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

[Chuck Guzis]

On 06/28/2018 01:42 PM, Eric Smith wrote:
> On Thu, Jun 28, 2018 at 2:20 PM, Chuck Guzis via cctalk
> <cctalk at classiccmp.org <mailto:cctalk at classiccmp.org>> wrote:
> 
>     So, on a TIA/EIA/RS-232C DB-25 connector, what's the official position
>     on pin 1?  The standards calls it PGND = Protective ground and most
>     reference seem to indicate that this is chassis/earth ground at both
>     ends of a cable.
> 
> 
> Which is exactly why this kind of problem routinely happens with
> TIA/EIA-232 and RS-232. People try to run it hundreds of feet to
> somewhere with a significantly different ground potential, and if
> they're _lucky_, it only works badly, but often equipment gets damaged.
> 
> Since -232 doesn't have any galvanic isolation, even if you don't ground
> pin 1 at one end, it will still have the same problem on signal lines,
> because ultimately they're referenced to ground at both ends. If you
> _must_ run a long -232 cable, you should use some kind of isolator at
> one end.

The original standard is very old--it dates form 1960--a very different
time; DCE was strictly under the control of the telcos, and I suspect
that connection to DTE had to be approved by them.  In that sense, pin 1
serving as a "protective ground" might have made some sense--and has
been grandfathered in.  Note that the DE-9 connector version doesn't
include this signal.

As a matter of fact, I'm not even sure that NEMA 5-15R receptacles
existed in 1960, so grounding may have been a moot issue.

Mostly, I ignore pin 1 and will occasionally make use if it if I'm using
shielded cable--but only on one end--usually the DCE side.

But perhaps someone with a copy of the current TIA-RS232C document might
see if the wording has changed.

--Chuck


--Chuck