选型注意事项

  S7-300PLC的选型原则是据生产工艺所需的功能和容量进行选型，并考虑维护的方便性、备件的通用性，以及是否易于扩展和有无特殊功能等要求。选型时具体注意以下几方面：

  (1)有关参数确定。一是输入/输出点数(I/O点数)确定。这是确定PLC规模的一个重要依据，一定要根据实际情况留出适当余量和扩展余地。二是PLC存储容量确定。注意当系统有模拟量信号存在或要进行大量数据处理时，其存储容量应选大一些。

  (2)系统软硬件选择。一是扩展方式选择，S7-300PLC有多种扩展方式，实际选用时，可通过控制系统接口模块扩展机架、Profibus-DP现场总线、通信模块、运程I/O及PLC子站等多种方式来扩展PLC或预留扩展口;二是PLC的联网，包括PLC与计算机联网和PLC之间相互联网两种方式。因S7-300PLC的工业通信网络淡化了PLC与DCS的界限，联网的解决方案很多，用户可根据企业的要求选用;三是CPU的选择，CPU的选型是合理配置系统资源的关键，选择时必须根据控制系统对CPU的要求(包括系统集成功能、程序块数量限制、各种位资源、MPI接口能力、是否有

  PROFIBUS-DP主从接口、RAM容量、温度范围等)，并好在西门子公司的技术支持下进行，以获得合理的选型;四是编程软件的选择，这主要考虑对CPU的支持状况，我们的体会是：STEP7V4.0对有些型号的CPU不支持，硬件组态时会发生故障出错，而STEP7V5.0则不存在这种问题。

  二、设计及使用注意事项

  1. 设计注意事项

  设计时主要应注意以下几方面：

  (1)PLC输出电路中没有保护，因此在外部电路中应设置串联熔断器等保护装置，以防止负载短路造成PLC损坏。熔断器容量一般为0.5A。

  (2)PLC存在I/O响应延迟问题，因此在快速响应设备中应加以注意。MPI通信协议虽简单易行，但响应速度较慢。

  (3)编制控制程序时，好用模块式结构程序。这样既可增强程序的可读性，方便调试和维护工作;又能使数据库结构统一，方便WinCC组态时变量标签的统一编制和设备状态的统一显示。

  (4)硬件资源。要合理配置硬件资源，以提高系统可靠性。如PLC电源配电系统要配备冗余的UPS不间断电源，以排除停电对全线运行的不利影响。又如对电机的控制回路要进行继电器隔离，以消除外部负载对I/O模块的可能损坏。另外，系统设备要采用独立的接地系统，以减少杂波干扰。

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