BECKHOFF FC7501 - Manual (Page 14)

Configuration
14 FC750x
The Timing (Online/Offline) tab
The Timing tab provides internal details of the time slot calculation. A distinction is made between an online and
an offline mode, since parameters are also read from the devices and included in the calculation of the time slot. In
offline mode (TwinCAT is stopped) default values are used for the timing values that are normally read from the
devices, so that the result of this calculation can differ slightly from that obtained with the true values. The offline
calculation does however as a rule provide a very good estimate of the bus timing. In online mode (TwinCAT is
running and the Sercos Bus is in phase 3 or 4) the exact values are displayed, and provide the expert user with
very precise information about the timing on the bus.
The diagram in the lower section provides a good summary of the bus loading being generated, and over the ca-
pacity still available. A Sercos cycle is displayed between the two red master control telegrams (MST). After the
MST the devices first send their drive telegrams (green), and after the NC shift time the NC access time (dotted
region) starts, in which the NC accepts the actual values and transmits new set values. The subsequent clear re-
gion leading up to the Master Data Telegram (MDT, blue) indicates bus capacity that is still free, and can be used
for other devices or for additional data associated with the existing devices.
Not long after the MDT the master sends another MST, so starting the next cycle. Times T3 and T4 indicate when
the devices should all simultaneously accept the set values or acquire the actual values.