29 Jun
2018
29 Jun
'18
1:27 a.m.
On 2018-06-28 at 17:05:32 -0700, Chuck Guzis via cctalk wrote:
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.
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.
Although I have never come across one, if such a surge protection device was
available for a -232 circuit, I could see pin 1 of the DTE or DCE (whichever
end the device is at) being a semi-plausable place to pick up the ground
connection for it. However, I can't see any reason for continuing the
protective ground connection any further on beyond this device.
Could the standards have just been making provision for devices like this,
in case they were later found to be necessary/useful? However, I think
requiring a separate, permanently connected ground wire would be more in
line with telephony practice and more helpful to prevent equipment damage.
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.
I'm guessing a NEMA 5-15R is a grounded power outlet?
The purpose of the power circuit ground conductor is to provide protection
from hazards presented by the power circuit concerned.
Any -232 pin 1 ground
connection cannot hope to contribute to that and should not
depend on it either
because it could be called on to provide protection when the power cable
is not connected to an outlet.
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.
If the shield is being used to reduce noise on the line, surely it should be
connected to pin 7 at both ends? If the shield is being used to protect
against induced voltage surges, I think it will only have a chance of doing
that if some sort of surge suppression equipment is also present. If the
shield is being used to protect against accidental contact with a power
conductor, I don't think connecting it to pin 1 is an appropriate way to
achieve that.
But perhaps someone with a copy of the current TIA-RS232C document might
see if the wording has changed.
I would be very interested to know what it says on the matter as well as
what was said in the earliest standards.
Regards,
Peter Coghlan.