Signal weakening

Hello guys

I have a question that seems to be an easy one, but I can't seem to find a solution for it:

I have to install sensors on a machine, and have to find a way to connect them to the PLC (S7-400) or to an ET200S island.

My question is: is here a way to calculate the effect of the length of the wires on the signal current? (or as the title of this threat says: the signal weakening)

Sure, how greater the distance is to the PLC, how lower the signal drops when it arrives at the PLC. But I need to have exact figures...

Is there someone who could help me?

Thanks in advance :-)

Hi, 

Check the Technical Specification of the card of you are willing to use, under the "Length of cable" section.

If you need any further help please do not hesitate to ask!

It's the 6ES7431-0HH00-0AB0.
The cable length there is 200m. Does that mean that over a length of 200 m, the transmitted current does not change?

I ask this because I'll use pressure sensors that transmit current between 4 - 20 mA, and I wonder if the current decreases when i use a long cable...

True.

So when I use a sensor which transmits current, th cable length is not a big problem, if I keep to the cable length mentionned in the data sheet of the card, right?

Hi, 

For long cable runs you should check for resistance against the sensor selection data. If the load is too high you  can burn the card. 

See the SIMATIC S7-400 S7-400 Automation System Module Data under chapter "5.22 Analog input module SM 431; AI 16 x 13 Bit (6ES7431-0HH00-0AB0) " for further info.

Current inputs do not measure current, they measure the voltage drop across a resistor.  The longer the cable, the more voltage drop in the cable, the less voltage drop across the resistor and the greater the inaccuracy.

Edit: Ignore above.

The higher resistance presented to a current source, the higher the voltage that it is required to source (V=IxR).  There is a limit to what voltage can be provided to provide the current, so there is a distance limitation.

Hi, 

 Check the sensor data in the SIMATIC S7-400 S7-400 Automation System Module Data under chapter "5.22 Analog input module SM 431; AI 16 x 13 Bit (6ES7431-0HH00-0AB0)", page 302, "Sensor Data".  

Anyways, it is common sense is to use a Remote IO when you need to have long cable runs. 

True. I just decided with my boss to use ET200S remote I/O's, so that I elliminate that risc.

it is the first time that I implement analogue sensors to a machine, so I've been searching for a week now how to do it. But now, it all seems to fall in the right place for me. :-)

Thanks for the answers! :-)