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From: phil@amdcad.UUCP (Phil Ngai)
Newsgroups: mod.computers.vax
Subject: cascading CONNECTED DELNIs is illegal
Message-ID: <8609062035.AA00803@amdcad.UUCP>
Date: Sat, 6-Sep-86 16:35:33 EDT
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Posted: Sat Sep  6 16:35:33 1986
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DEC seems to have been less than diligent in informing their customers
about what DELNI configurations are permissible. Granted, a lot of
customers are of the "real programmers don't read the manual" type,
but I have heard and heard of DEC employees giving bad information to
customers.  I personally have asked questions and gotten bad answers.
When I talk to people I've never met before, I usually start out with
questions I already know the answers to in order to "calibrate" the
quality of the information I get. Based on my experience, you would
not be wasting time if you confirm what someone tells you, even if
they are a DEC employee. Even if they are sure they are right. Ask for
references, page numbers from manuals or buyer's guides. Manuals
aren't perfect either but they provide more confidence that this
person has been trained in the subject they are talking about.

In the July - September _Networks and Communications Buyer's Guide_,
on page 2.44 (the page number depends on the date of your book, look
up DELNI in the index), the second paragraph says:

"The DELNI can be configured three ways: stand-alone, hierarchical
stand-alone, and connected."

Paragraph four says:

"Hierarchical stand-alone configurations are not connected to the
Ethernet coaxial cable."

A lot of people have told me "we tried it and it works for us".  This
is undoubtedly true. However, it is still illegal. The configuration
rules are conservative engineering guidelines. You can violate them in
a small enough network and get away with it.  If your network is a
large enough network, you may discover the hard way why the
configuration rules exist. I prefer to play it safe and follow the
rules. If you chose to violate the rules, at least know which rules
you are violating so that when something goes wrong, you know where to
begin debugging.

While we're talking about DELNI's, another poorly known rule is
that in a CONNECTED configuration, the transceiver cable lengths
add. If the maximum cable length for the controller is 40 meters
and it's 15 meters from the transceiver to the DELNI, you only
get 25 meters from the DELNI to the controller. See page 2.46.

There also seems to be some confusion on why I want to connect a DELNI
to a DEREP. We have 8 DEREP-RA (remote repeaters) coming together in
one rack. Currently we have a lot of transceivers and a lot of coax
coiled up on the wall. But gee, isn't that what a DELNI is for?
Sorry, you can't connect a DEREP to a DELNI.

	Phil Ngai