Safety Coding Standards for Siemens PLC Programs

Safety Coding Standards for Siemens PLC Programs: Want to make your Siemens PLC programs safer and more reliable? These safety coding standards and techniques will significantly enhance the quality of your programs!

Hello everyone, I am Hanhan. Today let’s talk about how to write safer and more reliable Siemens PLC programs. In the industrial control field, a small program bug can cause an entire production line to stop and even lead to safety accidents. Therefore, mastering PLC safety coding techniques is crucial for every automation engineer.

1.

Variable Naming Conventions

Imagine taking over a PLC program left by a predecessor, filled with nonsensical variable names like “a1” and “b2”. Wouldn’t you want to smash your keyboard? Good variable naming is the foundation of program readability and maintainability.

• Use meaningful names: For example, use “Motor_Speed” instead of “MS”.

• Adopt a uniform naming convention: For instance, Hungarian notation, using “bEmergencyStop” to indicate the emergency stop button.

• Avoid using easily confused names: Such as “O” (letter) and “0” (number).

• For global variables, you can add a “g_” prefix, such as “g_SystemStatus”.

Note: Although variable names should be meaningful, they shouldn’t be too long; otherwise, the ladder diagram will become cumbersome. Finding a balance is important.

2.

Program Structure Optimization

Many engineers like to cram all logic into a long network, which is like piling everything in the middle of the living room. A well-structured program is like a tidy room; it not only looks comfortable but also makes it easy to find things.

• Use subroutines (FC) and function blocks (FB): Encapsulate complex logic to improve code reusability.

• Reasonably divide program blocks: For example, classify logically by functional modules, device units, etc.

• Use comments to describe the function of each network.

• Keep each network concise, avoiding overly complex logic structures.

Let’s look at a simple example. Suppose we have a motor control program:

// Main program OB1
CALL "Motor_Control"
Enable := "g_SystemRunning"
Speed := "g_TargetSpeed"
Feedback := "g_MotorFeedback"
// Motor control function block FB1
FUNCTION_BLOCK "Motor_Control"
VAR_INPUT
Enable : BOOL;
Speed : INT;
END_VAR
VAR_OUTPUT
Feedback : BOOL;
END_VAR
VAR
Motor_On : BOOL;
END_VAR
// Start condition check
Network 1:
A #Enable
A "Safety_OK"
= #Motor_On
// Speed control
Network 2:
L #Speed
T "AO_MotorSpeed"
// Feedback signal processing
Network 3:
A "DI_MotorRunning"
= #Feedback
END_FUNCTION_BLOCK

This structure is clear, easy to understand, and maintain.

3.

Safety-Related Programming Techniques

In the industrial field, safety is always the top priority. I remember once debugging a PLC in a chemical plant, I forgot to add the emergency stop interlock, and almost caused equipment damage during testing. Since then, I have been very cautious about safety programming.

• Use redundant inputs: For critical safety signals, such as emergency stops, it’s best to use two independent input points.

• Implement watchdog functionality: Regularly check the system’s operating status to detect anomalies in a timely manner.

• Use rising/falling edge detection: Avoid false actions caused by signal jitter.

• Add reasonable delays: Some operations may require a certain response time.

• Implement safety interlocks: Prevent dangerous combinations of operations from occurring simultaneously.

For example:

// Emergency stop handling
Network 1:
A "DI_EmergencyStop1"
A "DI_EmergencyStop2"
= "Emergency_Active"
// Motor start conditions
Network 2:
A(
A "Start_Button"
AN "Emergency_Active"
A "Safety_Door_Closed"
)
S "Motor_Run"
// Motor stop conditions
Network 3:
O "Stop_Button"
O "Emergency_Active"
R "Motor_Run"
// Watchdog timer
Network 4:
A "Clock_1s"
L "Watchdog_Counter"
+ 1
T "Watchdog_Counter"
L "Watchdog_Counter"
L 10
>=I
R "Watchdog_Counter"
S "Watchdog_Alarm"

4.

Error Handling and Logging

I have a colleague who often gets woken up in the middle of the night to handle equipment failures. Later, he added error handling and logging functions to the PLC program, which not only allowed him to sleep soundly but also greatly improved the efficiency of fault handling.

