Locating Time-Capture Counters in TIA Portal Programs

When a TIA Portal project accumulates dozens of FB, FC, OB, and DB blocks written across LAD, FBD, SCL, and even VBScript (HMI side), locating the block that owns a single counter, time-capture sequence, or scheduled acquisition becomes a navigation problem rather than a logic problem. This guide gives a deterministic procedure for finding the block that drives a 20-point, 3-second time capture in a cyclic interrupt such as OB30, then for verifying the interval timing and correcting it.

1. Problem Definition and Engineering Scope

The reported symptom is twofold:

1. The capture logic is hidden inside a long program split across LAD, SCL, and VBScript, in a language the user does not read, so manual network-by-network review is impractical.
2. The intended 3-second interval between successive temperature captures is not being honored — successive readings are not 3000 ms apart.

Both symptoms share a root cause area: the cyclic interrupt organization block (OB30) that drives the capture sequence. The capture rate can drift or fail if (a) the OB30 cycle time is misconfigured, (b) the time base feeding the counter is a TON driven by OB1 cycle jitter rather than a hardware timer, or (c) the counter is reset by a condition other than completion of the 20-point window.

2. Prerequisites

• TIA Portal V15.1 or later (V16/V17 recommended for the Watch Table, Trace, and PLC Code Viewer features used below). Refer to the Siemens TIA Portal release notes for the features of your installed version.
• Project source files with write access (so cross-references can be re-generated on recompile).
• Online connection to the PLC for verification (or a PLCSIM instance mirroring the project).
• Operator rights to enable the HMI Runtime if VBScript is involved in the capture sequence.
• For interval measurement: a function-capable multimeter, scope on a DO flag, or the built-in TIA Portal Trace.

3. Navigation Strategy Overview

Use a four-step trace path. Each step is cheaper than the previous one to undo if it leads to a dead end.

1. Tag-driven cross-reference — start from the DB that stores the 20-row table; TIA Portal will list every block that reads or writes it.
2. Call structure (CFC-style) — from the candidate block, expand the call tree up to OB30 and down to any multi-instance DBs.
3. Search-in-project — sweep the project for the integer constant 3000 (ms), the tag named with a _count / _idx suffix, and the symbolic name of the temperature input.
4. Trace / watch verification — online, observe the counter increment, the array index advance, and the time delta between writes.

4. Step 1 — Open the Cross-Reference Browser

The cross-reference tool is the fastest path to the capture block because it does not require reading the code; it only requires knowing one tag.

Procedure:

1. Identify the data block that contains the 20-row temperature table. The user can usually find it in the HMI tag list (HMI → PLC tags), because the HMI is bound to the array tag, even if the HMI script is written in a foreign language.
2. In the project tree, right-click the DB and select Cross-references, or place the cursor on the array element and press Ctrl+Alt+F7.
3. TIA Portal opens the Cross-reference window, which lists every block that writes, reads, or accesses the array. Filter by Access type = Write to find the block that fills the table.
4. Double-click the entry under Access; TIA Portal jumps directly to the network that performs the write. Even if the block is in SCL, TIA Portal will highlight the line.

Shortcut reference:

5. Step 2 — Identify the Cyclic Interrupt Source (OB30)

The capture sequence is a 20-point loop. The 3-second period almost always originates from a cyclic interrupt. Open the Program info or Call structure to see which OBs call the candidate block.

Procedure:

1. Right-click the block found in step 1 → Call structure.
2. Expand the tree upward. The capture block is typically called from OB30 (cyclic interrupt), OB35 (100 ms default), or from OB1 (main cycle) — each tells you the time base.
3. Select OB30 in the tree → right-click → Properties. Verify the Cycle time and Phase offset. The factory default is 5 000 ms for OB30 on S7-300/400, but a project that uses OB30 for a 3 s capture will normally set the cycle to 3 000 ms (or 100 ms if a software counter inside the block divides the rate).

For S7-1200/S7-1500 the cyclic interrupt is configured under PLC → Program blocks → OB30 → Properties → Cyclic interrupt [OB30]. The Siemens S7-1500 OB30 specification states that OB30 is always synchronized to the configured phase offset, so a write occurring once per 3 s is best implemented as a 3 000 ms OB30 calling a single network that increments a 0..19 index.

6. Step 3 — Find the Counter or Index

Inside the candidate block, locate the variable that ticks once per acquisition. The tag name typically contains one of the suffixes below.

Click on the array write line found in step 1. Above the write, TIA Portal will show the read of the counter/index. Common SCL patterns are:

#iCount := #iCount + 1;
IF #iCount >= 20 THEN
  #iCount := 0;
END_IF;
"MyArray".Temp[#iCount] := "MyAI".PV;

or, more compactly, the project might use an IEC counter CTU instance DB_Counter. The cross-reference on the instance's CV (current value) tag jumps directly to the network where it is compared against the constant 20.

7. Step 4 — Search the Project for the 3-Second Constant

Open the project-wide search (Ctrl+F) and search for these tokens in order:

1. 3000 — direct ms constant for a 3 s timer.
2. T#3s — IEC time literal (more common in SCL).
3. 3.0 — may be a scaled time constant (seconds × 1000).
4. OB30_PERIOD or any constant DB entry named with a time suffix.

Match the search across All blocks, not just the current block, by clicking the small arrow at the right of the search box and selecting Search in project. The first match is normally the period source. If the constant is inside a tag (e.g. Config.tPeriodMs := 3000), double-click the constant to jump to the FB that owns the tag and inspect the default value.

8. Step 5 — Why the 3-Second Interval Is Wrong

After locating the counter, verify these common failure modes against the code. Each is a field-proven cause of interval drift in OB30-driven captures.

9. Step 6 — Verify with Watch Table and Trace

Online verification is mandatory because the offline cross-reference cannot reveal priority or timing bugs.

1. Create a new Watch Table (Watch table_1) and add:
   • The counter / index tag
   • The OB30 cycle time tag (read from System info)
   • The array element the code writes to (e.g. "MyTable".Temp[0])
   • A tag indicating the time of last write (often a TIME or TIMESTAMP in the DB)
2. Right-click the watch table → Modify if you need to manually reset the counter to 0 to start a clean 20-point run.
3. Use the Trace feature (PLC → Traces → New trace) to record the counter at 1 ms resolution. TIA Portal can decode the time delta between successive counter increments and overlay it on the configured OB30 cycle.

Acceptance criterion: the time between counter increments must equal the OB30 configured cycle ± 1 ms. The Siemens S7-1500 Trace configuration guide explains how to set a rising-edge trigger on the counter tag and capture a 60 s window.

10. Common Pitfalls and Field-Notes

• Two-phase capture in one FB. If the project uses 20 captures in two phases (10 + 10), the second phase is often a separate FC called by an IF #iPhase = 1 THEN. Use the Call structure to see both phases even if only the first FC is called from OB30.
• VBScript on the HMI side. If the HMI script reads SmartTags("DB") on a 100 ms schedule, the HMI can advance its own counter and write it back to the PLC, masking the OB30 logic. Check the HMI tag's Acquisition mode in the WinCC tag list.
• Indirect array access. SCL often uses a WORD_TO_BLOCK_DB pattern with an offset word to write into the array. Search for the literal DW#16# or the PEEK / POKE symbols.
• Multi-instance DBs. The counter may be a static tag of an FB that is called as a multi-instance; the name on the cross-reference will look like FB_PID_1.InstanceCounter. Search for the FB name and let the cross-reference descend into the instance DB.
• Rewiring after a Compile all. After a full compile, TIA Portal re-generates cross-references. If you change the period constant, the next compile is the only confirmation that no other block still uses the old value.

11. Summary Procedure (Cheat-Sheet)

1. Right-click the 20-row DB → Cross-references → filter Write.
2. Open the Call structure of the writing block; confirm OB30 is the parent.
3. Inspect the counter / index tag increment and reset condition.
4. Search the project for 3000, T#3s, or the period tag name.
5. Validate the OB30 cycle and phase offset in the PLC properties.
6. Run online: watch the counter, use Trace to record intervals, and verify 3 000 ms ± 1 ms between writes.