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Siemens AG
Entry type: FAQ Entry ID: 14340592, Entry date: 06/22/2010

Why is the voltage output of the S7-300 and S7-400 analog output group outside of the tolerance? What are the terminals S+ and S- used for?

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The following description applies to all SM 332 and SM 432 analog output modules.

The allocation for return inputs S+ and S- needs to be observed when using the SM 332 and SM 432 analog output modules. They serve the purpose of compensating the line resistance. When you connect the two contacts of the actuator with separate wires with the S+ and S-, the analog output adjusts the output voltage, so that the desired output voltage actually exists on the actuator. 

If the compensation is desired, the actuator must be connected with four wires. For example, for the analog output module 6ES7332-7ND02-0AB0 and for the first channel this means: 

  • You must conduct the output voltage from Pin 3 and Pin 6 to the actuator.

  • Sensor leads SO- and SO+ must be conducted directly from the actuator to Pin 4 and Pin 5. 

Fig. 01

If compensation is not desired, simply bridge the Pins 3-4 and 5-6 on the front connector.

In the case of open sensor leads (S+ and S-), the output voltage is readjusted by a maximum of 140 mV (for 10V).
For this connection configuration, the operational limit of ± 0.5 % for the voltage output cannot be kept.

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