当PLC运行时，是通过执行反映控制要求的用户程序来完成控制任务的，需要执行众多的操作，但CPU不可能同时去执行多个操作，它只能按分时操作（串行工作）方式，每一次执行一个操作，按顺序逐个执行。由于CPU的运算处理速度很快，所以从宏观上来看，PLC外部出现的结果似乎是同时（并行）完成的。这种串行工作过程称为PLC的扫描工作方式。
用扫描工作方式执行用户程序时，扫描是从第一条程序开始，在无中断或跳转控制的情况下，按程序存储顺序的先后，逐条执行用户程序，直到程序结束。然后再从头开始扫描执行，周而复始重复运行。
PLC的扫描工作方式与电器控制的工作原理明显不同。电器控制装置采用硬逻辑的并行工作方式，如果某个继电器的线圈通电或断电，那么该继电器的所有常开和常闭触点不论处在控制线路的哪个位置上，都会立即同时动作；而PLC采用扫描工作方式（串行工作方式），如果某个软继电器的线圈被接通或断开，其所有的触点不会立即动作，必须等扫描到该时才会动作。但由于PLC的扫描速度快，通常PLC与电器控制装置在I/O的处理结果上并没有什么差别。
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Product Information
The DS200DCFBG1B is a power supply board that was designed and manufactured by General Electric as part of their Mark V SPEEDTRONIC series of products. The DS200DCFBG1B board is versatile, with an ability to be used in many IGBT drive applications, including: AC2000, CB2000, DC200, EX2000, FC2000, GF2000, and ME2000. The DS200DCFBG1B board is designed to receive 38 and 115 VAC input power from the CPT, or control power transformer. The DS200DCFBG1B board then provides control level power to its drive and 115 VAC power to its enclosure fans. There are many circuits that are included on the DS200DCFBG1B board, including: Motor field power circuits, circuits to monitor AC line and DC motor signals, control level power supplies, and driver circuits that are made for use with the motor field SCR gate pulse. There are five test points located on the DS200DCFBG1B board that allow the user to check the voltage signals, the status of the voltage signals, and the DCFB board common. The DS200DCFBG1B board also has three fuses that are used to protect the power supply. When replacing a DS200DCFBG1B board, be sure that the drive’s power is shut off. Failure to follow this step can result in serious injury. Disconnect all cables from the DS200DCFBG1B board and push back on the plastic holders to release the board from its carrier. Position the new board so that it is in the same orientation as the old one and place it into the carrier. Then, you can reconnect the cables to the board as labeled. After that, you will need to adjust the board’s EE parameter for voltage feedback offset.
