PROFIBUS Manual
The class 1 DP master can be in various operating states.
State |
Meaning |
|---|---|
Off-Line |
Not switched on. No bus communication. |
Stop |
PROFIBUS running, detects other stations, DP slaves are not initialized. Class 2 communication (MS2) is possible. |
Clear |
DP slaves are initialized and blocked. Inputs are read, outputs remain empty. |
Operate |
Regular data communications with all DP slaves. The operating state is signalled. |
States of a class 1 DP master
Operate should be the normal state of the class 1 master. In this case, the master sends output data cyclically to the slaves and awaits input data. Operate is signalled as the operating state with a Global_Control telegram in an interval lasting 3-times the watchdog (Watchdog monitoring). This involves both bytes in the Global_Control telegram being set to 0.
The Clear state is intended for cases when, for example, a controller has been stopped and no valid output data is available any more. The master signals this state immediately with a Global_Control telegram in which the "Clear" bit has been set.
The behaviour of the class 1 master is different for each slave. DP slave possibilities are described in the GSD file:
Slave supports Failsafe:
Fail_Safe = 1
In this case a master in the Clear state sends no more data to the slave in cyclic data transmission. The slave independently sets replacement values or takes action, depending on definitions in the parameters.
Slave does not support Failsafe:
Fail_Safe = 0
In this case a class 1 master in the Clear state sends the necessary number of bytes to the slave, but with all values set to 0, regardless of what these values mean for the application.
A sequence including telegrams with Failsafe can appear as follows:
