Extracting Substring Characters and Linking Dynamic Text Signals Between WinCC OS and AS

In SIMATIC PCS 7 systems, extracting characters from an HMI output string—such as isolating substring 'AS2' from a composite identifier to generate a boolean supervisory input—can be executed in WinCC OS graphics via VBScript Mid() functions or in the Automation Station using standard S7 IEC string blocks (MID, FIND, EQ_STRING). Implementing string comparison requires configuring matching string lengths in Tag Management and selecting event-driven triggers to prevent excessive CPU or script execution loads.

String Data Handling Across SIMATIC PCS 7 OS and AS Layers

In process automation, alphanumeric strings represent batch identifiers, recipe codes, equipment IDs, and operator input strings. Frequently, supervisory logic must parse a subset of characters from a dynamic text string to trigger downstream automation actions (for example, detecting if a vessel code contains "AS2" to set an authorization bit):
* S7 String Format: In STEP 7 / CFC, a STRING variable is structured with two management header bytes: Byte 0 defines the maximum allocated length (up to 254 characters), Byte 1 defines the current actual length, followed by the ASCII character bytes.
* WinCC Text Variable: In WinCC OS, strings are mapped as Text tag 8-bit character set or Text tag 16-bit character set.
* Processing Location: String extraction can be performed at the supervisory OS layer (via VBScript or ANSI-C) or deterministically inside the Automation Station (via CFC IEC library blocks).

Method 1: WinCC OS VBScript Extraction and Comparison

To parse characters directly within a WinCC Graphics Designer screen:
1. Open the process picture (.pdl) in Graphics Designer.
2. Select the I/O field or text display object where the source string is displayed.
3. In Object Properties, navigate to Events -> Property Topics -> Output/Input -> Output Value.
4. Right-click the Change event and select VBS-Action.
5. Implement the extraction logic using the VBScript Mid() function:

Dim sourceStr, subStr, targetTag
sourceStr = ScreenItems("IOField_Source").OutputValue
' Extract 3 characters starting from character position 4
subStr = Mid(sourceStr, 4, 3)

' Compare substring against target string
Set targetTag = HMIRuntime.Tags("HMI_Input_Bool")
If subStr = "AS2" Then
    targetTag.Write 1
Else
    targetTag.Write 0
End If
Set targetTag = Nothing

Set the trigger to Upon Change of the source string tag. Never use a cyclic timer (e.g., 250 ms) for string scripts to avoid unnecessary script queue load.

Method 2: Automation Station (CFC) IEC String Extraction and Comparison

For safety-critical or deterministic interlocking, string parsing should execute inside the S7-400 or CPU 410 controller rather than in WinCC:
1. In the CFC chart, open the block catalog under Libraries -> Standard Library -> IEC Function Blocks.
2. Instantiate the following standard IEC blocks:
* MID (FC26): Extracts a substring of specified length (L) starting at character position (P) from input string (IN).
* EQ_STRNG (FC10): Compares two input strings (S1 and S2) and outputs a boolean 1 if they match identically.
* FIND (FC17): Searches for the presence of a substring within a larger string and returns the character position index (returns 0 if not found).
3. Connect the dynamic source string tag to input IN of MID.
4. Set input parameter L = 3 (length) and P = 4 (start position).
5. Feed the extracted output string from MID into input S1 of EQ_STRNG.
6. Assign constant 'AS2' to input S2.
7. Connect the boolean output OUT of EQ_STRNG directly to the target APL interlock or control block input pin.

Tag Management and AS-OS Data Transfer Configuration

When passing string variables between AS and OS:
* Length Consistency: Ensure the string length declared in the S7 data block matches the length configured in WinCC Tag Management (e.g., STRING[16]). Mismatched lengths can cause buffer truncations or corrupt adjacent data bytes.
* AS-OS Compilation: When creating the source string in a CFC block with attribute S7_m_c = 'true', execute an AS-OS compilation to generate the WinCC text tag automatically, guaranteeing matching address pointers.
* Data Consistency: Because string variables span multiple data bytes, update strings as atomic records in AS logic to prevent WinCC from reading half-updated text during execution cycles.

Verification

In WinCC runtime, enter a test string containing "PREFIX_AS2_SUFFIX" into the input field.
Verify that the VBScript or CFC logic isolates "AS2".
Confirm that the boolean status tag transitions from 0 to 1.
Enter an alternative string (e.g., "PREFIX_AS3_SUFFIX"): verify that the boolean output returns to 0.
Check the WinCC Global Script diagnostics window (apdiag.exe) to confirm no VBScript runtime errors or type mismatch exceptions occurred.

Limitations and Operational Precautions

Avoid executing string manipulation scripts in WinCC cyclic screen triggers; cyclic string operations cause high CPU utilization on OS clients.
S7-400 string operations in CFC consume significantly more CPU instruction cycles than standard integer or boolean logic; avoid executing string functions in fast cyclic interrupt OBs (e.g., OB35 at 100 ms) unless necessary.
VBScript string comparisons are case-sensitive by default; use UCase() in VBScript or normalized strings in AS to ensure reliable matching.

Engineer FAQ

What is the difference between MID and FIND in S7 IEC string blocks?
MID extracts a fixed-length substring starting at a known character offset, while FIND searches for a substring and returns its dynamic position index.
How are strings structured in Siemens S7-400 data blocks?
Byte 0 contains the maximum allocated length, Byte 1 contains the current actual string length, and subsequent bytes store the ASCII characters.
Can I use C-Script instead of VBScript in WinCC for string extraction?
Yes, using standard C library functions such as strncpy() or strstr() inside an action script triggered on tag change.
Why did my VBScript generate a 'Type Mismatch' error during string parsing?
This error occurs when the referenced tag is empty (VT_EMPTY), null, or cannot be implicitly cast to a string. Always validate tag quality before calling Mid().
Does writing to an HMI internal boolean tag automatically notify the AS controller?
Only if the tag is an external process tag mapped to an AS data block address; internal HMI tags exist solely within the WinCC memory space.
How do I make VBScript string comparison case-insensitive?
Wrap the variables in UCase(), for example: If UCase(subStr) = "AS2" Then.
Can CFC IEC string blocks handle strings longer than 254 characters?
No, standard STEP 7 STRING variables have a maximum architectural limit of 254 ASCII characters.
What happens if the start position P in MID exceeds the length of the string?
The MID block returns an empty string ('') and sets the actual length byte to 0 without causing a CPU crash.
How can I monitor active VBScript errors in WinCC runtime?
Launch the WinCC diagnosis tool C:/Program Files (x86)/Siemens/WinCC/bin/apdiag.exe and open the Output Window.
Is string comparison in CFC bumpless during redundant AS switchover?
Yes, string data in instance data blocks is synchronized across redundant fiber-optic links identically to other data types.

Technical References

SIMATIC STEP 7 V5.6 System and Standard Functions Reference Manual
SIMATIC HMI WinCC V7.5 VBScript for Process Communication Reference
Siemens Industry Online Support Entry: String Operations in STEP 7 and WinCC (Entry ID 298697)
SIMATIC PCS 7 Advanced Process Library (APL) Function Manual

Related Tags

PCS 7, WinCC, VBScript, String Manipulation, CFC, IEC Functions, MID, EQ_STRNG, Tag Management, S7-400

Extracting Substring Characters and Linking Dynamic Text Signals Between WinCC OS and AS