DELNI characteristics

[David Roode]

We have noticed that DELNI's have some non-intuitive limitations
on cable length. Could this perhaps have some bearing on your
problem? Unlike other multiport transceivers, the rule
for DELNI's is that the transceiver cable length limitation
of 40m (vs. 50m for other devices) applies to the sum of
lengths of the drop to the DELNI and the drop from the DELNI
to the host system.

It is also against the rules to plug one DELNI into another one
unless that DELNI is NOT connected to an Ether.
-------

[Edward J Cetron]

David Roode writes:

>It is also against the rules to plug one DELNI into another one

>unless that DELNI is NOT connected to an Ether[net].

WRONG!!!!!!!!!!!!!!!! Nowhere does it say that one DELNI cannot be
cascaded to another ethernet connected DELNI! And I've check every manual,
tech and sales, Digital has produced....

The above is a very common misconception...the real configuration
rule has to do with the fact the propagation delays build through the
ethernet having to do with things like cable lengths, numbers of repeaters
etc....

The one rule that is most apropos is the 'no more than two repeaters
between any two nodes....' This is (among other things) to prevent the
propagation delay between any two nodes to be less than the max... For very
conservative purposes, the DELNI acts with a propagation delay of 1 repeater
and this sort of implies the above misconception.

In reality:
1. A DELNI actually appears to introduce the delay of about .6-.7 that
of a repeater.
2. I have seen systems with 2, 3, and 4 cascaded DELNI's that work and
DO maintain in-spec propagation delays.
3. It has been pointed out that with the appropriate configuration, one
could even make 4 repeaters in a row work.
4. The configuration rules (like the rs232 max line length rules) are
very conservative to allow 100% probability of a network working when they are
strictly adhered to. This means that quite often, for networks not in a very
critical area (i.e. hospital ICU's controlling patient therapy) they are only
to be considered guidelines BY PEOPLE WHO UNDERSTAND WHY THE RULES ARE THERE.
Rules are there for a reason, but should be tested and verified before blindly
obeying them without knowing why

-ed cetron
Center for Engineering Design
Univ of Utah
cetron%utah...@utah-cs.arpa

[John Leong]

On the subject of cascading DELNI, one has to be careful about one quirk.

The Intel 82586 chip has, by default, a check that is not really in
conformance of the Ethernet spec. It measures the time between transmit and
receiving the signal back from the transceiver and signals whether the round
trip time equates roughly to 50 meter of drop cable (nothing to do with the
Ethernet slot time of 51.2 micro seconds or roughly 2.5Km). This test is
quite bogus but some earlier version of DECNET software actually check for
that return code and declares the world is sick accordingly. We have ran into
that problem earlier on when we cascaded DELNI's since they introduced
roughly 10 meter of cable worth of delay and we ran over the 50 meters.

[David Roode]

Reference on DELNI limitation to non-cascaded mode when connected
to an Ether cable:

EK-ETHV1-IN-002 Ethernet Installation Guide, Digital Equipment Corporation,
1984, Appendix E, p. E1:

"If DELNI interconnects are cascaded, the top DELNI interconnect
cannot connect to an Ethernet coaxial cable. A maximum of two layers
of DELNI interconnects are permitted, with the second layer of
DELNI interconnects operating in the global mode."
-------

[Scott Brim]

Date: Mon, 15 Sep 86 18:49:31 edt
From: le...@andrew.cmu.edu (John Leong)

.....

The Intel 82586 chip has, by default, a check that is not really in
conformance of the Ethernet spec. It measures the time between transmit and
receiving the signal back from the transceiver and signals whether the round
trip time equates roughly to 50 meter of drop cable (nothing to do with the
Ethernet slot time of 51.2 micro seconds or roughly 2.5Km). This test is

quite bogus ....

You can tell the chip to disable that feature. I think it's *on* at
power-up. Gould used to use the round-trip-to-transceiver check; we
asked them to disable it and I believe it's now off for all new Gould
ethernet boards.
Scott