BECKHOFF BK7500 - Manual (Page 9)

Basic information
BK7500 9
The operating modes of the bus coupler
Initialisation of the coupler When it is first switched on the bus coupler carries out a self-test to check
the functions of its components and the communications of the K bus, and
while this is going on the red I/O LED will flash. When the self-test has
been completed successfully, the bus coupler will begin to test the
attached bus terminals (the "bus terminal test”) and read in the
configuration from which it constructs an internal structure list, which is not
accessible from outside. If an error occurs the bus coupler will enter the
operating mode "STOP”. If the start-up sequence is completed without
errors the bus coupler will enter the mode "fieldbus start”.
The BK7500 is now in phase 0 (P0). In this phase it sends telegrams that it
has received to the next device in the ring (repeater function). In phase 0,
the master sends master sync. telegrams (MST). If these are received ten
times without interruption, the master switches to communication phase 1
(P1). Master data telegrams are now also sent. Each slave has its own
station address. The slave answers with a drive telegram (AT), and in this
way indicates its readiness for changing the phase to communication
phase 2 (P2). After the master has switched to phase 2, the SERCOS
interface is now in the stage of non-cyclic data exchange. Each cycle now
only involves communication with a device in the SERCOS ring. The time
slots required for cyclic data exchange are calculated, and are checked by
the connected devices. When all the slave devices have reported that they
are ready to switch-over, the master can switch to phase 3 with the MST.
In phase three (P3), the communication is already almost like that of phase
4. The only difference is that valid cyclic data is still not being transferred.
The time slots defined in phase 2 are valid, and are monitored. If the
communication is proceeding without error, phase 4 is entered.
Start-up behavior of the bus
coupler
Power on selftest
Bus terminal test Structure list
Cyclic data exchange Master Sync. Telegram (MST)
At the beginning of each cycle an MST is sent by the master to all the
slaves. The broadcast address is used for this. Each connected slave uses
this telegram to determine its send window.