Synchronization

The Master is responsible for sending a synchronization telegram (MST) at the beginning of each communication cycle. All Slaves (drives) will receive the MST and reset their clocks. In this way, all Slaves will run in phase lock with the Master's clock. Because all Slaves are in phase lock with the Master's clock, commands can be made active in all of the Slaves at the same instant. This means that the Master can coordinate motion between all axes without propagation effects distorting the motion profile. Feedback from the Slaves is handled in a similar manner, and is latched in all Slaves at the same instant.

Ring Timing

Data is sent and received by the Master and Slaves through communication telegrams. The communication telegrams are organized over the SERCOS cycle in the manner shown in the next figure.

The SERCOS ring timing is based on the data to be placed in the telegrams (AT, MDT), and on 6 drive parameters that are determined by the type and features of a drive (or drives), and 6 parameters that are written from the Master, with some of these parameters derived or calculated from the drive's timing parameters. The times at which the MDT and AT are sent (relative to the sending of the MST) are determined by the Master and sent to the Slaves during initialization.

During Phase 2, the Master reads parameters from the drives that determine what and when the drives are able to transmit and receive. Using this information and the desired telegram contents, baud rate and cycle time, the Master determines the timing and telegram parameters for each drive. The Master then writes these parameters to the drives.

Initialization

Before true synchronous data transmission can occur, the system must first be initialized. This is done through a series of communication phases (or just phases) in which data is first transmitted asynchronously. The data transmitted during these early phases is used to configure the Master and Slaves for synchronous data transmissions in later phases. The SERCOS protocol defines five phases.

On power-up, each drive or I/O module begins an initialization sequence. At this time, each drive and I/O module operates as a repeater, by simply passing received telegrams to the next device on the SERCOS ring.

The Master is only allowed to set the communication phase to the next logical communication Phase (Phase 0 -> Phase 1 -> Phase 2 -> Phase 3 -> Phase 4) or directly back to Phase 0. If at any time the Master attempts to switch a Slave into a Phase that is not the next logical Phase, then the Slave will immediately return to Phase 0. If at any time the Slave receives two invalid MSTs or MDTs consecutively, the Slave will also switch to Phase 0.

During Phase 0 no data is exchanged between the Master and the Slaves. All Slaves must be in "repeater" mode. This means that each Slave will retransmit any signal that it receives. In order to verify that the communications ring is intact and capable of sending telegrams, the Master begins sending Master Synchronization Telegrams (MST) through the SERCOS ring.

The Slaves (drives) simply pass the MST to the next drive in the daisy-chained ring, and eventually because of the ring topology, the MST returns to the Master (i.e., Master will receive its own MST). Phase 0 is completed when the Master receives 10 consecutive MSTs. After the tenth consecutive MST, the Master changes the phase information in the MST to 1, which commands all Slaves to switch into Phase 1 operations.

During Phase 1, the Master sends out an MDT with the address of a specific Slave in the system. If present within the system, the Slave with the specified address responds by sending an AT back to the Master. The AT that is sent is rudimentary and is intended solely as a confirmation that the addressed Slave is in the system. The Master then repeats this query for all target Slaves. (Note that not all Slaves in the system will be target Slaves. Target Slaves are chosen by the application before initialization begins.)

When the Master receives an AT from each target Slave (in response to a query), the Master changes the phase information in the MST to 2, which commands all Slaves to switch into Phase 2 operations.

During Phase 2, the Master sets up the configurable data portion and calculates the duration and time slots within the SERCOS cycle for all telegrams to be used in Phases 3 and 4. The Master also determines the slave operation mode for all Slaves. In order to do this, the Master requires certain data from the Slaves. The Master obtains this data by sending an MDT addressed to a specific Slave, that uses the MDT's Service Channel to query the Slave for the required data

The Slave responds by sending an AT containing the appropriate data, in the AT's Service Channel. Once the Master has determined all parameters, it sends them to each Slave via the MDT's Service Channel. In Phase 2, the Service Channel is active in both MDTs and ATs.

Once all data is transmitted to the Slaves, the Master will initiate the Communication Phase 3 Transition Check S-0-127 procedure for each Slave.

The Communication Phase 3 Transition Check checks the validity of all the data, and if all the data is valid, then the procedure executes successfully. If any data is not valid, the procedure fails and the IDN number of the invalid data is placed in IDN-List of Invalid Operation Data for CP2 S-0-21.

After all Slaves have completed the procedure successfully, the Master changes the phase information in the MST to 3, which commands all Slaves to switch into communication Phase 3.

In Phase 3, the real-time synchronous and asynchronous communication starts, and the Master uses the Service Channel to configure and parameterize (write parameters to) the Slaves. The parameters set in Phase 3 are application-oriented (e.g., conversion factors). During Phase 3, all communication telegram parameters sent in Phase 2 become active. Note that although Configurable Data (in ATs and MDTs) is present in the communication telegrams, some of that Configurable Data may not be valid until Phase 4.

The Master will still send the MST at the beginning of the SERCOS cycle, but will now also send an MDT with a global address at a specific time (all Slaves will receive a global telegram). Each Slave will transmit its AT during its specified time slot. The communication telegrams now contain the Control/Status Word, a Service Channel, and the Configurable Data.

When the Master has finished sending parameters to the Slaves, it will initiate the Communication Phase 4 Transition Check S-0-128 procedure for each Slave.

The Communication Phase 4 Transition Check checks the validity of all the data, and if all the data is valid, then the procedure executes successfully. If any data is not valid, the procedure fails and the IDN number of the invalid data is placed in IDN-List of Invalid Operation Data for CP3 S-0-22.

If the Communication Phase 4 Transition Check executes successfully and the Phase information in the MST is equal to 4, the Master switches the drives to Phase 4.