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Re: Serial Connection

Moczik Gabor put forth on 3/28/2011 12:01 AM:
> Stan Hoeppner wrote:
>> We bought DB25 plugs in bags of 100, and used spooled CAT5 as the noise
>> rejection is many times that of CAT3, allowing greater distances across
>> sprawling warehouses.
> RS-232 uses single-ended signaling and requires shielded cable, twisted
> pair doesn't help either. 

Interesting to see you state this, considering the experience of many IT
folk stuck supporting legacy serial terminals and printers proves the
opposite to be true.  Back in 1996/7 I worked for such a legacy
equipment using company, a chemical producer in St. Louis.  We were
routinely (weekly) making 200+ ft. CAT5 UTP (24 AWG solid plenum cable)
runs across warehouses and production buildings.  The connections were
between RS232 Wyse terminals, thermal label printers, and ethernet
terminal servers at 9600 bps, using 3 conductors, one twisted pair
(orange/white-orange) and one single conductor of another pair (green)
for ground.  We could reliably run 115,200 bps on runs up to 50 ft.
using CAT5 UTP.  The cables distances we routinely achieved were
obviously many times the RS232 spec maximums.

> The common-mode noise rejection of twisted
> pair only works if the signals connected to a balanced input receiver.

Twisted pair cabled achieves its rejection performance against EMI/RFI
simply due to its design, not whether the signal is balanced or not.  If
any non-negligible length of the cable is in proximity to a noise
source, having twisted conductors minimizes the potential pick up area
of any one conductor.

The single largest user of twisted pair cable, its inventor and
progenitor actually, was AT&T (the old monopoly AT&T), now known as many
different companies after the 1980s breakup of the AT&T monpoly and the
creation of the "baby Bells", now known as Verizon et al.  CAT1 and CAT3
UTP have been used for telephones for more than 6 decades, and I'm
pretty sure everyone knows telephones don't use balanced signals.

You should read about the history of UTP cabling.  It's pretty
interesting, for a geek anyway.


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