serial realtime communication system (SERCOS)

The sercos interface (Serial Realtime Communication System) is a standardized, digital interface between control, drive and peripheral devices. Development work began in the mid-1980s and can be traced back to an industry consortium that drew up the specifications for the real-time capable sercos interface. The activities, which led to the IEC standard 61491 in 1995, were accompanied by the ZVEI.

In the first version, the sercos interface had data transfer rates of 2 Mbit/s and 4 Mbit/s; the following generation, which followed in 1999, already had 8 Mbit/s and 16 Mbit/s. Further activities are reflected in Ethernet with the aim of achieving a continuity of transport protocols from commercial enterprise areas via enterprise networks and the Internet to production and automation.

The Ethernet implementation realized in sercos III supports a real-time function. Since Ethernet is not real-time capable by approach and uses a non-deterministic access method, namely CSMA/ CD, a time slot method was selected for collision-free transmission in sercos III and the Ethernet telegram, which is the Ethernet frame, was modified accordingly. Sercos III is characterized by data rates of 100 Mbit/s. The delay times of the transmitted data depend on the number of connected drives and are between 31.25 µs and 1 ms for 250 drives.

The sercos data frame is composed of telegrams for cyclic and non-cyclic communication. Sercos III networks can have a ring or line topology with a maximum of 511 nodes. Several sercos III networks can be synchronized with each other.