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Entry type: FAQ Entry ID: 42087579, Entry date: 03/29/2010
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How can you use analog 0-20 mA signal modules and signal boards with 4-20 mA signals?

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Description
You can use analog 4-20 mA input and output signals with the analog 0-20 mA signal modules and signal boards. You can scale the signal range of the analog inputs and outputs using the "Scale_current_input" and "Scale_current_output" predefined functions (FCs), which you can find attached to this FAQ.

Adding the "Scale_current" Global library

  1. Download the attached library, and unzip it. Open the "Libraries" task card.
  2. Click the "Open global library" button. The "Open global library" dialog window opens.
  3. Browse for the "Scale_current" library folder, and open the "Scale_current.al10" file. Now, the library is displayed in the "Global libraries" pane in the "Libraries" task card.


Fig. 01

Scaling of the analog input  
The range of 0-20 mA without the "Scale_current_input" instruction corresponds to the PLC’s internal signal range of 0-27648. The "Scale_current_input" instruction adapts this internal range to 4-20 mA linearly, starting with "0" for 4 mA and ending up with "27648" for 20 mA. A limit for wire break monitoring can be chosen by hand.


Fig. 02
 

No. Scaling of the analog input
1. Insert the "Scale_Current_input" instruction into a network
Browse the Global libraries tree for the "Scale_current_input" FC. Drag and drop this FC into a network of your S7-1200 PLC program.


Fig. 03

2. Scaling the analog input signal
Connect the contact "w_input_0-20mA" (2) to your analog hardware input (for example, IW96). You receive the scaled value at the contact "r_input_4-20mA" (4).

Note
For any input value lower than 4 mA the signal at "r_input_4-20mA" (4) will be set to "0". Accordingly, any input values higher than 20 mA will not exceed the maximum of "27648".


Fig. 04

3. Wire break monitoring
At the contact "r_wire_break_limit" (1), enter the limit for the wire break monitoring. The value "2764.8" for example will set the wire break limit to 2 mA. This means, if your input current is lower than 2 mA, the contact "b_wire_break" (3) will be set to "1".
Table 01

Scaling of the analog output   
The range of 0-20 mA without the "Scale_current_input" instruction corresponds to the PLC´1 internal signal range of 0-27648.
The Scale_current_output" instruction adapts this internal range to 4-20 mA linearly, starting with 4 mA for "0" and ending up with 20 mA for "27648".

Fig. 05
 

No. Scaling of the analog output
1. Insert the "Scale_current_output" instruction into a network
Browse the Global libraries tree for the "Scale_current_output" FC. Drag and drop this FC into a network of your S7-1200 PLC program.


Fig. 06

2. Scaling the analog input signal
Connect the contact "r_output_4-20mA" (1) to your software value. You receive the scaled value at the contact "w_output_0-20mA" (2) to send it to your analog hardware output (for example, QW96).

Note
For any input value lower than "0", the signal at "w_output_0-20mA" will be set to 4mA.
Accordingly, any input values higher than "27648"will not exceed the maximum of 20mA.


Fig. 07

Table 02

Requirements

  • S7-1200 PLC
  • Ethernet cable
  • signal board / signal module for analog output / input
  • STEP 7 Basic V10.5

   S7-1200_Scale_current.zip ( 348 KB )  

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