28 Jun
2018
28 Jun
'18
6:18 a.m.
On Jun 28, 2018, at 4:52 AM, Peter Coghlan via cctalk
<cctalk at classiccmp.org> wrote:
On 2018-06-27 19:34:38 -07:00, Chuck Guzis via cctalk wrote:
Yes, and indeed the Ethernet spec does that.
On 06/27/2018 04:19 PM, Antonio Carlini via
cctalk wrote:
I guess one of the keys to a successful networking technology is that it
should be possible to specify how to install it in a way that people not
familiar with the inner workings of the technology can readily deal with.
There are advantages to keeping the instructions as simple, short and easy
to follow as possible with a minimum of exceptions and special cases. No idea. But on thickwire the taps were all
supposed to be made at
specifically marked positions (for the reason given earlier).
Perhaps someone (incorrectly) thought that the terminator should also be
at such a position and so a terminator could not be located at a tap?
It had occurred to me to wonder if some poor tech had measured out, say,
151 meters of cable where 152.5 was called for by the "exactly every
2.5M spec). It would seem that any attempt to add an extra 1.5 was
believed to be called for in order to install a terminator would have
resulted in a "cure" worse than simple adding the terminator at the end
of the cable.
It might make sense to state that everything should be
spaced 2.5m apart
even when there is no advantage to this in the special case of terminators.
The advantage is in reducing the complexity of the instructions. The
disadvantage is it might lead to difficult cases like this one.
The spec is fine. What seems to happen is that people who don't understand EE made up
their own additional rules for no good reason. Section 7.6.1 talks about cable lengths;
7.6.2 describes transceiver placement. Those rules are clear and sufficient, but neither
says anything about terminator placement.
On a slightly different point, didn't the
thickwire spec call for the outer
conductor of the cable to be earthed at exactly one point, presumably for
safety reasons in case the cable contacted something at high voltage?
Yes, Ethernet spec section 7.6.3. Also for static discharge, though it doesn't say
that explicitly.
This requirement was somehow not carried forward into
thinwire, perhaps
because the entirity of a thinwire network, including the connectors was
supposed to be insulated and therefore not a danger to anyone? DEC produced
insulated thinwire connectors and terminators but other than that I think
this requirement was honoured more in the breach.
Looking at 802.3, it says that a Thinwire segment MAY be grounded at one point, but not at
multiple points. It also requires a static discharge path at each transceiver, 1 Mohm to
ground. So you don't necessarily have a hard ground for the case where the cable is
shorted to an AC power line -- I assume the reasoning was that this is unlikely enough it
doesn't need to be considered. The Ethernet spec doesn't have anything analogous
for 10Base-5 transceivers, so there the hard ground is necessary for a static discharge
path.
Both coax types, of course, require termination at each end. And both have a stated
requirement for all connectors to be insulated. In practice, you can be a little loose
with that if you place things so they stay away from other metal objects.
You may not see the terminator at both ends on thinwire, if you're dealing with
repeaters that are designed to sit at the segment end. Those have the terminator for that
end inside the box.
paul