How to Use Bitwise Operators and Bit Shifting in WinCC C-Scripts for PCS 7 Status Tag Decoding

How to Use Bitwise Operators and Bit Shifting in WinCC C-Scripts for PCS 7 Status Tag Decoding

In WinCC ANSI C scripting, the ampersand symbol '&' functions as the bitwise AND operator, while '>>' is the bitwise right shift operator. Unlike arithmetic addition or logical evaluation, bitwise operations evaluate integers at the binary level. In PCS 7 picture navigation scripts, the expression 'nCallFrom & 0xF' applies a bitmask (hexadecimal 0x0F, binary 0000 1111) to isolate the lower 4 bits representing a button number, whereas '(nCallFrom & 0xF0) >> 4' masks the upper 4 bits and shifts them right by 4 positions to decode the operating area number.

Table of Contents

• Binary Arithmetic and Bitwise Operators in ANSI C for WinCC
• Bitwise AND (&) vs Logical AND (&&)
• Bitwise Shift Operators (>> and <<) Explained with Numerical Examples
• Application in PCS 7 Faceplates and Picture Hierarchy Scripts
• Summary of C-Script Bitwise and Shift Operators
• Verification
• Limitations and Operational Precautions
• Engineer FAQ
• Technical References
• Related Articles
• Related Tags

Binary Arithmetic and Bitwise Operators in ANSI C for WinCC

WinCC Graphics Designer utilizes an ANSI C script interpreter (pdlrt.exe) to execute dynamic graphic actions, button clicks, and picture navigation calls. In PCS 7 system scripts—such as @PCS7Typicals.pdl, @screen.pdl, or custom faceplate callers—multiple pieces of state information are frequently packed into a single integer or double-word tag to optimize communications across the terminal bus and plant bus.

To extract or encode packed information, C scripts rely on bitwise operators:
* & (Bitwise AND): Compares bits of two integer operands; each resulting bit is 1 only if both corresponding bits are 1.
* | (Bitwise OR): Resulting bit is 1 if either corresponding bit is 1.
* ^ (Bitwise XOR): Resulting bit is 1 if the corresponding bits differ.
* ~ (Bitwise NOT / One's Complement): Inverts all bits.
* >> (Bitwise Right Shift): Shifts bits to the right by a specified number of positions.
* << (Bitwise Left Shift): Shifts bits to the left by a specified number of positions.

Bitwise AND (&) vs Logical AND (&&)

A frequent source of confusion among automation engineers is distinguishing bitwise operators from logical operators:

1. Bitwise AND (&):
   * Evaluates two numbers bit-by-bit.
   * Example: 51 & 15
   • 51 decimal = 0x33 hex = 0011 0011 binary
   • 15 decimal = 0x0F hex = 0000 1111 binary
   • 0011 0011 & 0000 1111 = 0000 0011 binary = 3 decimal (or 0x03 hex).
   • This operation acts as a bitmask: bits set to 1 in the mask (0x0F) allow the corresponding bits of the source number to pass through, while bits set to 0 are forced to zero.
2. Logical AND (&&):
   * Evaluates boolean truth values (non-zero = TRUE, zero = FALSE).
   * Example: (x > 10) && (y < 20)
   * Returns purely boolean 1 (TRUE) or 0 (FALSE) and employs short-circuit evaluation.
   * Using & where && is intended evaluates every operand without short-circuiting, whereas using && where & is intended destroys integer bitmasks.

Bitwise Shift Operators (>> and <<) Explained with Numerical Examples

The bitwise shift operators move the binary representation of a value left or right:

1. Right Shift (>>):
   * Shifts bits to the right by N positions. Upper bits are filled with zeros (for unsigned integers), and lower bits are discarded.
   * Shifting right by 1 is mathematically equivalent to integer division by 2 (x / 2).
   * Shifting right by 4 is mathematically equivalent to dividing by 16 (x / 16 or x / 2^4).
   * Example from PCS 7 script: nOpArea = (nCallFrom & 0xF0) >> 4;
   • Suppose nCallFrom = 51 (0x33 hex, 0011 0011 binary).
   • 51 & 0xF0: Masking with 0xF0 (1111 0000 binary) yields 0011 0000 binary (48 decimal, 0x30 hex).
   • Shifting right by 4 (0011 0000 >> 4): The bits 0011 move to the lower 4 positions: 0000 0011 binary (3 decimal).
   • Thus, the upper nibble (representing the Operating Area ID) is extracted cleanly as the integer 3.
2. Left Shift (<<):
   * Shifts bits to the left by N positions, filling vacated lower bits with zeros.
   * Shifting left by 1 multiplies the integer by 2 (x * 2).
   * Shifting left by 4 multiplies the integer by 16 (x * 16).

