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@classiccmp.org> wrote: > > > > On Jun 28, 2018, at 4:52 AM, Peter Coghlan via cctalk < > cctalk@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 > >
