28 Jun
2018
28 Jun
'18
5:05 p.m.
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