On 06/28/2018 01:42 PM, Eric Smith wrote: > On Thu, Jun 28, 2018 at 2:20 PM, Chuck Guzis via cctalk > <cctalk@classiccmp.org <mailto:cctalk@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
