adapter|
T==|=============|="Extra" wide SCSI lines=|T | +---------------+
T==|"Wide" device|==="Narrow" SCSI lines===|=======|==|"Narrow" device|===T
+-------------+ +-------+ +---------------+
In this picture, "T" represents a terminator. The terminators at the
devices may be internal or external.
The Adaptec has an internal wide connector, an internal narrow connector,
and an external wide connector. This handles many situations gracefully.
The glaring exception is when you need to use narrow devices (or, worse, a
mix of narrow and wide) externally, in which case you'll need adapters or
special cables to get the narrow external devices connected. In this case,
it's often easier and cheaper to get another SCSI adapter -- a bargain one
will do in most cases -- than to mess with the extra cables and gadgetry.
This is what I do for my external Exabyte backup tape drive.
There's also another advantage to having a second adapter: it can operate
concurrently with the first. This can speed backups up a lot, with one
exception: if your backup software is doing direct device-to-device
transfers over the SCSI bus. UNIX doesn't have provisions in the OS for
this, so I doubt that any UNIX backup utility does it.
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