Application in PCS 7 Faceplates and Picture Hierarchy Scripts

In PCS 7 Picture Tree Manager and standard APL faceplate scripts, bit manipulation is heavily utilized to optimize communication and standardize function calls:

// Example WinCC C-Action: Decoding packed parameter nCallFrom
#include "apdefap.h"

void OnClick(char* lpszPictureName, char* lpszObjectName, char* lpszPropertyName)
{
    long nCallFrom;
    long nButtonNumber;
    long nOpArea;

    // Retrieve packed tag representing picture call context
    nCallFrom = GetTagWord("System_PictureCallContext"); // Assume value is 51 (0x33)

    // Extract Button Number (Lower 4 bits: 0..15)
    nButtonNumber = nCallFrom & 0x0F; 

    // Extract Operating Area (Upper 4 bits: 0..15)
    nOpArea = (nCallFrom & 0xF0) >> 4;

    // Diagnostic trace to apdiag window
    printf("Decoded Context - Operating Area: %ld, Button: %ld\r\n", nOpArea, nButtonNumber);

    // Call picture based on decoded area
    if (nOpArea == 3)
    {
        OpenPictureArea3(nButtonNumber);
    }
}

This packing technique allows a single 16-bit or 32-bit tag to transfer multiple status indicators (alarm state, maintenance lock, manual mode, operator authorization) simultaneously across the network in a single read cycle.

Summary of C-Script Bitwise and Shift Operators

The table below summarizes the ANSI C bitwise operators supported by the WinCC C-Script engine and their functional behaviors.

Verification

To verify bitwise calculations in WinCC C-scripts:
1. Open the WinCC Graphics Designer and create a temporary test button.
2. In the Mouse Click event, write a C-script utilizing printf statements to print the results of the bitwise expressions.
3. Open apdiag.exe (C:\Program Files (x86)\Siemens\WinCC\uTools\apdiag.exe) and set focus on the Output Window.
4. Launch WinCC Runtime, click the test button, and observe the output in APDIAG:
* Verify that 51 & 0xF prints 3.
* Verify that (51 & 0xF0) >> 4 prints 3.
5. Verify with edge cases (e.g., input values 0, 255, 0xFFFFFFFF) to ensure no sign-extension errors occur.

Limitations and Operational Precautions

• Always use unsigned integer types (unsigned int, DWORD, BYTE) when performing bit shifts; right-shifting signed negative integers triggers arithmetic shift behavior (filling upper bits with 1s rather than 0s).
• Do not confuse bitwise AND (&) with the address-of operator (& before a variable name like &myVar), which retrieves a memory pointer in C.
• Avoid heavy bit-manipulation loops inside cyclic C actions (e.g., 250ms triggers); execute bitwise decoding only on event triggers (such as tag changes or mouse clicks) to prevent WinCC runtime thread exhaustion.
• Ensure operator precedence is clearly defined using parentheses; shift operators (>>, <<) have lower precedence than arithmetic operators (+, -).

Engineer FAQ

Why did 65 + 15 not equal 65 & 15?

+ is arithmetic addition, so 65 + 15 = 80. & is bitwise AND: 65 decimal is 0100 0001 binary and 15 decimal is 0000 1111 binary. Comparing them bit-by-bit yields 0000 0001 binary, which equals 1 decimal. Bitwise AND filters overlapping active bits, whereas addition sums quantities.

What is the difference between & and && in an if condition?

&& is a logical AND that evaluates whether both conditional statements are true, stopping evaluation immediately if the first expression is false (short-circuit). & is a bitwise operator that computes the binary intersection of two integer values.

Why does WinCC pack multiple values into a single integer tag?

Packing multiple boolean status flags or small integers into a single 16-bit or 32-bit tag significantly reduces communication bandwidth and WinCC tag license consumption, allowing rapid synchronization between the AS and OS.

Technical References

• WinCC Scripting: VBS, ANSI-C, VBA Operating Manual
• Siemens Industry Online Support FAQ: Bitwise Operations in WinCC C-Scripts

Related Articles

• Writing High-Performance WinCC C-Actions and Avoiding Thread Deadlocks
• Decoding PCS 7 APL Status Words Status1, Status2, and Status3
• Debugging WinCC Scripts Using APDIAG and Diagnostics Tracing

Related Tags

• WinCC C-Script
• Bitwise Operators
• Bit Shift
• Graphics Designer
• APDIAG