Limit of cable lenght for Analog Inputs – Are the limits only applicable in Voltage mode?

Danw,

In SIEMENS documentation for S7 200, S7 300 and S7 1200 they put a limit of cable length for Analog inputs . In some cases the limit is 100m and in some cases the limit is 200m.

Now you could argue that this limits is only applicable in Voltage mode because the input impedance is very high. 

But when you talk to the SIEMEN support people they don't give any calculation to support the above limit. they simply say whether it is a 4-20mA transmitter or 0-10 V the limit is 200m. Above this there is no guarantee.

The reason i am asking all of this is because we have a existing old system which is working with AI  loops of at least 1300m. Now i need to be able to make a case if a change is necessary for the reliability of the system.

regards

Rashid IQbal

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Split from Permitted transfer distance for current signal 4-20mA.

hi,

The limitation is also because of interferences you will get on the wires. And each cable is also a capacitor.

The cable length in the technical data is the length which the development department of the manufacturer can say, yes it works. For anything else, they can´t give you a guarantee. If it works anyway, everything is fine. They can´t test any use-case. It is not common to have cable length more than 200m.

hope this helps,

regards,