Common Functions

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This guide describes the functions commonly used in Siemens S7 Series PLC ladder diagrams, as shown in the following table.

Ladder diagram	Function	Description
	Normally open contact	If IN is True, the normally open contact will close; otherwise, the normally open contact will open.
	Normally closed contact	If IN is False, the normally closed contact will close; otherwise, the normally closed contact will open.
	Set	If S (Set) is activated, OUT will be set to True. If S is not activated, OUT will not change.
	Reset	If R (Reset) is activated, OUT will be set to False. If R is not activated, OUT will not change.
	Positive result of logical output (RLO) edge detection	When a positive transition (0 → 1) is detected on the CLK power flow input, the Q output power flow or logic state is True. This function compares the current signal state of CLK with the signal state from the last scan of CLK, which is stored in the edge memory bit Tag_M.
	Move	Copy a single data element from the source address specified by parameter IN to the destination address specified by parameter OUT1.
	Equal	If Tag_Value1 equals Tag_Value2, the output will be True; otherwise, the output will be False.
	Not equal	If Tag_Value1 is not equal to Tag_Value2, the output will be True; otherwise, the output will be False.
	In range	If the value of VAL is within the range [MIN, MAX], the output will be True; otherwise, the output will be False.
	Add	Perform an addition operation on the input values (IN1 and IN2), and store the result in the memory address specified by the output parameter OUT.
	TON	When IN changes from False to True, the timer starts. When ET equals PT, Q immediately outputs True, and ET immediately stops timing and holds the value. At any time, as soon as IN becomes False, ET immediately stops timing and resets to 0, and Q outputs False.

Ladder diagram

Function

Description

basic command 1

Normally open contact

If IN is True, the normally open contact will close; otherwise, the normally open contact will open.

basic command 2

Normally closed contact

If IN is False, the normally closed contact will close; otherwise, the normally closed contact will open.

basic command 3

Set

If S (Set) is activated, OUT will be set to True.

If S is not activated, OUT will not change.

basic command 4

Reset

If R (Reset) is activated, OUT will be set to False.

If R is not activated, OUT will not change.

basic command 5

Positive result of logical output (RLO) edge detection

When a positive transition (0 → 1) is detected on the CLK power flow input, the Q output power flow or logic state is True.
This function compares the current signal state of CLK with the signal state from the last scan of CLK, which is stored in the edge memory bit Tag_M.

basic command 6

Move

Copy a single data element from the source address specified by parameter IN to the destination address specified by parameter OUT1.

basic command 7

Equal

If Tag_Value1 equals Tag_Value2, the output will be True; otherwise, the output will be False.

basic command 8

Not equal

If Tag_Value1 is not equal to Tag_Value2, the output will be True; otherwise, the output will be False.

basic command 9

In range

If the value of VAL is within the range [MIN, MAX], the output will be True; otherwise, the output will be False.

basic command 10

Add

Perform an addition operation on the input values (IN1 and IN2), and store the result in the memory address specified by the output parameter OUT.

basic command 11

TON

When IN changes from False to True, the timer starts.
When ET equals PT, Q immediately outputs True, and ET immediately stops timing and holds the value.
At any time, as soon as IN becomes False, ET immediately stops timing and resets to 0, and Q outputs False.

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