Technology : Fieldbus
Fieldbus is an industrial network system for real-time distributed control. It is a way to connect instruments in a manufacturing plant. Fieldbus works on a network structure which typically allows daisy-chain, star, ring, branch, and tree network topologies. Previously, computers were connected using RS-232 (serial connections) by which only two devices could communicate. This would be the equivalent of the currently used 4-20 mA communication scheme which requires that each device has its own communication point at the controller level, while the fieldbus is the equivalent of the current LAN-type connections, which require only one communication point at the controller level and allow multiple (hundreds) of analog and digital points to be connected at the same time. This reduces both the length of the cable required and the number of cables required. Furthermore, since devices that communicate through fieldbus require a microprocessor, multiple points are typically provided by the same device. Some fieldbus devices now support control schemes such as PID control on the device side instead of forcing the controller to do the processing.
There were many competing technologies for fieldbus and the original hope for one single unified communications mechanism has not been realized. This should not be unexpected since fieldbus technology needs to be implemented differently in different applications; automotive fieldbus is functionally different from process plant control.
Ethernet & Fieldbus
Recently a number of Ethernet-based industrial communication systems have been established, most of them with extensions for real-time communication. These have the potential to replace the traditional fieldbuses in the long term. One of the most prosperously booming technology is EtherCAT.
Modbus
● developed with industrial applications in mind
● openly published and royalty-free
● easy to deploy and maintain
● moves raw bits or words without placing many restrictions on vendors
Modbus enables communication between many (approximately 240) devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in SCADA systems. Many of the data types are named from its use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.
CANopen
CANopen is a communication protocol and device profile specification for embedded systems used in automation. In terms of the 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.
The basic CANopen device and communication profiles are given in the CiA 301 specification released by CAN in Automation. Profiles for more specialized devices are built on top of this basic profile, and are specified in numerous other standards released by CAN in Automation, such as CiA 401 for I/O-modules and CiA 402 for motion control.
Feature Comparison
Different fieldbuses offer different sets of features and performance. It is difficult to make a general comparison of fieldbus performance because of fundamental differences in data transfer methodology. The table below just gives a simple comparison for some popular fieldbuses.
| Fieldbus | Bus power | Cabling redundancy | Max devices | Synchronisation | Sub millisecond cycle |
|---|---|---|---|---|---|
| CANopen | No | No | 127 | Yes | No |
| DeviceNet | Yes | No | 64 | No | No |
| EtherCAT | No | Yes | 65,536 | Yes | Yes |
| Ethernet Powerlink | No | Optional | 240 | Yes | Yes |
| EtherNet/IP | No | Optional | Almost unlimited | Yes | No |
| Modbus | No | No | 246 | No | No |
| PROFIBUS DP | No | Optional | 126 | Yes | No |
| PROFIBUS PA | Yes | No | 126 | No | No |
| PROFINET IO | No | Optional | Almost unlimited | No | No |
| PROFINET IRT | No | Optional | Almost unlimited | Yes | Yes |
EtherCAT ( Ethernet for Control Automation Technology )
EtherCAT is an open high performance Ethernet-based fieldbus system. The development goal of EtherCAT was to apply Ethernet to automation applications which require short data update times (also called cycle times) with low communication jitter (for synchronization purposes) and low hardware costs. EtherCAT was originally developed by Beckhoff Automation GmbH but is now overseen by the EtherCAT Technology Group that was set up to help with proliferation of the EtherCAT standard. Today, there are over 1,900 member companies from 52 countries that create and deploy EtherCAT-compatible products. Ethernet has seen unparalleled adoption in diverse applications, but in industrial environments, it is still not effi cient enough for small amounts of data exchange, it has low determinism for real-time operation, and it works with only star topology in which the network nodes much be connected through switches. EtherCAT technology adds certain features on Ethernet and enforces certain confi gurations to make it a very effi cient network technology for automation while fully conforming to the Ethernet specifications. The design of EtherCAT enables any standard PC to be used as an EtherCAT master and communicate with EtherCAT slaves, which are specialized devices compliant with the EtherCAT specification. Together, the master and slave EtherCAT devices can be used in all devices in the factory network – automation controllers, operator interfaces, remote input/output units, sensors, actuators, drives and others.
