SDCS-POW-1/3ADT220090R0003
咨询***：18506919598
PLC的工作原理
在每一扫描周期内，用户程序是按梯形图，从头开始由左→右，由上→下，逐条执行，直至程序结束。根据梯形图逻辑
每个周期程序执行的结果是：
表2-1 状态表
|
I/O 周期号 |
X00 |
Y00 |
Y01 |
Y02 |
|
0 |
0 |
0 |
0 |
0 |
|
1 |
1 |
0 |
1 |
1 |
|
2 |
1 |
1 |
1 |
1 |
|
3 |
0 |
1 |
0 |
0 |
|
4 |
0 |
0 |
0 |
0 |
|
5 |
0 |
0 |
0 |
0 |
Y00(N)=Y01(N-1) Y01(N)=X00(N) Y02(N)=Y01(N)
已知： X00(0)=1 X00(1)=1 X00(2)=1
X00(3)、(4)、(5)=0
所以：***周期的结果是
Y00(1)—0 Y01(1)—1 Y02(1)—1
( Y00(1)=Y01(0)=0; Y01(1)=X00(1)=1; Y02(1)=Y01(1)=1)
同理可得：第2--5周期的输出结果是：
Y00(2)=1 Y01(2)=1 Y02(2)=1
Y00(3)=1 Y01(3)=0 Y02(3)=0
Y00(4)=0 Y01(4)=0 Y02(4)=0
Y00(5)=0 Y01(5)=0 Y02(5)=0
用状态表列出（见表2-1）。
扫描周期的计算方法
扫描周期的长短，对PLC系统的性能有一定的影响，例如较长的扫描时间对I/O响应时间，对系统运行的***性均会产生不利的影响。见表2-2。
表2-2 PLC扫描时间对内部功能的影响
|
扫描时间ms |
产生的不利影响 |
|
>10 |
内部0.01s时钟脉冲不起作用 |
|
>100 |
内部0.1s时钟脉冲不起作用 |
|
>200 |
内部0.2s时钟脉冲不起作用 |
|
>6500 |
超过WDT定时值，迫使CPU停机 |
扫描周期(T)的计算公式是：
扫描周期（T）= 内部处理时间 + 通信服务时间 + 输入刷新时间
+ 用户程序时间 + 输出刷新时间
其中：
①内部处理时间：是固定的（例OMRON C200H为2.6ms）
②通信服务时间：若系统安装了外设、网络通信等模块，则有固定的时间（例OMRON C200H为0.8ms(max) 、8ms(max)），否则为0。
③输入刷新时间：将接在输入端子上元件的状态读入，并保存在“输入状态表” (I/O映像存储器)中所耗费的时间。（例OMRON C200H输入0.07ms/8点，三菱FX2N输入50μs/8点）。
④用户程序执行时间：取决于程序的长度和指令的种类，一般PLC均提供各指令的执行时间表。（例OMRON C200H LD、OUT指令，其执行时间分别为0.75、1.13μs；三菱FX2N基本指令0.08μs/条，应用指令1.52 ~ 数100μs/条）
⑤输出刷新时间：将“输出状态表” (I/O映像存储器)中的内容输出到接口电路中所耗费的时间。（例OMRON C200H输出0.04ms/8点）
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