Industrial networking is the key function to process control. it should be recognized that that networking goes beyond the interfacing of factory systems, to interface with design and documentation systems to form a virtual organisation and other related systems. The actual selection of an industrial network is based on a number of criteria including:
- The detailed requirements of the application.
- The capabilities of the network technologies, including speed and timing requirements.
- Integration with existing equipment.
- Availability of components.
- Costs of installation, devices, training, and maintenance.
Within the industrial networking domain, the lack of standardisation is the major problem in inter-computer communication. A large number of signal characteristics and topologies is just part of the problem.
The exchange of data over a network can be either asynchronous or synchronous, depending on the protocol being used.
n asynchronous communications, each word of data is sent as a separate message. Asynchronous communication is usually adequate if only two computers are connected to each other.
Online programming and monitoring are often performed asynchronously with a single computer directly connected to a PLC, robot or other controllers.
Synchronous communication data messages consist of many data words that are preceded by a header containing information about the data ’packet‘, and followed by a footer containing error-checking information. Synchronous communication is appropriate where large amounts of data are to be transmitted quickly.
Local area networks (LAN) use synchronous data communications.
The hierarchy of an industrial network, ranging from the enterprise network for corporate information to the interconnection of individual sensors through a Sensor bus.
At sensor/actuator level the signals of the binary sensors and actuators are transmitted via a sensor/ actuator bus. At this level, a low-cost technique, through which data and a 24-volt power supply for the end devices are transmitted using a common medium, is an important requirement. The data are transmitted purely cyclically.
At field level, the distributed peripherals, including I/O modules, measuring transducers, drive units, valves and operator terminals require communication with the automation systems via real-time communication systems.
At the automated cell level, the programmable controllers such as PLC and IPC communicate with each other. The information flow requires large data packets and a large number of powerful communication functions. Smooth integration into company-wide communication systems, such as Intranet and Internet via TCP/IP and Ethernet are now becoming important requirements.
In order to show the key features at each level of the hierarchy, an overview of a number of systems is given below.
Ethernet is considered to be the most widely-installed local area network (LAN) technology, and as such is found throughout most industrial organisations, but normally restricted to the office environment. The Ethernet protocol is formally specified in the IEEE 802.3 standard.
An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires as the transmission medium. One of the key features of the system is that no power is transmitted in the bus, hence all hubs and peripherals need to be independently powered.
DeviceNet, (DeviceNet, 2005), is a digital, multi-drop network that is capable of operating between industrial controllers and I/O devices and is considered to be the de facto standard in the US semiconductor industry.
In the DeviceNet architecture, each device and/or controller is considered to be a node on the network. One of the
important features of this technology is that power is provided on the network. This allows devices with limited power requirements to be powered directly from the network, reducing connection points, physical size and cost. DeviceNet conforms to the OSI model, and as such is an open standard.
PROFIBUS (PROFIBUS, 2005) or the Process Field Bus is based on the IEC 61158 standard and is mainly used at field level with capabilities to operate down to the sensors, or up to the production levels.
A number of variants have been designed for factory operation (PROFIBUS DP), motion control (PROFIdrive), process operation (PROFIBUS PA) or safety applications (PROFIsafe). In PROFIBUS DP and PROFIdrive, the communication is based on RS485, and has the capability of speed up to 31.25 Kbps up to distances of 1900 m, and uses a logical token ring with a master/slave paradigm.