SIEMENS SIMATIC S7 + WINCC
Profibus
Profibus is an open fieldbus standard for a wide range of applications in manufacturing, process and construction automation. Their independence of ownership and openness are guaranteed by the Profibus EN 50 170 standard. With Profibus, devices from different manufacturers can communicate without special interface settings. Profibus can be used for critical high-speed data transmission and extensive complex communication tasks.
Profibus specifies technical and functional characteristics of a serial fieldbus system, in which decentralized digital controllers can be interconnected from the field level to the cell level.
The Profibus family consists of three compatible versions.
Profibus DP
With optimization for high speeds, this Profibus version was specially developed for communication between automation control systems and I/O distributed in devices. Profibus-DP can be used to replace 24 V or 4-20 mA parallel signal transmission.
To use these functions, several service levels of the DP protocol are defined:
• DP-V0 provides basic DP functionality, including cyclic data exchange and specific station, module and channel diagnostics.
• DP-V1 allows parameter assignment via acyclic data communication and alarm control.
• DP-V2 for asynchronous mode and data exchange broadcast (slave-to-slave communication).
Profibus PA
Profibus PA was developed especially for process automation. It allows sensors and actuators to be connected on a common bus, through a dedicated DP/PA gateway. Profibus PA allows data and power communication on the same bus, using 2-wire technology, in accordance with the international standard IEC 1158-2.
Profibus FMS
Profibus FMS is a general purpose solution for cell-level communication tasks. FMS services reach a wide range of applications and high flexibility. Profibus FMS can also be used for extensive and complex communication tasks. This protocol was the first to be developed for Profibus, but it is not normally used.
Profibus Master and Slave
Master devices determine the communication data on the bus. A master can send messages without an external request when he has access rights to the bus (the token). Masters are also called ‘active stations’ in the Profibus protocol.
Slave devices are peripheral devices. Typical slave devices include input/output devices, valves, drives and measurement transmitters. They do not have access rights to the bus and they can only acknowledge incoming messages or send messages to the master when asked to do so. Slaves are also called 'passive stations'.
Profibus DP Master class 1 (DPM1)
A class 1 master controls normal communication or data exchange with the slaves assigned to it. This is typically a PLC.
It uses cyclic communication in the process of exchanging data with its associated slaves.
The class 1 master sets the baud rate and the slave detects this speed.
Each slave device is assigned to a master and only that master can write output data to the slave. Other masters can read information from any slave, but can only write output data to their own assigned slaves.
Profibus DP Master class 2 (DPM2)
A class 2 master is a special device primarily used to commission slaves and for the purpose of diagnosis. He is typically a Supervisor.
It uses acyclic communication on what is known as the MS2 channel.
A DPM2 does not have to be permanently connected to the system bus.
Cyclic communication
DP Master class 1 cyclically exchanges data with all slaves assigned to it. This service is configured. During the configuration process, addresses are assigned to masters and slaves, bus parameters are defined, module types and numbers (in the case of modular slaves) modules are specified, etc.
Before the data exchange takes place, the master will send parameterization and configuration telegrams to all his slaves. These parameters and configuration data are checked by the slaves. If these two steps are valid, the master will initiate the communication of cyclic I/O data with the slave devices.
Acyclic communication
In addition to cyclic data exchange, the Profibus protocol has the option of acyclic communication. This service is not configured.
There are two different communication channels between the master and the slave:
• MS1 channel (MS1 connection): can only be established if cyclic data is exchanged between the master (DPM1) and the slave.
• MS2 channel (MS2 connection): it is possible with several masters simultaneously, but the communication must be established explicitly by the master.
Reading or writing acyclic data requires an MS1 or MS2 connection to be established.
For the MS1 channel, three conditions must be met:
• the slave device must support the MS1 channel (key C1_Read_Write_supp at 1 in the GSD file).
• The DPV1_enable bit must be set during parameter assignment.
• Data exchange is taking place.
For the MS2 channel, the connection must be explicitly initiated by the master.
Profibus DPV1
Profibus DPV1 is an extension of the DP protocol. With it, it is possible to exchange acyclic data of parameters, diagnostics, control and test data.
DPV1 slave
A slave is characterized as a DPV1 slave if it is operated on the Profibus bus with the Profibus DP protocol and works according to the EN 50 170 standard.
GSD (device data)
Device data (GSD) includes descriptions of the DP slave in a uniform format. The use of GSD makes it easier to parameterize the DP slave in a DP master system.