Hart Protocol Extension Interface Design And Its On The Radar Level Gauge

As a type of industrial fieldbus protocol,HART has been popularly used in much area. Nowadays, though the market share of HART smart instruments is approaching 50 percents,there are still 40 percents of analog or non-digital instruments in practical use.If we want to update those analog or non-digital instruments to HART instruments, typically a kind of HART protocol expanding module (electric module) should be custom-designed for them. In this subject, we are commissioned by TOKYO KEISO co.of Japan to custom-design a HART Unit for a type of radar level meter. With the module, this product will become a HART instrument and its competition ability will be increased in all aspects.The HART Unit connects to the radar level meter through a UART interface, provided by the level meter. In this way, it can get information of the level meter, including the process variable, device status and so on, through MODBUS transmission. The information will be saved in the internal storage of MCU.The master device of HART can reach those useful information by sending request commands to the module.The hardware of the system consists of the MCU sub-system, which is the core part of the module, the HART modem sub-system, the analog-digital convertor sub-system and the power sub-system. Since the HART Unit is a two-wire instrument, it is required that the module can be used in low current situation.In order to make the type of each electrical part certain and demonstrate our solution, we evaluate the power consumption of the system at first.During the procedure of designing the MCU program, we divide the program into four task. They are the MODBUS communication task, the main program task, the EEPROM programming manager task and the HART communication task. Besides, we introduce the embedded operation system, which provides the core task mechanism to achieve the link and synchronization of all parts.The tough thing in this subject is how to achieve the HART communication function. To insure the clarity and standardization of the program code, we take a multi-documents structure.Each document achieves function of the physical layer, the datalink layer and the application layer separately. During the design process, we made some modification to the device status machine of the HART datalink layer, so as to make the change of status more understandable.To make our customers configure the instruments, on which the HART Unit is mounted, and read data in easy way, we designed a master PC software.This software is also provided to our customers, to increase the efficiency of configuration and data reading.Finally, in order to validate the communication reliability and function integrality of the module, we test it both in simulation way and in practical way.