CPU是PLC的核心，起神经中枢的作用，每套PLC至少有一个CPU，它按PLC的系统程序赋予的功能接收并存贮用户程序和数据，用扫描的方式采集由现场输入装置送来的状态或数据，并存入规定的寄存器中，同时，诊断电源和PLC内部电路的工作状态和编程过程中的语法错误等。进入运行后，从用户程序存贮器中逐条读取指令，经分析后再按指令规定的任务产生相应的控制信号，去指挥有关的控制电路。
Parameter MaxNumOfPOUs:
The number of POUs contained in a project is composed as follows:
-planned programs-1 POU each
-planned functions-1 POU each
-planned function blocks-2 POUs each
-planned data types-1 POU each
-functions in libraries-1 POU each
-function blocks in libraries-2 POUs each
In the older 907 AC 1131 versions up to V4.2,the maximum number of POUs was set using the
parameter to the runtime system of the AC31 controllers it was
fixed to 1024.
From 907 AC 1131 version V4.30n,the maximum number of POUs is no longer read from the
file,but entered for each controller in the corresponding controller configuration file.
(Refer to chapter 2.1.3'Configuration files for the controllers'.)
3.2.2 Parameters for configuring the communication with the PLC
Parameter KT94WaitTime:
The parameter KT94WaitTime is used to set the wait time until a communication error is
reported in case of serial communication disturbances.
This parameter is0nly effective for drivers which do not have an own'Timeout'parameter.
Default value:KT94WaitTime=2200(corresponds to approx.10 sec.)
Parameter UpdateRate:
The parameter UpdateRate is used to set the refresh time in milliseconds for the variables.
Default value:UpdateRate=200(smaller values are not possible)
Parameter DownloadWaittime:
The parameter DownloadWaittime is used to set the download timeout interval.
This parameter is available from 907 AC 1131 version V5.00nwards.
Default value:DownloadWaittime=22222(corresponds to approx.20 sec.)
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