Sercos III (Serial Realtime Communication System) is a hard real-time, full duplex bus with a transfer rate of 100 Mbit/s for communication between controllers, motion devices such as servo drives or frequency inverters, input/output devices such as bus couplers, other equipment such as encoders or other sensors, as well as standard Ethernet nodes. Sercos is based on the Ethernet standards IEEE 802.3 and ISO/IEC 8802-3. Interface definition, profile specification and mapping of profiles to network technologies are standardized in the IEC 61800-7 series.
Sercos uses a master/slave arrangement to exchange cyclic and non-cyclic data between bus devices in ring or line topologies.
Communication on a Sercos bus takes place at cyclic intervals. During each cycle, real-time data is interchanged between the bus devices using two types of Sercos telegrams:
oMDT: Master Data Telegram
oAT: Acknowledge Telegram
MDTs and ATs are sent by the master. The slaves read the data in the MDTs. The ATs from the master are initially “empty” with reserved slots for each slave. As an AT is passed on from one to the next slave, each slave writes to its reserved slot. Once the MDTs and ATs of a cycle have reached the last slave in the line or ring, they return to the master on the same way back through each slave (loopback). In the case of a ring topology, the master sends the telegrams simultaneously in both directions so that they pass through the slaves only once. This part of the communication cycle is referred to as the Real-Time Channel (RTC).
Sercos devices provide access at the communication ports for other protocols between cyclic real-time telegrams. The time slot of a cycle not required by real-time MDTs and ATs is reserved for the Unified Communication Channel (UCC). The UCC can be used for transmission of Ethernet telegrams (ETH) with IP-based protocols (such as TCP/IP and UDP/IP).
This way, any Sercos device can use and transmit data via other protocols regardless of whether Sercos is in cyclic mode or not, and without additional hardware to process tunneling. The Sercos specifications require Sercos devices to provide a store and forward method for buffering non-Sercos messages that cannot be processed because cyclic data is currently being processed.
