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

[Paul Koning]

> 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