Sercos communication is based on a time slot process with a cyclical transmission of telegrams based on a master-slave principle. Cycle times are 31.25 μs, 62.5 μs, 125 μs, and multiples of 250 μs up to a maximum value of 65 ms. In addition to automation concepts with centralized signal processing, decentralized automation solutions are also created as a result of this bandwidth of cycle times.
In order to meet hard real-time demands despite using non-deterministic Ethernet, a communication cycle is divided into two time slots (channels). Sercos-defined real-time telegrams (Ethertype 0x88CD) are transmitted through a collision-free real-time channel. Parallel to this real-time channel, a UC channel can be configured, in which all other Ethernet telegrams (Ethertype <> 0x88CD) and IP-based protocols such as TCP/IP and UDP/IP can be transmitted.
Cycle times and the division of the bandwidth or bus cycle in the real-time and UC channels can be adjusted for each application.
With Sercos, real-time data is sent according to the IEEE 802.3 standard in cyclical telegrams with Ethernet protocol type 0x88CD. These make M/S, DCC and SVC, SMP and Safety communication mechanisms available. The exchanged data is addressable via standardized functional groups, classes and profiles.
Sercos differentiates between the following kinds of telegrams:
- Master Data Telegram (MDT): The master sends schedule data to the slave devices.
- Acknowledge Telegram (AT): The slaves send their status data to the master and to other slave devices.
The connected devices are recognized in the initialization phase (phase start-up with communication phases CP0 – CP4), and they are addressed and configured for application. Each slave is assigned a device channel in the MDT and the AT, which the slave either uses to read from or to write into. Depending on the amount of data, several MDT and several AT telegrams are sent by the master per communication cycle. The telegram transports data from device to device. The relevant schedule data is read at each device or the required status data is written in.