基于RS232C协议的并发帧碰撞模型与呼叫策略研究

Collision Model and Communication Strategy of Multiple Callings with RS232C Protocol

RS232C协议的主从通讯模式,限制了分布式控制系统(DCS)中的控制中心对来自现场终端的随机事件的实时响应能力,降低了整个DCS系统的系统扩展能力和系统可靠性.研究了RS232C协议接口电气特性和DCS系统网络的工作特性,分析了基于RS232C协议的并发呼叫帧发生碰撞的机理,并研究了由并发碰撞引起的帧错误类型.引入了泊松流以描述系统中的多主呼叫过程,建立了基于RS232C串行通讯协议的并发帧碰撞模型.呼叫帧格式、传输速率和呼叫密度共同决定了发生帧碰撞的概率.平均呼叫密度小于10次/秒时,发生帧碰撞的概率小于0.01%,并可以被多主呼叫策略所标识,连续两次呼叫失败的概率小于0.0001%,表明:基于RS232C的多主呼叫协议是可行的.为即插拔多主RS232C通讯协议设计提供了理论依据.

The communication model of one Master to Multiple Slaves in DCS, which is out of RS232c serial protocol, limits the ability for DCS to response and deal with the random requests from terminals. The circuit and topology of RS232C interface are analyzed to reveal the mechanism of multiple callings from terminals. Poisson distribution is introduced to describe the calling process and develop the collision model. A collision probability depends on calling frame format, Baud rate and calling frequency. The collision probability of calling from two or more terminals is less than 0. 01% when mean calling frequency is less than 10 times per second. The collision can be listened, identified and compensated with another calling, which collision probability is less than 0. 0001%. It is showed that the communication strategy of multiple terminals calling to central computer with RS232C orotocol is feasible.