S0 interface

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S0 interface with timing diagram

The S0 interface (pronounced S-zero interface) is a hardware interface for the transmission of measured consumption values ​​in building automation . The interface is defined in EN 62053-31. While standard signals are suitable for values ​​such as current, voltage, temperature or power, meter readings are transmitted with the help of pulses. The interface should not be confused with the S 0 bus within an ISDN installation.

The interface is used in the following measuring devices:

Such counters with pulse output are used to transmit pulses corresponding to a certain value to a receiver. The identification of the pulse value is the responsibility of the meter manufacturer. It can indicate the pulse valency: x (energy units) / imp or the pulse output frequency: x imp / (energy units).

Two different current / voltage levels are permitted. Class A: 25V / 8mA Class B: 14V / 2mA

This pulse output has a Lifezero signal . This means that the receiver and transmitter are able to differentiate between a broken cable and a 0 signal.

The pulse is defined as follows: 30 ms ≤ t_ON ≤ 120 ms 30 ms ≤ t_OFF

The output is technically considered an open collector (engl. Open collector ) of a transistor of one optocoupler is connected downstream. When connecting the interface, you must therefore pay attention to the polarity. There are classes A and B, A for long and B for short transmission paths. In class B, up to 15 V DC voltage can be connected , in class A up to 27 V DC voltage. The maximum current is specified as 15 mA or 27 mA, which corresponds to a resistance of 1 kΩ. A current of less than 2 mA corresponds to a LOW value, a current higher than 10 mA corresponds to a HIGH value. Common DDCs can work with this threshold. The signal is output by the counter as a pulse sequence, whereby a single pulse must last at least 30 ms.

The time of the HIGH phase can be problematic for further processing, as not all DDC inputs can process very short counting pulses (e.g. 70 ms).

An alternative to the S0 interface is the potential-free contact , as this is less problematic in use.