• Implement an error code system: Define unique error codes for different types of errors.

• Use state machines: Convenient for tracking the current state of the system and possible anomalies.

• Log key events and errors: Including timestamps, error codes, related variable values, etc.

• Implement a remote diagnostic interface: Facilitate remote troubleshooting.

Let’s look at this simple error handling example:

DATA_BLOCK "ErrorLog"
STRUCT
ErrorCode : ARRAY[0..9] OF INT;
Timestamp : ARRAY[0..9] OF DATE_AND_TIME;
CurrentIndex : INT;
END_STRUCT
BEGIN
CurrentIndex := 0;
END_DATA_BLOCK
// Error logging function block
FUNCTION "LogError" : VOID
VAR_INPUT
ErrorCode : INT;
END_VAR
// Update error log
L "ErrorLog".CurrentIndex
L 10
MOD
T #TempIndex
L #ErrorCode
T "ErrorLog".ErrorCode[#TempIndex]
L "Clock_DateTime"
T "ErrorLog".Timestamp[#TempIndex]
L "ErrorLog".CurrentIndex
+ 1
T "ErrorLog".CurrentIndex
END_FUNCTION

Note: Although logging functions are very useful, do not forget that PLC storage space is limited. Plan the log size reasonably and consider periodic cleaning or exporting if necessary.

5.

Communication Security

In this interconnected age, PLCs usually do not work alone but need to interact with upper-level machines and other controllers. However, communication security cannot be taken lightly.

• Implement a communication handshake mechanism: Ensure both parties are online and ready.

• Add data verification: For example, use CRC checks to prevent data transmission errors.

• Set communication timeouts: Prevent system hangs due to communication interruptions.

• Add permission control for sensitive operations: Prevent unauthorized operations.

Let’s look at a simple Modbus communication example:

// Modbus communication handling
Network 1:// Communication handshake
A "Modbus_Connected"
A "Data_Valid"
= "Communication_OK"
// Data reception and verification
A "New_Data_Received"
CALL "CRC_Check"
Data := "Receive_Buffer"
Length := "Data_Length"
CRC_OK => "Data_Valid"
// Communication timeout detection
A "Communication_OK"
L "Communication_Timer"
SE T5, 100   // 10 seconds timeout
NOP 0
AN T5
R "Communication_OK"
S "Communication_Timeout_Alarm"

6.

Version Control and Comments

Let’s talk about version control and comments, which are often overlooked but actually super important. I once took over a project where the predecessor left nothing behind, which was quite confusing. Since then, I have placed great importance on version control and comments.

• Add version information and modification history at the beginning of the program.

• Add detailed comments to key function blocks and networks.

• Use version control tools (such as TIA Portal’s version control feature) to manage code.

• When making significant changes, back up first before proceeding.

Let’s look at an example of a program header:

// ====================================
// Program Name: Injection Molding Machine Control System
// Version: V1.2.3
// Author: Hanhan
// Date: 2025-01-24
// Modification History:
// V1.2.3 - 2025-01-24 - Hanhan
//   - Optimized temperature control algorithm
//   - Added automatic recovery function for faults
// V1.2.2 - 2024-12-15 - Zhang San
//   - Fixed high-temperature alarm delay bug
// ====================================
// Main program loop
Network 1:// System initialization
// ... (specific code omitted)
Network 2:// Temperature control
// Use PID algorithm to control heater power
// Input: Current temperature, target temperature
// Output: Heater power (0-100%)
// ... (specific code omitted)
// ... (more networks)

Note:

• When updating the version number, follow certain rules; for example, major revisions, minor changes, and bug fixes correspond to changes in different positions of the number.

• Comments should be concise, explaining “what” and “why,” rather than “how” (the code itself explains how).

Practical Exercise Suggestions:

1. Take an existing PLC program and try to refactor it according to the above standards. You will find that this process not only improves program quality but also deepens your understanding of the system.

2. Simulate a simple industrial process (such as level control) and write a PLC program from scratch, ensuring it covers all safety coding aspects mentioned in this article.

3. Conduct code reviews with colleagues or friends in online communities to learn from each other and improve together.

Remember, writing safe and reliable PLC programs is a continuous improvement process. Keep coding, stay safe!