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Introduction to Sercos interface
Applications Types
Sercos III
Sercos I and II
Fiber Optics
Timing & Interface
Sercos IDNs
I/O Functions
Cycle Times
Cyclic Operation
System Safety
Packaging Profile
Sercos Standardization

1. Telegram Format / Telegram Sequence
  2. Master Synchronization Telegram
  3. Amplifier (Drive) Telegram
  4. Master Data Telegram
  5. Service Channel / Cyclic Operation Example / Error Correction

Cyclic Operation

Sercos interface devices communicate and stay in sync by sending each other a series of telegrams. A telegram is a rigidly defined bit steam carrying data and timing information.

Telegram Format

Standard Telegram Format for Sercos I and II

All Sercos I & II telegrams consist of five major fields, as illustrated. Telegrams are transmitted in the NRZI (non return to zero inverted) format. The transmitted signal remains at a 0 or 1 as long as the bit is a 0 or 1. In order to ensure edge synchronization and to prevent the delimiter pattern from reoccurring, the master forces a signal change every six bits by utilizing a “bit stuffing” technique of inserting a zero after five consecutive ones.

Telegram Sequence

A cycle involves three different types of telegrams.

  1. First the master (CNC or motion control) transmits a Master Synchronization Telegram (MST). The MST is used as a time mark for all slaves (drives) to determine when to talk on the bus, when to acquire feedback signals, and so forth.

  2. At a predetermined time after the end of the MST, the first drive in the system places its data on the bus in an Amplifier (Drive) Telegram (AT). Each drive follows in turn, all synchronized off the MST. During the initialization phase, the drives are instructed when they should transmit their message with respect to the MST.

  3. Finally, the master sends out a Master Data Telegram (MDT), which is one long message with space set aside for each drive in the ring. The drives have been previously instructed where their data is located within the MDT. As the MDT is received by a drive, it "fast forwards" to the start location for its information, and retrieves its data.

Timing Diagram for Cyclic Operation

After the MDT is sent, another MST is transmitted by the master control, signaling the beginning of another cycle, as illustrated.


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