Profibus network using AWG#20 Shielded twisted pair

recycled_mind · Jan 2, 2013

Dear Automation experts,
I want to use the existing lonworks network which uses AWG#20 Shielded twisted pair for my Profibus system because I find difficulty in setting up a new Profibus cabling. This shielded twisted pair lonworks networks are properly installed in a 1/2 inch metal piping.

My question is can I use this without any issues?
Need your advise on my plans.

Thanks,
APS

JesperMP · Jan 2, 2013

You cant just exchange one type of shielded cable for another when using it with diverse bus networks.
It may work ok, or maybe not. Noone can give you a reassurance that it will work.

If you Profibus speed is 19.6k or 187.5k then maybe it will work.
If you Profibus speed is 1.5M then probably it will NOT work.
If you Profibus speed is 12M then certainly it will NOT work.

manmeetvirdi · Jan 2, 2013

Profibus cable specification

Hi
Just check out specification of profibus cable over here. Reproduced here for ease of reference

Cable specifications: What should I use?

The PROFIBUS standard defines two variations of the bus cable. However it is recommended to use cable Type A in all new installations.

Type A is recommended for high transmission speeds and permits a doubling of the network distance in comparison to Type B.

Type A Technical specification:

Impedance: 35 up to 165 Ohm at frequencies from 3 to 20 Mhz.
Cable capacity: < 30 pF per meter.
Core diameter: > 0,34 mm², corresponds to AWG 22.
Cable type: twisted pair cable. 1x2 or 2x2 or 1x4 lines.
Resistance: < 110 Ohm per km.
Signal attenuation: max. 9 dB over total length of line section.
Shielding: CU shielding braid or shielding braid and shielding foil.
Max. Bus length: 200 m at 1500 kbit/s, up to 1,2 km at 93,75 kbit/s. Extendable by repeaters.

You have to see if your cable matches above specs, if not then behavior cant be foretold. Just as a side note what I have seen is that if there are sharp bends then signal (voltage) drop is high.

Regards
Manmeet

RussB · Jan 2, 2013

Or just paint the shield "purple", that should do it.

Even if it meets those specs, remember if it is not purple (Profibus certified), it will not be supported by any of your device manufactures, so if you have a problem you will be on your own.

The Plc Kid · Jan 2, 2013

You may know what you did but think of the poor basta*d that has to come behind you and figure this out at 2:00 am 10 years down the road. Just get the correct cable and do it right the first time.Taking shortcuts like this will always come back to bite you in the arse.

As my grandpa always told me. "If you can't do it right just don't do it at all"

Rant Off.

recycled_mind · Jan 2, 2013

Thanks for the advises, I will be using the existing AWG#20 shielded twisted pair in Modbus network and will leave Profibus plans.

recycled_mind · Jan 3, 2013

The cable is Belden 8762.

Profibus network using AWG#20 Shielded twisted pair

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