Overview of the Sercos Standard
The Sercos interface is a standardized interface (IEC 6149) for real-time communication between controllers, drives, servo drives, I/O devices, encoders, and other equipment requiring real-time services.
For motion control, the Sercos standard describes the internationally standardized digital interface for communication between a control unit and associated servo drives. It defines standardization of operating data, parameters, and scaling for machines with multiple drives that can be operated in torque, velocity, or position interface operation modes.
The main features of the Sercos interface are:
oRing topology (redundancy)
oMaster / slave system
oBaud rate 100 MBaud
oMinimum synchronization time of 1 ms (8 axes), 2 ms (16 axes), or 4 ms (24 axes)
oSynchronization (jitter < 1 µs)
Communication with Sercos interface is divided into two types:
oCyclical communication:
The cyclical communication is used for exchanging real-time data (for example, position) and is executed once in every communication cycle (CycleTime). Certain specified data are transferred from the controller to all drives and from all drives to the controller in every cycle.
The exchange of information between the motion controller (Sercos master) and the servo drives (slaves), is accomplished via a message structure known as a telegram. There are three telegrams defined by IEC 61491:
oMST (Master Synchronization Telegram): An MST telegram is broadcast by the master at the beginning of each transmission cycle to synchronize the timing of the cycle.
oMDT (Master Data Telegram): An MDT telegram is sent by the master once during each transmission cycle to transmit data (command values) to the servo drives (slaves).
oAT (Acknowledge Telegram): AT telegrams are sent by the slaves to the master (feedback values).
oNon-cyclical communication with function blocks.
Non-cyclical communication is used to exchange data such as parameters for configuring communication, the drive parameters, status, and so on, where time is not a critical factor. The controller controls non-cyclical communication. All of the parameters in the system can be contacted using this channel, even parameters that are configured cyclically.
NOTE: The two types of communication can be used simultaneously.
IEC 61491 assigns identification numbers (IDNs) to all the operation data in a Sercos drive. Operation data includes parameters, interface procedure commands, and command and feedback values.
There are two categories of IDNs available:
oStandards IDNs (S): They are defined by the Sercos standard IEC 61491. Standards IDNs, if supported by a Sercos drive, behave the same, irrespective of the drive manufacturer;
oProprietary IDNs (P): They are reserved for product-specific data that can be defined by the manufacturers of control units and servo drives.
