29 Jun
2018
29 Jun
'18
6:31 a.m.
On Jun 29, 2018, at 7:32 AM, Tony Duell via cctalk
<cctalk at classiccmp.org> wrote:
On Fri, Jun 29, 2018 at 9:27 AM, Peter Coghlan via cctalk
<cctalk at classiccmp.org> wrote:
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
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!).