Logical Expressions in Siemens SCL (Structured Control Language) with Practical Examples
Published on Jun11, 2025 | Category: tia
Logical expressions in Siemens SCL (Structured Control Language) are used to evaluate conditions based on Boolean logic. These expressions are essential for controlling the program flow in PLC programming, especially in decision-making processes.
Logical expressions typically involve more than two operands combined using logical operators like AND, OR, XOR, and NOT. The result of a logical expression is always a Boolean value — either TRUE or FALSE.
Logical operations are commonly used in conditional statements such as IF...THEN...ELSE, WHILE, or CASE. They help make decisions within the program based on multiple inputs. For example, if you have two switches and want the motor to start when either switch is ON, you would use an OR expression.
Logical expressions evaluate multiple values or conditions and determine a single TRUE or FALSE output based on the logic applied.
These expressions are useful in automation scenarios like:
- Ensuring safety conditions are met before starting a machine.
- Verifying sensor states.
- Controlling actuator outputs based on multiple inputs.
- Handling alarms or fault conditions by combining several signals.
Types of Logical Expressions in SCL
The following logical expressions are available in Siemens SCL (Structured Control Language):
- AND (AND or &) – Logical AND expression
- OR (OR) – Logical OR expression
- NOT (NOT) – Logical NOT expression
- XOR (XOR) – Logical Exclusive OR expression
Siemens SCL AND Logical Expression
The AND logical expression is used when you want to check if all conditions are true. If every condition in the expression is true, the result will also be true. If any one condition is false, the final result becomes false.
This logic is commonly used in PLC programs where an action should occur only when multiple input signals are active. For example, you may want to stop a motor only when all level sensors confirm that the tank is full.
Example 1: Two Level Sensors
- Check if both level 1 and level 2 sensors are active.
- If both are true, the condition is satisfied.
- The motor will be stopped when both levels are active.
- If even one sensor is inactive, the motor will continue running.
Example 2: Three Level Sensors
- Check if level 1, level 2, and level 3 sensors are all active.
- If all three are true, the motor will stop.
- If any one of them is false, the motor will keep running.
This approach is useful for safety and process control, where multiple conditions must be met before allowing or stopping a machine action.
Siemens SCL OR Logical Expression
The OR logical expression in Siemens SCL is used when you want an action to occur if at least one condition is true. The result of an OR operation is TRUE if any of the conditions (operands) are TRUE. It only returns FALSE if all conditions are FALSE.
This is similar to how a logical OR gate works in electronics. The output of an OR gate is high (true) if any of its inputs are high. In PLC programs, OR logic is helpful when you want to trigger an action based on multiple alternative inputs.
Example 1: Two Level Sensors
- Check if level 1 or level 2 sensor is active.
- If either sensor is active (true), the condition is satisfied.
- The motor will start (motor is set to true).
- If both sensors are inactive (false), the motor will not start.
Example 2: Mixed OR and AND Condition
- The condition checks: level 1 OR (level 2 AND level 3).
- This follows standard logical precedence where AND is evaluated before OR.
- If level 1 is true, the motor will start — regardless of other levels.
- If level 1 is false, then both level 2 and level 3 must be true to start the motor.
- If none of the conditions are met, the motor remains off.
The OR logical operator is ideal when you have multiple input options for activating a single output. It provides flexibility in control logic, allowing actions to be triggered from any one of several sources.
Siemens SCL XOR Logical Expression
The XOR (Exclusive OR) logical expression returns TRUE when an odd number of inputs are TRUE. If an even number (including zero) of inputs are TRUE, the result is FALSE.
This logic is similar to an XOR gate in digital electronics. The XOR gate produces a HIGH output only when the number of HIGH (TRUE) inputs is odd. This makes XOR useful when you want to trigger an action based on exactly one condition being active—or an odd number of conditions.
Example 1: Two Switches
- Condition checks if either switch 1 or switch 2 is active, but not both.
- If only one switch is ON, then the XOR expression returns TRUE.
- Motor 1 starts running.
- Motor 2 remains OFF.
- If both switches are ON or both are OFF, the XOR result is FALSE, and both motors are OFF.
Example 2: Three Inputs – Two Switches and One Level Sensor
- This condition checks: switch 1 XOR switch 2 XOR level 1.
- The expression returns TRUE if an odd number (1 or 3) of inputs are TRUE.
- In that case, motor 1 turns ON, and motor 2 stays OFF.
- If 0 or 2 of the inputs are TRUE (even number), the result is FALSE, and both motors stay OFF.
XOR logic is useful in control systems where exactly one (or an odd number) of inputs should activate an output. It helps avoid conflicts caused by multiple simultaneous signals.
Siemens SCL NOT Logical Expression
The NOT logical expression is used to invert a Boolean value. It returns TRUE when the input is FALSE, and returns FALSE when the input is TRUE. This is called negation or generating the one's complement of a logical value.
The NOT operator is useful when you want to trigger an action when something is not true. For example, you may want to run a motor when a switch is not pressed or a condition is not active.
Example 1: One Switch Must Be OFF
- This logic checks if switch 1 is ON and switch 2 is NOT ON.
- If switch 1 is ON and switch 2 is OFF, the result is TRUE.
- Motor 1 will turn ON, and motor 2 will remain OFF.
- If both switches are ON, or both are OFF, the result is FALSE, and both motors remain OFF.
Example 2: Using NOT with OR Expression
- This logic checks whether level 1 or level 2 is active (OR condition).
- If any one of them is active, the result is TRUE.
- Then, motor 1 is assigned NOT TRUE → FALSE, and motor 2 is assigned NOT FALSE → TRUE.
- If neither level 1 nor level 2 is active, the result is FALSE.
- Then, motor 1 is assigned NOT FALSE → TRUE, and motor 2 is also assigned NOT FALSE → TRUE.
The NOT operator is often used in conditions where you want to reverse the logic or stop a process when a condition becomes active. It is essential for creating fail-safe logic or handling default inactive states.
