Fieldbus technology is widely used in modern intelligent building monitoring systems. There are currently more than 40 types of fieldbus, but there are two main types for intelligent buildings and they are promoted in China: CAN (ControlAreaNetwork) bus and Lonworks bus. CAN bus technology has great application potential due to its high reliability, simple structure, long transmission distance and low cost.
Control LAN is the most mature and promising microprocessor LAN in fieldbus technology. The protocol adopts the bus topology. By studying the CAN2.0B protocol specification, the communication protocol conforming to the intelligent building monitoring system is developed, and the communication node software is designed.
1 Intelligent building network topology based on CAN bus
The intelligent building monitoring system of CAN bus consists of three parts: upper computer, CAN communication node and various on-site intelligent devices. The number of communication nodes can be increased or decreased according to the scale of the building. The CAN bus acts as a communication network to connect the various nodes into a distributed intelligent monitoring system.
(1) Host computer: It consists of computer and monitoring software. The management and control of the entire intelligent building monitoring system is the center of the whole system.
(2) CAN communication node: Each communication node has the same function, and mainly completes the real-time data collected by the field device and sends it to the CAN bus to receive the control information sent by the CAN bus. The hardware of the CAN communication node is composed of a microprocessor, a CAN controller SJAl000, and a CAN transceiver PCA82C250.
(3) On-site intelligent equipment: consists of a microprocessor, and field function devices. Complete functions such as detection, alarm, control, and display. The intelligent building monitoring system field equipment consists of lighting, air conditioning, elevator, safety monitoring, fire monitoring, water supply and drainage, and power distribution.
Hardware base of 2CAN bus communication protocol
The CAN bus protocol describes the rules for the transfer of information between devices. The definition of the layer is consistent with the open system interconnection model OSI. The CAN is divided into the application layer, the data link layer and the physical layer. The layers are mutually Transparent, each layer only communicates with the same layer on another device. The actual communication takes place between two adjacent layers on each device, and each device is only connected through the communication medium of the physical layer.
The CAN bus specification defines the bottom two layers of the model: the physical layer and the data link layer. The CAN bus driver and communication medium implement the main functions of the physical layer. The CAN bus controller implements the data link layer transmission tasks specified in the bus protocol. The commonly used CAN bus driver is PHILIPS' PCA82C250, the bus controller is SJAl000, and the communication medium is twisted pair or coaxial cable.
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