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.
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.
A cycle involves three different
types of telegrams.
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.
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.
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.
for Cyclic Operation
After the MDT is sent, another MST is
transmitted by the master control, signaling the beginning
of another cycle, as illustrated.