Design of RTU based on DSP and CAN bus

The remote measurement and control terminal (RTU) has been introduced to China since the 1980s and has been widely used since its introduction in the 1980s. The RTU applied to the substation is mainly to realize remote collection and control of on-site power parameters. The remote release of the command, and the information or results are assembled into a message, sent to the control center or dispatcher. Throughout the domestic and international RTU products, the development of a centralized, control structure to a modular, decentralized, and open system control structure. Due to the large amount of data and information in the substation and high real-time requirements, DSPs with powerful and efficient computing power and rich peripheral interface circuits are applied to the RTU design scheme. At the same time, the CAN fieldbus introduction with open structure is introduced. It is used in the communication of substation field data and constitutes an open, reliable and real-time monitoring system.

1 system overall structural design

The RTU system uses a distributed modular structure with DSP as the microprocessor and CAN as the field communication bus. Because the distributed modular structure is easy to implement functional decomposition, it can be concentrated and distributed according to needs, so the system has great flexibility. The system structure is shown in Figure 1.


Figure 1 System structure

RTU can be divided into communication main control module and signal measurement and control module. The CAN bus enables communication of field data. On one hand, the signal measurement and control module monitors the position of the remote substation circuit breaker (switch), the position of the knife gate, the position of the tap of the on-load tap-changer, the accident displacement signal, the alarm signal, etc. (remote signal), and monitors the active power of the main transformer and the line. Power, reactive power, voltage, current, power factor, active energy, reactive power, main frequency, etc. (telemetry) and converted into corresponding electrical parameters; on the other hand, the command issued by the host computer or dispatch center passes the signal The control signal output port of the measurement and control module is used to control the switching position of the circuit breaker and the tap position of the load regulating transformer (remote control and remote adjustment).

Considering the versatility and flexibility of the RTU, the communication master module has multiple communication port modes:

(1) CAN bus, connected with the lower signal measurement and control module to realize data request and command release; (2) RS232 port, realize communication with PC, transfer RTU processed data to PC for further processing or realized by PC Remote operation and control; (3) remote communication port for communication with remote control center; (4) RS485 port for compatibility with traditional RTU equipment or other lower intelligent instrument interfaces.

2 hardware circuit design of communication main control module

The TMS320LF2407A (LF2407A) with built-in CAN controller is used as the processing core of the communication main control module. The peripheral digital circuit of the processor is integrated with the EPM7128STC100-7 CPLD of ALTERA's MAX7000S series. The system structure is shown in Figure 2. From the functional communication main control module is divided into three parts: CAN bus interface part; parallel and serial conversion part; keyboard display part and power-down data saving part.


Figure 2 System structure diagram

Since the LF2407A integrates a CAN bus controller, it is not necessary to add a CAN controller to implement the underlying protocol of the CAN bus. Simply add an anti-interference optical isolation and CAN bus transceiver between the CAN output terminal and the physical channel. The optical isolation uses TI's 8-pin dual-channel high-speed optical isolation HCPL2631. The CAN transceiver uses PHILIPS's PCA82C250 differential driver with various protection and anti-interference capabilities as the bus interface. In order to better solve the radio frequency interference problem, the communication medium adopts Shielded cable, in order to overcome the long-line effect, reduce the reflection of signals in the communication medium, and connect two 120 Ω matching resistors in parallel across the transmission line.

The communication main control module system has multiple serial ports for data communication with the outside world, and the real-time communication requirement is also high. The use of a general I/O line to form a serial port is not applicable here, and four asynchronous communication units are selected. The TL16C554A chip implements parallel-to-serial conversion. Because the PC and remote communication ports are all RS232 ports, and in order to be compatible with the traditional 485 devices, the MAX232 chip is used to convert the TTL level of the TL16C554A serial output signal into RS232 and RS485 levels.

In order to maintain the communication main control module has a certain degree of independence in function, the Chinese character LCD module with the T6963C controller and four buttons are combined to form a human-machine interface. The display shows the communication status of the system, and the four buttons complete the setting of the user function, including the address of the module and the baud rate of various communication ports. The X5045 is designed to save some system parameters when the system is powered down. It can be recalled when the system is started again. Extended high-speed static RAM as external data storage space and debugger storage space.

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