Interest Check: Belden Thicknet 10base5 Ethernet Coax

[Paul Koning]

> On Feb 2, 2018, at 5:22 PM, Grant Taylor via cctalk <cctalk at classiccmp.org> wrote:
> 
> On 02/02/2018 03:10 PM, Paul Koning via cctalk wrote:
>> More precisely: ground the cable at exactly one point.  Any point will do, but it must be grounded (because none of the taps provide a ground).
> 
> Why is that?
> 
> Is it in an attempt to avoid current loops / sneak current paths?

Yes, exactly.  And if the cable crosses between buildings, which at least for 10Base5 is plausible, they might have different ground systems.  If so, grounding both ends might produce a LARGE current through the cable, possibly enough to be hazardous.

(Somewhat different but similar: there's a story about the DEC building at Marlborough, which apparently had two separate power sources at the two ends, from different external supplies.  Each was grounded at the service entry, but the two were not bonded together (a code violation).  One of the machine rooms had branch circuits from both.  One system had a string of large disk drives, RP04 or the like, some fed from one branch, some from the other.  As required by the book, the drives were bonded together by substantial braided wire jumpers.  One of those got hot, possibly enough to melt it, because the two grounds were at different voltages and the "sneak current" was many amps.  I'm not sure if the story is true, but it sounded somewhat plausible.)

> I thought the outside of BNC connectors (et al) was typically bonded to the card edge connector, which is (ideally) bonded to the system chassis, which should be grounded either directly or indirectly.

Many coax connectors have the shell connected to the chassis.  But 10Base2 Ethernet connectors are required to be insulated: if you look closely you will (should) see a plastic sleeve between the jack shell and the mounting flange.  I don't remember for sure, but it may be that 10Base2 repeaters ground that end, or have an option to do so.  That would make sense because you usually have just one repeater on a 10Base2 segment, so grounding there is a logical thing to do.

The requirement for controlling grounding is also why 10Base2 connectors are often made with insulating sleeves.  For example, the ones DEC sold had plastic shells as an integral part of the connector assemblies (T connectors too).   Similarly, you might find plastic shells covering 10Base5 barrel splices or terminators (those were separate from the connectors themselves).

	paul