The Analysis And Realization Of Communication In Two Kinds Of Control Networks With Bus Topology

Today, bus topology has been more and more popular in control network, while there are still several ways of media access control (MAC). Thus how to choose a effective way and get it into effect is discussed particularity in this paper using the example of two typical kinds of control network.The first kind of network is a single-layer bus involved in a railway project to control many isolating switches carefully. The process of switch control is divided into a series of steps: 1, ID recognition; 2, application to operate; 3, authorization to the application by remote monitor; 4, field operation. But this is premised on the perfect communication between field machines and remote monitor. Thus a polling way was defined to control the media access to the bus built with RS-485 interface. Within the long period of test run, the whole system was proved to be effective. Besides, the problem of timing and disturbing in the communication is also discussed.The other is train communication network. Based on the protocol of "IEC 61375-1", published by IEC in 1999, the data to be transmitted is classified into urgent periodic variables and deferrable random messages. In this paper the whole net is decomposed into two layers: train bus and vehicle bus. At the sublayer of MAC, vehicle bus connects all kinds of control devices within a vehicle using the protocol of polling, while train bus connects all the vehicle nodes using a new protocol. This new protocol derives from the standard token-bus protocol, IEEE802.4. But the ways to initialize, transmit and maintain token are much different. To make sure its efficiency and reliability, a demo was particularily finished to run on a bus built with RS-485 interface in our lab. What's more, some proposals are also discussed on such issues as how to initialize the train bus, how to exchange data between the two nets and how to speed up the communication rate.