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I have 4 analog signal 4-20 mA, and i have to connect them to the nearest RTU ( S7-400 PLC), but the problem is the distance between the transmitter and the RTU, which is 3000 m.??? so the instrument cable will no go for such big distance, i don't know what to do, any help please !!

There are all kinds of options. Retransmission and wireless being the most obvious.

Omalaheem,
The cable length should not be the problem. You can increase the voltage to overcome voltage drop. The cable cost may make it more feasible to use a wireless link

Regards
Roy

I would suggest that you do the following:

1. setup an S7-300 PLC close to the point where you are picking the analog signals.

2. install two optical link modules (OLM) one at the S7-300 plc end and the other at the S7-400 plc end, and connect the two with fibre optic cable.

3. send/receive you analog signals in digital format between the two plcs, and conver to analog using the S7-300 plc.

This should work. Siemens have a extensive info on their website re such setups.

The most common solution is to install I/I stations every 500 meters. This breaks up the loop into six loops, all individually powered. I/I converters are inexpensive, and all you have to do is provide power and a very small enclosure to house the converter.

Walt Boyes
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If you have line of sight Spread Spectrum Radio will be much more cost effective than running a cable. You would need a wireless system with 4 inputs. If you already have a cable installed measure the resistance of one pair (short at far end) and let us know, up to 1500 Ohms shouldn't be a problem.

Another option is a Data Concentrator, Moore Industries, these convert a number of analogs into a digital signal that is re-constituted at the far end.

Roy