In today’s article, we will briefly discuss some similarities/differences that may exist between Modbus and Profibus communication protocols. As you probably know, Modbus and Profibus have widely used communication protocols in industrial automation, whether it is to ensure communication between instrumentation equipment in process industries or to ensure exchanges between supervision software and PLC in the building management field.
Knowing the fundamentals of these two communication protocols is essential when you have to set up exchange networks between programmable controllers, sensors/actuators and man-machine interfaces.
What is the Modbus protocol?
The Modbus protocol is one of the oldest industrial communication protocols. It is a communication protocol initiated by Modicon (now Schneider Electric) in 1979. Now an open protocol, it is compatible with many devices industrial ranging from variable speed drives, energy meters, PLCs etc …
The Modbus protocol is available in 3 variants:
– Modbus ASCII: uses ASCII characters for data encoding
– Modbus RTU: uses a binary format for data encoding
– Modbus TCP / IP: communication via TCP / IP
Modbus via RS-232
In Modbus protocol, whether ASCII or RTU, the following transmission standards are used: RS-232, RS-422 and RS-485. RS-232 allows point-to-point communication, that is to say between two single devices. The exchange takes place via the master/slave communication mode.
Modbus via RS-422 or RS-485
Unlike Modbus via RS-232 where you can only have two devices on the line, with Modbus via RS-422 and Modbus via RS-485, you can have up to 32 devices on a segment of a network over a maximum length of 1200 meters.
With the use of repeaters, you can have up to 247 slaves on an RS-485 or RS-422 Modbus line as can be seen in the image below.
In the master/slave configuration, only the master is active, the slave devices are completely passive. The slaves therefore only communicate on the initiative of the master.
What is the Profibus protocol?
PROFIBUS or Process Fieldbus is a master/slave communication protocol. Unlike serial Modbus which is mono-master, Profibus is a multi-master protocol with a mode of access to the network by passing a token as is possible. see on the picture below.
There are several versions of Profibus, among which we can mention:
- PROFIBUS PA - PROFIBUS DP - PROFIBUS FMS
The Profibus uses the physical RS-485 medium to ensure the transmission of information on the network.
There are several similarities between the Modbus protocol (particularly the serial Modbus) and the Profibus protocol, in particular with regard to the physical transmission medium: we find RS-485 for example for the Modbus RTU as well for the Profibus DP. the cabling is also used shielded twisted pair. For serial Modbus like Profibus, terminating resistors are used at the ends of the network lines.