PROFIBUS Manual
A controller designated by PROFIBUS as master class 1 will control its remote peripherals in a cyclic exchange of data. This is called an MS0 relationship and is explained in detail in chapter 4.
In an initialization phase the controller will initialize each peripheral device and also check during cyclic data transmission whether the peripheral device, which has been designated by PROFIBUS as a slave, remains addressable.
For its part, the slave monitors whether the controller remains active with a response monitoring watchdog.
Phase: |
Actions: |
Diagnosis |
The controller sends diagnostic requests to all project slaves |
Initialization |
The controller sends parameters to and checks the configuration of all project slaves |
Data exchange |
The controller sends output data and receives input data. Response monitoring and evaluation of diagnosis. |
Phases in an MS0 relationship
During initialization the controller delivers initial parameters to the slave and checks the slave configuration. This configuration defines how much data will be exchanged in cyclic data traffic between master and slave.
In cyclic data exchange the master sends its output data to the slave and receives input data in reply. The PROFIBUS network for cyclic process data is therefore like a distributed process image of the controller.
The minimum cycle time can easily be estimated if the number of input and output bytes is known:
TBCycle = (380 + Slaves x 300 + Bytes x 11 )/Bitrate + 75 μs
Where
Slaves = number of slaves in the network
Bytes = total number of input and output bytes
N.B.: More precise formulae can be found in the chapter on cyclic data.
In a PROFIBUS network, there must always be at least one class 1 master present. However, several class 1 masters may also be on the same cable. This means that they will share the bit rate, i.e. the cycle time will be longer accordingly. A slave can only be controlled by one class 1 master!
These basic functions for cyclic process data are known as DP-V0. With protocol extensions DP-V1 and DP-V2, it is possible to fix the controller's cycle time to a specific value (equidistant cycle) and, with special telegrams, even synchronize the slave's cycle time with the PROFIBUS cycle (isochronous cycle).
If an application requires two slaves to exchange data directly with each other, the DP-V2 protocol will be needed. These protocol extensions allow data to be exchanged directly between slaves.
