基于Codesys的PLC自动控制及IEC相关标准

始需为二个项式一一进行内容分解，

其次在计算 分解的结果，

最后再得到总体处

理结果。这样很显然势必会影响 程序的可读性。

图

5 ST

（结构文本编程语言）

---- -------------------------------------------------- ---------------------------------------

2.2 IEC 61131-3 Program Organization Units (POUs)?

?

Function (FUN). POU that can be assigned parameters, but has no static

variables (without memory), which, when invoked with the same input

parameters, always yields the same result as its function value (output).

?

Function block (FB). POU that can be assigned parameters and has static

variables (with memory). An FB (for example a counter or timer block), when

invoked with the same input parameters, will yield values which also depend

on the state of its internal (VAR) and external (VAR_EXTERNAL) variables,

which are retained from one execution of the function block to the next.

?

Program (PROG). This type of POU represents the “main program”. All

variables of the whole program, that are assigned to physical addresses (for

example PLC inputs and outputs) must be declared in this POU or above it

(Resource, Configuration). In all other respects it behaves like an FB.

2.3 Variables(

变量

)

Variables are only assigned to explicit hardware addresses(e.g. input and outputs)

in configurations, resources or programs. In this way a high level of hardware

independency is created, supporting the reusability of the scopes of the

variables are normally limited to the organization unit in which they are declared, e.g.

means that their names can be reused in other parts without any conflict,

eliminating another source of errors.