Using 2 Core Shielded Cable in Place of 3 Core

I don't think so; the common would be subject to electrical interference.

"... in RS485, one wire is for data transmitting, one wire for data receiving and one for common." - in fact, RS485 is a differential interface; two wires are for transmitting and the same two wires are for receiving - just not at the same time.

for RS485 there is no common, no 3wire concept, only 2wire + shield, ie 2core is enough.

Hello,

But in the wiring diagram, there are 3 wires with shield mentioned, TX+, TX- and GND with GND of drives.

I think its related with convertor, check the output terminals for RS485 only

While 2 core is usually enough, you will need to know more about the local installation.

There is an optional 3 SC connection on some devices that provides a common potential connection for the communications which becomes useful when the devices are a large distance apart. Once again, it really depends on the installation as to whether the SC connection is necessary. Note that the wire shield connection should not be connected to the SC terminal, rather it should be connected to earth to shield noise from the communications signal.

Wiki provides further information to shed some light on this: http://en.wikipedia.org/wiki/RS-485

If you are using Modbus RTU over RS485, then section 3.3.2 of the document in this link would also provide useful information. Note that even if it is not Modbus RTU, the theory for the hardware is still the same: http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf

Hope this helps.

http://www.chipkin.com/rs485-cables-why-you-need-3-wires-for-2-two-wire-rs485/

It would no longer be a shielded cable!

Yes you should be OK.

As others have pointed out RS485 is differential signalling: with common mode immunity from -7V to +12V. if the grounds at either end of the cable are reasonably coupled you don't need any common. If one of the grounds is noisy (turning fluorescent lights on and off on the same ring main as one end is a good way of demonstrating this) then even with a common wire you may run into problems: sticking a one to one transformer in at both ends with a centre tap coupled through a big capacitor to 0V will allow you to get away with almost anything.

See almost any RS485 data sheet:- http://www.ti.com/lit/ds/snls096b/snls096b.pdf - (sorry, link no longer available) for a better understanding of RS485.

2cores with shield is correct. The shield is used as common ground.

What I have concluded, I can use 2core shielded cable in place of 3core for my application.

did u check with peterjh's reply? based on that cant conclude like that, it mentioned common is not as shield, its common power supply, check with communication specialists.

How on earth can this be off topic: it's from the OP declaring the conclusion he has come to following the discussion. This is feedback we often don't get; is extremely welcome, and, is clearly on topic.

I have countered the vote.

Please be sure to give us feedback on how well it works.

As I stated earlier:
If you use the shield as your signal return, you no longer have a shielded cable. If your environment is quiet (EMI) you should be ok, but any noise pick up on the cable will be on the common return line(shield) and will be directly imposed on the signals- If you use 3 core shielded, with the shield earthed at one end only (standard practice) it will earth the environmental noise, while the twisted triplet in side will remain relatively noise free. If you want to use a three core cable, an unscreened twisted triplet would give better performance than a screened twisted pair with the shield used as return. The best solution is two twisted pairs with an overall shield, one twisted pair for each signal, and an overall shield grounded at one end only.

Interesting.

"If you use the shield as your signal return, you no longer have a shielded cable."

What would you describe as the main characteristic of differential signalling?

As one colleague suggested, i talked with a communication expert and here are his words;

"You will have to connect the shield to the data common on both ends, which will eliminate any shielding functionality it might have been providing.

A servo communications link is the sort of environment where you should be trying to implement the most noise-resistant cabling possible. I very strongly recommend that you use three-conductor cable with a separate shield".

So now I would not use 2core SH cable in place of 3 core SH cable.

Thanks for all participants in this thread.

Amazing! Here we are, right back at the beginning....

Yes Tornado, you are right but after discussions, every thing is clear now.

Sorry: I have to disagree.

First I should point out that with differential signalling, each wire effectively acts as the return for the other.

Now lets consider two situations.

1.) The noise between the grounds at either end of the link is less than the immunity provided by the RS485 receivers. You're good to go with or without shield or common.

2.) The noise between the grounds at either end of the link is greater than the immunity provided by the receivers. In this case your best bet is to connect the shield earth "at both ends" to try to pull the two grounds closer to each other. Connecting a common 0V wire in this instance could be catastrophic because you would be imposing the ground noise from each far end on the 0V of each local end.

If you are in this latter situation, and you don't want to try to connect the grounds, then your best bet is to provide a fully isolated comms. port at the slave end using optocouplers and a floating DC-DC supply. Or use signal transformers with their "end facing" centre taps AC coupled to the local 0V at both ends.

If u want to use 2 core cable due to availability, use two runs of the cable.

Gajanan Phadte