15. Programming Languages of PLCs (Text-Based Languages)

4. Structured Text (ST)

• Description: A high-level programming language with syntax similar to Pascal or C. Very powerful and flexible.
• Applicable Scenarios: Complex mathematical calculations, algorithms, data processing.
• Example: Analog Temperature PID Control (Simplified)
• Control Requirements: Read the temperature value, compare it with the set point, and output a control signal through PID computation.
• Character Diagram (Code Block):

+—————————————–+ | // Temperature PID Control Program | | VAR | | ActualTemp : REAL; // Actual Temperature | | SetPoint : REAL := 80.0; // Set Temperature | | Output : REAL; // Output Value | | END_VAR | | | | ActualTemp := AI_Temp; // Read analog input | | | | // Simple Proportional Control | | IF ActualTemp < SetPoint THEN | | Output := (SetPoint – ActualTemp) * 0.5;| | ELSE | | Output := 0.0; // Stop heating when temperature is reached | | END_IF; | | | | AQ_Heater := Output; // Output to analog | +—————————————–+

• Program Explanation:

1. This is a simplified control logic, not a complete PID.
2. The program reads the actual temperature ActualTemp.
3. It uses the IF…THEN…ELSE statement to determine: if the actual temperature is lower than the set point, it outputs a signal proportional to the temperature difference (proportional control); otherwise, it outputs 0.
4. Finally, the calculated result Output is sent to the analog output address AQ_Heater (e.g., heater).

5. Instruction List (IL)

• Description: A low-level programming language similar to assembly language. Composed of a series of instructions.
• Applicable Scenarios: Now rarely used, mainly for maintaining legacy systems or in cases with extreme size constraints on code.
• Example: Implementing Basic AND Logic
• Control Requirements: When I0.0 and I0.1 are both 1, let Q0.0 output 1.
• Character Diagram (Code List):

+—————-+ | LD I0.0 | // Load (read) the value of I0.0 to the current result | AND I0.1 | // Perform AND operation with I0.1’s value | ST Q0.0 | // Store (output) the current result to Q0.0 +—————-+

• Program Explanation:

1. LD I0.0: Loads the state (0 or 1) of input I0.0 into an invisible “accumulator”.
2. AND I0.1: Performs a logical AND operation between the value in the “accumulator” and the value of I0.1, placing the result back in the “accumulator”. The result is 1 only if both are 1.
3. ST Q0.0: Sends the final result of the “accumulator” to output Q0.0.

Summary