Thicknet/10base5 Test Segment: The Cable is In!

[systems_glitch]

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.

Thanks,
Jonathan

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 <
> 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
> 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.
> >>
> >
> > 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.
>
> 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
> 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
>
>