RS232 over CAT - shielding

I need to go ~10 meters (33ft) from the Arduino to a motherboard's COM port.
I know RS485 would be better but I did read that people managed this distance with RS232.

I'm not sure if it would be a good idea to ground one twist or not.
The angel tells me it would shield but the devil says if there is current traveling
in the ground then it injects noise in the data lines.

First a correction to your thinking. A shield does not carry any current. It is connected to ground at one end only! Otherwise it is not a shield!

Second, CAT5 cable consists of 4 twisted pairs of solid wires. One should be signal and the second of the pair should be a ground connected closest to the source of the signal.

In real life, you can do whatever you want. For RS-232 signals, use one wire of a twisted pair for signal and the second wire of the pair for ground. that will give you the best noise immunity. You do not need another wire for ground.

Paul

See my schematic. Isn't it what you describe except i connected both ends?

But what do you mean by "You do not need another wire for ground."?

And you say "It is connected to ground at one end only! Otherwise it is not a shield!" I have a book that is called "EMC - effective construction of low voltage installation" and is from the VDE (german association for Electrical, Electronic & Information).

Cable shielding needs to be connected on both ends at the termination technique"

I have another book called "EMC-handbook for electronics engineer".

Earthing of screens: In former times in control engineering the screening was connected only on one end. That had the advantage that no balancing currents could travel. But this method shields only against up to 16 kHz. Cables in IT should be earthed on both ends.

If i remember right i did read somewhere that if you connect only one end then you have a antenna.

Nobody here in Germany is using UTP. Most installations are done with S/FTP. So there are two things. 1. One wire of a pair to ground 2. The CAT screen/shield

Normally the ground of the power supply is connected to the shield/case. That means CAT screen and GND are linked.

And any current in the shield results in a transformer primary and all wires inside are the secondary windings.

Paul

MrGlasspoole: Nobody here in Germany is using UTP.

No one at all ?

I find that hard to believe.

Paul_KD7HB:
And any current in the shield results in a transformer primary and all wires inside are the secondary windings.

What to you mean, want to tell me?

srnet:
No one at all ? I find that hard to believe.

It is rare.

I try to translate from a german electronic site:

SFTP cables require that the electrician also takes care of the potential equalization to discharge the currents that arise in the cable shielding. This requires a corresponding training.
In Germany and most parts of Europe this is general knowledge of electricians.
Worldwide it looks very different. There twisted pair cable is laid and mounted from non-professionals and the shielding will be cut off.
If the shielding is not earthed/grounded it can behave like a antenna and makes statics/EMC worse.

Even the old telephone cables here have a foil screen. And our house is build 1986.
cable.jpg

cable.jpg

Phone cables are audio, ie low voltage analog, not digital.

Screening requirements are very different for the two.

Found some PDF from a german electronics manufacturer where it's also in English: http://xfel.desy.de/localfsExplorer_read?currentPath=/afs/desy.de/group/xfel/wof/WPG/WPG04-operation/wp39-emc/Meetings/060710_BuK_Erdung.pdf

Page 40: Shields and protective conduit must, if possible, be connected to ground at both ends.

Or also in english under 1.4.5 (When screens cannot be connected at both ends): http://www.compliance-club.com/archive/keitharmstrong/design_techniques1.html

And another EMC german engineer site says also: Shielded data cable (Buscabel, Ethernet, Ethercat...) are connected on both ends.

MrGlasspoole: What to you mean, want to tell me?

A varying current in the shield will produce a varying magnetic field. That will induce a voltage in the wires that are in the field, according to Faraday's law of induction.

aarg: A varying current in the shield will produce a varying magnetic field. That will induce a voltage in the wires that are in the field, according to Faraday's law of induction.

The question was more if he wants to tell me i should not connect the shielding on booth ends if the books they the opposite.

Read the links and what i translated in all my postings. In Germany we have the VDE (german association for Electrical, Electronic & Information). They make the policy every craftsman with a company or manufacturer has to follow. If they don't and something happens then no insurance will pay.

Then there is the CE marking: https://en.wikipedia.org/wiki/CE_marking If you want to sell something and it needs to fulfill the EMC Directive: https://en.wikipedia.org/wiki/List_of_common_EMC_test_standards

You have to bring the stuff to a laboratory where it gets tested. If you don't and sell it and it does disturb your neighbors radio then you have to pay a lot off penalty.

So how does the statement "A shield is connected to ground at one end only! Otherwise it is not a shield!" match all that if the books say "Cable shielding needs to be connected on both ends"?

Again the English site: 1.4.5 (When screens cannot be connected at both ends): http://www.compliance-club.com/archive/keitharmstrong/design_techniques1.html

If you connect shields at both ends for a high speed(*) unbalanced digital signal cable, you will get
a lot of problems with induced noise spikes being injected into the grounds, which for an unbalanced
signal can have similar effects as if you put it on the signal wire. I am talking high frequencies here
where the stray inductance dominates the situation.

Induced voltages are the same for all conductors in the cable, but the current depends on the
impedance of the conductor going into the equipment - typically the shield is the lowest impedance
carries significant current - connecting this to ground at the input of a circuit will play
havoc with that ground voltage relative to other parts of the circuit at short time-scales if there
are noise spikes around.

You often see ferrite beads on a cable at each end, these help block induced currents on the shield
as well as reducing RFI emitted by the cable. Ferrite acts as a high impedance at high frequencies,
but only to the common-mode currents, differential is unaffected.

The other function of a shield, to protect against capacitive pickup, is completely different, and
only requires connection in at least one place to ground. It too can inject currents into ground.

Best practice is to ground the shield at the transmitting end, but not the receiving end (an RFC
can be used for DC continuity). Bidirectional cables are more problematic and the real solution
is to use differential signalling (like USB, Firewire, ethernet, LVDS video cables, etc etc).