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

[Paul Koning via cctalk]

> On Jun 29, 2018, at 7:32 AM, Tony Duell via cctalk <cctalk@classiccmp.org> 
> wrote:
> 
> On Fri, Jun 29, 2018 at 9:27 AM, Peter Coghlan via cctalk
> <cctalk@classiccmp.org> wrote:
>> 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.
>>> 
> 
> I thought the original use of the RS232 interface was between a terminal and
> a modem sitting right next to it. In which case there is not likely to be much
> voltage between the protective grounds of the 2 devices (well, unless there 
> are
> serious problems with your mains wiring!).

Yes, and the specified distance limit for RS232 is quite short, 50 feet or so.

People have used RS232 over longer distances, of course, and gotten away with 
it.  Ditto for current loop; I remember our  college RSTS timesharing system 
which had current loop terminal connections running between buildings, the 
longest run was probably 1000 feet or so.  Worked well enough, even though one 
of the terminals on the long runs was a Beehive video terminal, probably 1200 
baud.  (Most of the rest were either ASR33 or TI Silent 733.)  Current loop of 
course doesn't have ground differential issues, but signal integrity on runs 
that long is still a concern.

        paul