zemon@fritz.UUCP (Art Zemon) (01/17/85)
I just installed a cable between two buildings (under a
parking lot) and am very mystified by something. There is
180 Hz, 0.5 V peak-to-peak triangular wave on the lines.
Does anybody have any idea what might cause this or how to
get rid of it? Is this a "common" problem?
More information on my installation:
The cable is 12 twisted pairs of 22 guage wire. Although
each pair is not shielded, the cable has a shield
consisting of braid and foil. The cable shield is grounded
to AC ground at both ends.
The cable is about 600 feet long.
The cable runs through two electrical power distribution
rooms and past an industrial 480 V -> 208/115 V
transformer. Remember, however, that the interferance is
triangular, not sinusoidal.
A 60 Hz signal is also apparent on the cable but is of much
lower magnitude (about 0.2 V peak-to-peak).
The total voltage of the interferance is up to about 0.7 V
and this is too high for me to run my Micom 400 local
datasets above 2400 baud. What really bothers me about
this is that I had been running the same datasets over 6
*miles* of telephone company lines at 4800 baud without
problems.
Any help would be greatly appreciated! Should you wish to
call, my phone number is (714)966-2344.
--
-- Art Zemon
FileNet Corp.
...! {decvax, ihnp4, ucbvax} !trwrb!felix!zemonzemon@fritz.UUCP (Art Zemon) (01/19/85)
The problem is solved. Thanks to all of you that offered
helpful suggestions.
The cause was a simple open wire!
--
-- Art Zemon
FileNet Corp.
...! {decvax, ihnp4, ucbvax} !trwrb!felix!zemonrf@wu1.UUCP (01/23/85)
180 Hz sawtooths are common noises when three-phase power is involved. The
coupling between the noise source and your cable could be converting it from
a screwy triple sine wave to a sawtooth - especially if there are some
rectifiers around.
Some rules of cable installation:
- *Never ground both ends of a shield!* Ground potential very likely
differs at the opposite ends. Ground one end only.
- Make sure the ground you use is a good one -- if possible, check to
see that it is actually earth (cold water pipes are good). You
sometimes can use building green-wire ground - but check for ground
currents first.
- *Never use electrical neutral for grounding!* This one can literally
kill you -- be careful.
- Never ground either side of a pair. Keep your pairs isolated - that's
what line drivers are for.
- Don't use RS-232 between buildings - it is possible to damage your
computers if there are substantial ground currents.
Uucp: {decvax,ihnp4}!philabs!wu1!rf Randolph Fritz
Easylink: 62905318 Western Union Telegraph
phone: 1-201-529-6238 90 McKee Dr.
1-201-529-6221 Mahwah, NJ 07430