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

[Brent Hilpert]

On 2018-Jun-29, at 1:27 AM, Peter Coghlan via cctalk wrote:
> 
> A telephony connection is the most plausable theory I have come across yet.
> I can remember devices that looked like large junction boxes with a ground
> connection that were installed where an overhead telephone line entered a
> building.  They contained a fuse in series with each line conductor and
> a surge arrestor consisting of a spark gap and/or a VDR from each conductor
> to ground.  I think the theory was that they might provide some protection
> against brief high voltage spikes induced onto the line by thunderstorm
> activity.  I think they might have been more trouble than they were worth.

Over here at least, they are still a standard part of any telephone landline.

Many decades or a century ago they were a big white 4*5 inch ceramic block mounted in the basement, with two long red (fusible?) resistors in series with each pair-wire and a double carbon-block spark gap to ground underneath a black bakelite knob.

Now they're built into the house-side connection box and you might not even know there is something besides the connection terminals there.

How often they actually come into active play I have no idea, but they still strike me as a good idea when you have miles of overhead line back to the CO.



> If the shield is being used to reduce noise on the line, surely it should be
> connected to pin 7 at both ends?

Surely you'll often get away with such, but "surely it should be connected to pin 7 . ."? No.

I don't know whether the -232 spec actually refers to pin 1 as "protective" ground, but regardless, "protective" here doesn't have to mean the same thing as "protective" in the mains wiring arena (or provide the identical functions).

Protective ground in the mains arena serves both to flatten voltages and bleed off slight currents from leakage and L/C coupling, and to carry enough current to blow the mains breaker in the event of shorts to chassis.

The "protective" ground on a -232 connection could be performing at least two functions: the afore-mentioned neutralisation of leakage, and noise suppression.
It is not at all insensible to separate those functions into a separate wire from the wire providing signal-circuit continuity (signal common / signal ground).
You don't want currents or voltages from leakage or noise upsetting the signal-circuit common level.
I don't think anyone was intending the -232 protective ground to provide the 'blow the mains breaker' function.

Yes, sometimes the noise/leakage/signal-common functions are all combined into one wire, or stated alternatively, sometimes the environment and needs allow one to combine them into one wire.
Consumer audio for instance - where the shield is commonly both the signal common and noise/hum shield.
But even in that low-requirements arena you might remember turntables - where there were two channel-cable common/ground shields but there was also a separate chassis ground wire.

Equipment, esp. back in the 60's, might not have a mains ground. Even in equipment that does, signal common/ground and chassis ground may be separated.
Relying on the signal ground wire to provide all these functions when connecting equipment with varying grounding policies is asking for problems. 

-

Speaking of abusing RS-232, in our CS dept (UBC) in the 80s when we moved our project offices to another room some ways away from the machine room we were faced with getting terminal cabling between them.
'Properly' this would involve calling in the physical plant dept or the telco to run new wire or make twisted-pair connections on the telephone wiring.
However, a little OCD observation of wall panels over my time there, some speculation, and some continuity testing allowed me to figure out there were some
little-to-unused 100-pair cables terminating in punch-blocks in panels in various rooms, so with a little punching between blocks within a panel I was able to get
continuity between the machine room and our new room.
No new wire-runs and no bureaucracy involved.

We used -232 over those connections. Something makes me think I reserved 3 pair per line but used 2 (4 wires): XMT, RCV, DTR to let the terminal switch know a terminal was present, and signal-GND. 

The university timesharing Computing Center, to provide terminals across the large campus, used 422-style balanced-line signaling.
Every CC-connected terminal around campus had a little 3*4*6 box sitting with it, with 3 LEDS (power, XMT, RCV), and containing dual power supplies (232 terminal side & 422 'campus' side), two opto-isolators (XMT & RCV) and assorted drivers. Each connection was 2-pair (XMT,RCV) over the telephone wiring physical plant.
There were various manufacturers (Gandalf and Develcon come to mind) that made such devices, but in UBC's case the computing centre made their own.
I saved what is probably the last existent one when I ran across it in a radio-musuem donation pile years later but years ago now.