Thanks for the clarifications, Paul!
Indeed, some thinnet devices do have terminators built in. On a fair bit of
Allied Telesyn gear, there's a switch for it.
On Thu, Jun 28, 2018 at 9:18 AM, Paul Koning via cctalk <
cctalk at classiccmp.org> wrote:
On Jun 28, 2018, at 4:52 AM, Peter Coghlan via
cctalk at classiccmp.org> wrote:
On 2018-06-27 19:34:38 -07:00, Chuck Guzis via cctalk wrote:
> On 06/27/2018 04:19 PM, Antonio Carlini via cctalk wrote:
>> 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
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.
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
to follow as possible with a minimum of
exceptions and special cases.
Yes, and indeed the Ethernet spec does that.
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
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
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
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
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.