Based on PLC of gateway communication research

1 introduction in view of the automatic control field, the field bus in the bottom control network to get a number of applications, but with the development of network technology and embedded and technological change, the growing need for intelligent, networked and openness of the control system become the development trend of industry. At present, the field bus technology itself also has some limitations, especially in the existing international standards of fieldbus communication protocol and the bus standards incompatible, competition coexist, cannot achieve interoperability, which severely limits the development of fieldbus in the control field. In the process of control network system upgrade, due to the different field embedded equipment manufacturers adopt different ways of communication, there is no a general software agreement between each other, make on the industry of intelligent network is used to connect the embedded device hardware communication media differ in thousands ways, field bus and communication protocol are also each are not identical, to form a heterogeneous network incompatible, lack of openness and compatibility [ 1] 。 2 network communication related technology introduces PLC controller has some characteristics of general computer, but it works with general computer has the very big difference, it USES circular scanning way of working. The principle can be explained by the handling of the task program is according to certain access order and the order of execution work, each completed a scan cycle, again from the beginning loop execution of user programs, the operation of the cycle. Its working process is shown in figure 1, mainly includes three stages: the input sampling stage, stage of program execution, scanning output stage [ 2] 。 Figure 1 PLC controller working principle diagram as communication protocol becomes a key component of embedded control network, the new agreement that could satisfy the requirement of high bandwidth and predictability. Have a variety of real-time communication protocol, in the protocol can satisfy the vehicle reuse: controller area network ( 可以) , local interconnect network ( 林) And the SAE J1850 currently in widespread use. Also only a few agreement for predictability mandatory security applications, documents to satisfy the safety of the embedded system communication protocols, according to a survey and comparison of data communication activity must be able to be triggered, both based on the dynamic response of events, and based on the static time polling trigger [ 3] 。 On the one hand, is based on the time to deal with the static scheduling information protocol, for example, used in avionics industry SAFE BUS and spiders protocol, used in automobile industry TTCAN and time triggered protocol TTP. On the other hand, some information is through the dynamic scheduling of communication protocols, such as, controller area network ( 可以) Is widely used in various fields, LONWORKS protocol and PROFIBUS protocol widely used in the real-time system. In the midst of this, the CAN bus in distributed embedded real-time system is one of the most well-known and widely used in the field of event driven communication protocol [ 4] 。 3 congestion control algorithm in the embedded gateway on 3. 1 active queue management strategy control model in order to solve the data communication activities must be triggered by the uncertainty problem, take the initiative, rather than responsive packet discard is a kind of effective means. Corresponding queue management strategy is called active queue management is the mainstream of the buffer queue management algorithm. The basic train of thought is, according to the changes of the queue length, before the queue buffer overflow, discarding the tag, to reach the grouped data with probability of grouping discarding the tag signals after time delay has been detected by the source side, the source side so the state of the network, according to the different status, call the corresponding control algorithm to adjust the sending rate, so that the queue length in router cache control [ 5] 。 In this way, the strategy to form a closed loop control system, which is the controller, the output for the system control signals, and the source of the rate control algorithm is the executor system, its features and router queue length and link delay together, formed a system of generalized object, as shown in figure 2. Figure 2 active queue management strategy control model link algorithm research is focus on 'active queue management (' Active Queue Management,AQM) With the traditional algorithm, 'of the discarded' ( Droptail) Compared with AQM in network equipment before they discard or tag message buffer overflow. As a member, read on.
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