Communication networks are used in the process industry to exchange information between sensors, actuators and controllers.
Controller Area Network (CAN) bus generally being used for communications between vital controllers and a slower network such as the Local Interconnect Network (LIN) being used for less critical tasks.
Control Area Networking Working:
The CAN network has a serial multi-master bus for linking electronic control units, also known as nodes, each of which has a microprocessor and possibly sensors and actuators.
In serial transmission, each bit is sent one at a time on essentially a single wire cable, the parallel transmission would mean each bit having its own wire so that all the bits can be sent concurrently and would provide quicker transmission.
If the bus is free and no other messages are on it, messages are sent serially to the bus. When a message is received by the CAN controller, the received serial bits will be stored bit by bit until a complete message is received and its microcontroller can act on it.
Format of messaging:
The bit partisan of CAN message is as follow: firstly 1 bit to denote the start of transmission, followed by an arbitration field of 11 bits to identify the data, followed by 8 control bits, followed by 8 data bits, followed by 16 bits for the cyclic redundancy check (CRC) field, followed by a 2 bit acknowledge field to indicate the message has been received and finally 7 bits to indicate the end of the message.
The CRC bits are used to detect for transmission errors in a message.
Before they can try to send a message, all electronic control units have to monitor the bus for a period of inactivity. However, since there is the possibility of multiple access to the bus, it is possible that at a given time more than one control unit will try to send a message.
The network must have a way of identifying collisions in order to avoid collisions between messages. When messages are simultaneously transmitted, each transmitting node checks whether the bus data reflects the status of its newly transmitted bit.
Programmable Logic Controllers (PLCs) can have a communication module to enable them to be included in the factory floor and wider networks.
ControlNet is used to control and manage real-time input / output between systems, while applying supervisory monitor over lower control units. The DeviceNet used is called a Fieldbus and is used to exchange data between a PLC and the various devices input / output.
Supervisory Control and Data Acquisition (SCADA) systems are used to control distributed systems that require hierarchical data acquisition. The term is generally used for operations of large scale that may include multiple sites and large distances.
The information collected by the field sites is collected, analyzed and logged. The server software is programmed to tell the system what and when to monitor, which parameter ranges are acceptable, and when to initiate actions when parameters change outside the acceptable values.
Field sites track sensors and local actuator control. Telephone lines, optical fibres and radio frequencies such as television, microwave and satellite can be used to communicate between field sites and the control centre.
The term Human Machine Interface (HMI) is used for the input-output device through which a human operator is able to control the process and which presents process data to the operator.
Distributed Control Systems:
A distributed control system (DCS) is used to control production systems within the same geographic location.
DCS systems tend to have a controller hierarchy spread through a plant and linked to a command and tracking communications network, while SCADAs provide central control. There are often DCS modules in SCADA systems.