Design Of Field-Monitoring Unit Based On CAN-Bus

In recent years, with the development of economy, all kinds of basic constructions can be seen everywhere. For the large scale construction, it is better that machinery machines distribute rationally and work together, single machines have not met the requirements yet. It needs to strengthen the information flow between engineering machineries. So joining real-time communication function is very important and neccesary.This paper introduces a kind of communication equipment, which includes wired and wireless communications. It is used to monitor the location information and environment information of the construction site, which is transported to the host in wired way. After simple analysis, the information is transported to engineering machines in order to guide the next work. When the engineering machine breaks, it can transport the fault information to the control center in wireless way, then the control center can make reasonable dispatch according to the fault informations. The host also can transport the location information and environment information to the control center, then the control center can make reasonable allocation to shorten engineering period and improve efficiency.The hardware design is mainly composed of LPC2129 ARM7 microprocessor, CAN- Bus controller, GPRS module and SIM card, GPS Locator, E2PROM memory, LCD display and keyboard. The software design usesμC/OS-Ⅱoperating system, which is a kind of multi-task real-time hard-core based on priority. It can support at most 64 tasks, includeing 2 system tasks.μC/OS-Ⅱallocates a only priority to every task to achieve the dispatching and switching between diffirent tasks. Most ofμC/OS-Ⅱ's code is writed in C, just a little code close with processor writed in assemble language, so it is easy to port.CAN Bus is a kind of multi-host, serial data transmission way. CAN Bus use differential signal transmission, so it has stronger anti-jamming. The farthest transmission distance of CAN Bus is 10km, and the biggest transmission speed is 1Mbps, it's transmission speed reduces with distance's increasing. There are 120 nodes at most in a CAN network, every node has a only identifier as its mark. In CAN2.0A standard, one CAN network can have 211 identifier, but in CAN2.0B standard, one CAN network can have 229 identifier. The identifier is saved in E2PROM in this design to avoid losed when power off. CAN Bus just has the physical layer and data link layer, and its communication media can be twisted pair,coaxial-cable, and optical fiber. CAN Bus transmit data in different form. There are four kings of forms,data frame,far frame,error frame and overload frame, and the first two kinds frame can be send by user program, the other two kinds frame send by hardware to manage error arbitration. CAN Bus decide whether receive a frame through filter and acceptance mechanism. The node will accept the frame if the filtering result is the same with node's identifier, or else the node will give up the frame. Every CAN Bus node have a send fault counter and a receive fault counter that users can set the upper limit. When there is a fault, the corresponding fault counter will add one, until up to the upper limit then close the node's bus, not to influence the other nodes. Because of all of the above, CAN Bus has very good reliability and stability, and CAN Bus has been expanding to variety areas.GPRS is the short of general packet radio serves, the transition between the second mobile communication and the third mobile communication. In order to realize the transition from GSM to GPRS, it add two support nodes, GGSN and SGSN. GGSN is interface of GPRS network and outside data netwok, as like a router that makes GPRS network to be a subnet of outside network. SGSN is the packet switch whose wok is sending and receiveing data between GGSN and mobile teiminal, remembering mobile terminal's current location, in charge of mobile terminal's moving and safety management. It is needed of TCP/IP protocol for communication, so mobile terminals must have IP address. Mobile terminal must get a IP address from GGSN to get in Internet, so it has to make a point-to-point link with GGSN. The PPP protocol provides a whole set of schemes to build link, keep link, backout link, negotiate upper layer protocol, authenticate. PPP protocol includes LCP, NCP, authenticate protocol and so on. LCP protocol is used to configure and test data communicating link. NCP is used to configure the chosed network control protocol. Authenticate protocol is used to configure the authenticating mothod. MC55 has embedded PPP and TCP/IP protocol stack, so the micrprocessor can just send AT command to build PPP connect. After mobile terminal obtains its IP address, It can make Internet connect and then communicate.Besides, this design has used GPS positioning system to monitor CAN node's location. Now there are a lot of kinds of very small GPS locators on market, some of which are just U-memory size, and read positioning data through USB or serial port. The GPS receiver used in this design is DH2000e which uses GN-80-H receive board of Japanese GuYe, made by DaHeng company. It is a kind of high stability and hign efficiency GPS receiver. DH2000e uses RS-232 serial port to connect with microprocessor. The user can set its time slot of receive data, and the data's format in accordance with NMEA0183 protocol standard. The microprocessor LPC2129 can just get the location information to reach the target of position. It is necessary to add person-and-computer contact to embed system. So this design adds SMG240128A LCD displayer to display received data and menu, keyboard to set destination address.This design is lightweight, low cost. If installed the system, wherever the car is the main control station can get its real-time position in order to make scheduling decition. With the development of communication industry, GPRS signal coverage wider, plus the GPS positioning system mature, it is believed that engineering machinery with communications system will become the further development trends.