Hello,

I have cpu 1214 dc/dc/dc and when I go online I see that monitor value of analog input display some numbers from 0 to 34 (as you see in attachment) despite it is disconnected.

is it normal to see this value or it must be 0 ?

Hello.

Thank you for the simple animated GIF you shared. the animation is simple, short and to the point.
 

Med_selmi

Hello,

I have cpu 1214 dc/dc/dc and when I go online I see that monitor value of analog input display some numbers from 0 to 34 (as you see in attachment) despite it is disconnected.

is it normal to see this value or it must be 0 ?

The varying analog is expected, especially because it is not connected to anything. Recognise that the pins used by the analog signals are not connected (anchored) to anything; these are floating.

It is recommended to bridge the analog input pin to the reference pin. Connect also the reference pin to the reference of the power source. (Negative of the 24V power supply.)

A floating unused analog input is not - if the potential between the pins remains small. The best is to have the pins anchored.

What do you see when you follow the recommendation?

BR

#dP

Hello.

Thank you for the simple animated GIF you shared. the animation is simple, short and to the point.
 

Med_selmi

Hello,

I have cpu 1214 dc/dc/dc and when I go online I see that monitor value of analog input display some numbers from 0 to 34 (as you see in attachment) despite it is disconnected.

is it normal to see this value or it must be 0 ?

The varying analog is expected, especially because it is not connected to anything. Recognise that the pins used by the analog signals are not connected (anchored) to anything; these are floating.

It is recommended to bridge the analog input pin to the reference pin. Connect also the reference pin to the reference of the power source. (Negative of the 24V power supply.)

A floating unused analog input is not - if the potential between the pins remains small. The best is to have the pins anchored.

What do you see when you follow the recommendation?

BR

 Thank you very much, very useful recommendation. I tried first to bridge the analog pin to the reference pin (for having the same potentiel) but it doesn't work. So in the second time I connected only the refernce pin to the negative of power source (without connecting analiog pin) then when I go online I see anlog input value is 0.

But shall I retain this connection even when I use 0..10v source power?

Hello.

<sub>reference voltage:
The negative pole of the 24Vdc power supply that is connected to the PLC. These are connected to the M ("minus") pin of PNP inputs.

reference for 0-10V analog:
The base reference from which the analog voltage is measured from. Depending in the hardware the pin could be marked with an xM or AIx- (x is a number).</sub>

Med_selmi

Thank you very much, very useful recommendation. I tried first to bridge the analog pin to the reference pin (for having the same potentiel) but it doesn't work. So in the second time I connected only the refernce pin to the negative of power source (without connecting analiog pin) then when I go online I see anlog input value is 0.

But shall I retain this connection even when I use 0..10v source power?

Your instinct is correct. Assuming the analog inputs on the S7-1200 CPU. The analog input on the CPU is most stable if the reference pin (2M) is connected to the reference voltage. It implies further that the device supplying the analog is connected to the same reference voltage.

Not always is it possible to connect an external device to reference our PLC uses. The device might be powered by another power source than the PLC or is a far distance away. The analog reference is only connected to the analog source. This cause some problem if you would use the onboard to CPU analog inputs. Extension cards are better suited for this task.

As alternative a potential isolation device may be used. It is especially helpful to use if an analog signal source comes from an external machine. For this case the analog reference can be connected to the reference voltage and the reference of the analog source.

BR 

Thank you very much #dP, as usually you are the best one on the best moment for giving support.

Can you, please, give me example of a potential isolation device that may be used with external voltage source?

Hello.

Med_selmi

Thank you very much #dP, as usually you are the best one on the best moment for giving support.

Can you, please, give me example of a potential isolation device that may be used with external voltage source?

Do not worry - I will send you the bill for all the support you are receiving.

The following link should be most helpful to you: SIGNAL CONDITIONERS. Read the detail text also.

Follow this information up with 'walking' through the factory where you are working. See how the analog signals are treated. Also identify the places where the service personal is regular having problems with analog signals. Think if you can identify possible causes.

#dP

Hello.

Med_selmi

Thank you very much #dP, as usually you are the best one on the best moment for giving support.

Can you, please, give me example of a potential isolation device that may be used with external voltage source?

Do not worry - I will send you the bill for all the support you are receiving.

 I think that I need to work 100 years to pay your bill 

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