A fieldbus is a serial bus (communication) system used in machines and systems to connect a number of
sensors and actuators (motors) to each other and to one or multiple masters (IPCs, PACs, PLCs). Fieldbuses make it possible to exchange data between different system components over long distances and under high external load. They operate in master-slave mode: While the master is responsible for controlling processes, the slave handles single subtasks.
Fieldbuses differ in the "physical layer" they use, i.e. the hardware level (e.g. CAN, RS485, Ethernet), and in the protocol level that defines the form of the exchanged information.
There are fieldbuses, such as CANopen and DeviceNet, that use the same physical medium (CAN) but operate with different protocols. Equally, there are protocols that can be implemented on varying types of hardware for example Modbus /TCPon Ethernet and as Modbus on RS232.
The hardware i.e. cable lengths and transmission capacity often determines the fundamental bus properties. The protocol level determines which standardized messages and functions are available between the master and slave.
Depending on the application, each of the established fieldbuses has its strengths and weaknesses. Often it is the availability of fieldbus interface components that determines the selection of the specific fieldbus.
CANopen (Controller Area Network) is a communication protocol and device profile specification for embedded systems used in automation, healthcare, agricultural and automotive systems. In terms of the Open Systems Interconnection (OSI) model, CANopen implements the layers above and including the network layer. The
CANopen standard consists of an addressing scheme, several small communication protocols and an application layer defined by a device profile. The communication protocols have support for network management, device monitoring and communication between nodes, including a simple transport layer for message segmentation/desegmentation. The lower level protocol implementing the data link and physical layers is usually Controller Area Network (CAN), although devices using some other means of communication (such as Ethernet Powerlink, EtherCAT) can also implement the CANopen device profile.
Modbus is a serial communications protocol originally published by US company Gould-Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Using Modbus, a master can exchange data with multiple slaves. In principle, each node can send messages via the bus but communication is usually initiated by the master. Modbus has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices.
In general, there are three Modbus variants:
Modbus RTU and ASCII use a serial interface (RS485 or RS232) as physical transmission layer; Modbus TCP uses Ethernet (TCP/IP).
Modbus TCP/IP uses Transmission Control Protocol and Internet Protocol for the transmission of messages from Modbus between compatible devices over various systems. That is, Modbus TCP/IP uses a physical network (Ethernet), with a networking standard (TCP/IP), and itself offers a method of representing data (Modbus as the application protocol). A Modbus TCP/IP message is basically just a Modbus communication data message compressed in an Ethernet TCP/IP cover. TCP/IP is simply a transport layer protocol and does not change the way data is stored or interpreted inside the message.
The main reasons for the use of Modbus in a variety of many market segments are:
Ethernet for Control Automation Technology (EtherCAT) was developed by Beckhoff. It’s based on the CANopen protocol and on Ethernet but differs from internet communication or network communications in being specifically optimized for industrial automation control. The standards are defined and maintained by the EtherCAT technology group. EtherCAT is a fast and deterministic network, and processes data using dedicated hardware and software. It uses a full duplex, master-slave configuration, and accommodates any topology. It can process 1,000 I/O points in 30 microseconds and communicate with 100 servo axes in 100 microseconds. The axes receive set values and control data and report actual position and status. Axes are synchronized using a distributed clock technique that’s a simple version of IEEE 1588 and reduce jitter to less than 1 microsecond.
Advantages of EtherCAT: