TIA Portal HMI Multiplexing for Indexed PLC Data Arrays

Overview: The Indexed HMI Variable Problem

When a SIMATIC PLC exposes a data block that contains an ARRAY of identical structures (for example, DB_Drive.DRV[0..31] holding Goal_Pos, Actual_Pos, Enable and other parameters per drive), engineers naturally want to bind a single HMI faceplate or a single set of E/A fields to "whichever drive the operator currently selects". The selection is driven by an index variable such as DriveNumber typed as INT or DINT.

The intuitive attempt is to type the symbolic path directly into the HMI variable table, for example "DB_Drive".DRV[DriveNumber].Goal_Pos. The HMI compiler rejects the path because TIA Portal HMI tags require a static, fully resolved symbolic name at compile time; only the PLC's SCL/ STL/ LAD/FBD editors evaluate index expressions at runtime. The same restriction applies to Drive.DRV_[Index].Goal_Pos or any bracketed indirection embedded inside an HMI tag's PLC name field.

Three production-grade methods solve the problem without changing the existing DB_Drive structure or breaking the installed base of running machines:

• Method 1 – Native HMI tag multiplexing (WinCC Unified, WinCC Professional, WinCC Comfort/Advanced where supported).
• Method 2 – PLC-side mirror DB that copies the indexed element into a single-instance shadow structure that the HMI addresses directly.
• Method 3 – VBScript or C-script runtime indirection on legacy panels that lack multiplexing support.

Why Direct Indexed Addressing Fails in TIA Portal HMI Tags

The HMI tag table in TIA Portal produces a fixed-point list of absolute PLC addresses at compile time. The runtime on the panel resolves each tag only by its symbolic name to a single byte/bit offset inside a known DB. The HMI does not contain the SCL compiler; expressions such as DRV[DriveNumber] cannot be evaluated by the panel firmware. The PLC accepts the index because the SCL/STL instruction set implements DB[DBD].DRV[i] at runtime, but the HMI side has no equivalent engine.

The result on screen is one of the following diagnostics in the TIA Portal "Compile" output or directly on the panel:

• "The PLC tag does not exist or is not accessible."
• "Invalid address or unsupported data type."
• Field remains greyed out and shows #### or ??? at runtime.

Prerequisites

1. TIA Portal V16 or later recommended. Multiplexing for WinCC Unified became broadly available from V15.1; legacy Comfort/Advanced panels require the multiplexing add-on from V14 SP1. Refer to the Siemens Industry Online Support entry for "Multiplexing in WinCC" for the exact panel firmware / TIA Portal compatibility matrix.
2. SIMATIC panel: Comfort, Unified Comfort, or WinCC Runtime Professional. Basic panels (KTP400 mono and KTP700/1000 mono) historically did not support multiplexing; use the mirror-DB method instead.
3. PLC: SIMATIC S7-1200 (firmware V4.2 or later) or S7-1500 (any standard firmware). Symbolic addressing must be enabled and the DB must be downloaded with optimized access or with standard access as appropriate for the chosen method.
4. Source DB structure already defined (do not modify it because the installed base depends on it). A typical pattern:

TYPE UDT_Drive :
  STRUCT
    Goal_Pos       : REAL;
    Actual_Pos     : REAL;
    Enable         : BOOL;
    Run            : BOOL;
    Error          : BOOL;
  END_STRUCT;
END_TYPE

DATA_BLOCK DB_Drive
  STRUCT
    DRV : ARRAY[0..31] OF UDT_Drive;
  END_STRUCT
END_DATA_BLOCK

Method 1 – Native HMI Tag Multiplexing (WinCC)

Native multiplexing in WinCC Unified and WinCC Comfort/Advanced lets a single HMI tag point to one of N configured PLC tags. The selection is driven by a separate HMI index tag whose value the runtime reads to choose which physical connection is active. Bidirectional read/write is supported, which makes this method the cleanest answer to "I want to see and modify the value with one E/A box".

Configuration Steps

1. In the TIA Portal project tree, open HMI Tags and create a new tag, e.g. HMI_Goal_Pos, with data type REAL and the desired acquisition cycle (250 ms default is acceptable for axis position).
2. Open the tag's properties and select Multiplexing. Enable the option and add one row per drive index you want to address. Each row contains the fully qualified symbolic PLC tag: "DB_Drive".DRV[0].Goal_Pos, "DB_Drive".DRV[1].Goal_Pos, … "DB_Drive".DRV[31].Goal_Pos.
3. Bind the Index tag property to the PLC tag that carries the current drive selection, typically "DB_Drive".DriveNumber (INT) or any HMI tag that mirrors it.
4. Drag the multiplexed tag HMI_Goal_Pos onto the screen and bind an E/A field to it. Set the field to Input/Output mode. The field now reads from and writes to whichever drive the index variable points at.
5. Repeat the procedure for each member of the UDT you want to expose (for example HMI_Enable, HMI_Actual_Pos).

Runtime Behaviour and Limits

• Index out of range (for example DriveNumber = 32 in a 0..31 array) causes the HMI to keep the previous value and raise a connection diagnostic in the alarm log. Always clamp the index in the PLC: IF DriveNumber > 31 OR DriveNumber < 0 THEN DriveNumber := 0; END_IF;
• Multiplexing is processed on every acquisition cycle of the tag. A 100 ms cycle is the practical lower bound; faster rates consume significant CPU on Comfort panels.
• WinCC Unified supports multiplexing across multiple PLCs as well, useful for cells that are split across controllers.

Method 2 – PLC-Side Mirror DB Approach

If the panel does not support multiplexing (Basic panels, older Multi panels, third-party SCADA, or WinCC Runtime on a PC that needs indirect addressing without tag lists), build a "mirror" or "shadow" DB on the PLC that always contains exactly one drive's data. The HMI addresses only this single-instance structure; a PLC routine copies the indexed element in and out on every scan.

Mirror DB Definition

DATA_BLOCK DB_DriveMirror
  STRUCT
    SelectedIndex : INT;
    DRV : UDT_Drive;          // single instance, not an array
    NewValue       : REAL;     // staging area for incoming writes
    Cmd_Apply      : BOOL;     // rising edge triggers a copy back
    Cmd_Read       : BOOL;     // rising edge triggers a refresh from indexed source
  END_STRUCT
END_DATA_BLOCK

Mirror Logic in SCL (S7-1500 / S7-1200)

// Called from OB35 every 100 ms or from a cyclic interrupt OB
IF #Cmd_Read OR (mirror.SelectedIndex <> last_index) THEN
  IF (#SelectedIndex >= 0) AND (#SelectedIndex <= 31) THEN
    #DRV := "DB_Drive".DRV[#SelectedIndex];    // copy indexed element to mirror
    last_index := #SelectedIndex;
  END_IF;
  #Cmd_Read := FALSE;
END_IF;

IF #Cmd_Apply THEN
  IF (#SelectedIndex >= 0) AND (#SelectedIndex <= 31) THEN
    "DB_Drive".DRV[#SelectedIndex].Goal_Pos := #NewValue;
  END_IF;
  #Cmd_Apply := FALSE;
END_IF;

Bidirectional Operation

The mirror makes read flow trivial: the HMI binds an E/A field to DB_DriveMirror.DRV.Goal_Pos and the field always reflects the indexed drive. Writes need a small handshake because the panel cannot write directly into an array element. Typical pattern:

1. Operator edits DB_DriveMirror.NewValue in an HMI input field.
2. Operator presses Apply; the HMI button sets DB_DriveMirror.Cmd_Apply = TRUE.
3. The PLC detects the rising edge and copies NewValue into the indexed DB_Drive.DRV[SelectedIndex].Goal_Pos, then clears Cmd_Apply.
4. On the next cycle, DB_DriveMirror.DRV.Goal_Pos is refreshed by the read direction, so the field shows the new value.

Method 3 – VBScript Runtime Multiplexing (Legacy Panels)

On Comfort panels running V14 firmware or earlier, and on WinCC Runtime Advanced, scripting is the alternative when neither native multiplexing nor a PLC mirror is desired. Two scripts, executed on tag-value-change or on button events, achieve the indirection.

Read Script (cyclically executed)

' VBScript – bound to the "Change value" event of the index HMI tag
Dim idx, path
idx = SmartTags("DriveNumber")
path = "DB_Drive".DRV[" & idx & "].Goal_Pos"   ' symbolic path
SmartTags("HMI_Goal_Pos").Read path            ' runtime resolve

Write Script (button "Apply" click)

Dim idx, newVal
idx = SmartTags("DriveNumber")
newVal = SmartTags("HMI_Goal_Pos_New")
SmartTags("HMI_Goal_Pos").Write "DB_Drive".DRV[" & idx & "].Goal_Pos", newVal
SmartTags("Cmd_Apply") = True

The script approach is the most fragile of the three because tag paths are concatenated at runtime; a typographical error in the DB name or a typo in the UDT element name produces a silent fault that only shows up in the panel's diagnostic buffer. Reserve this method for situations where Method 1 and Method 2 are not options.

Known Bugs and Field-Proven Caveats

Several issues are reported by field engineers when implementing indexed HMI bindings. The following list captures the most reproducible ones and the verified workaround for each.

Verification Checklist

1. Compile the TIA Portal project. The "Compile" log must contain no "The tag is not assigned to a PLC tag" entries for multiplexed tags.
2. Download the HMI and PLC programs. Open an online watch table on the PLC and force DriveNumber to 0, 1, 2, … 31. Verify that DB_Drive.DRV[i].Goal_Pos writes from the HMI land in the correct array element.
3. On the panel, open the diagnostics view (Diagnostics > Connections on Unified, System > Diagnostics on Comfort). Confirm that no multiplexed tag reports a connection error.
4. Run a 24-hour soak test with an automated index sweep in OB35: i := (i + 1) MOD 32;. Verify that no alarm is raised and that the E/A field always shows the value from the currently indexed drive.
5. For BOOL tags, verify both directions: write from the panel, observe in the watch table; force from the watch table, observe the panel update.
6. For sliders, change the index through the full 0..31 range while the panel is in runtime. The slider must remain visible at all times.

Troubleshooting Matrix

Field-Engineering Notes

• Always keep the original DB_Drive structure intact. The mirror DB is a non-invasive add-on, which is critical when many running machines already use the original layout.
• Document the multiplexing lists in the project documentation. Each row must be listed with the symbolic path, the data type, and the acquisition cycle.
• When migrating from a Comfort panel to a Unified panel, re-test the multiplexing behaviour: the acquisition cycle default is different (250 ms vs 100 ms), and some bug fixes (slider visibility) are panel-firmware-specific.
• For S7-1500, prefer symbolic addressing on the HMI connection. This is required for any multiplexed tag whose source is an optimized-access DB.
• Use the Siemens Industry Online Support entry "Multiplexing HMI Tags" as the canonical reference. Cross-check the panel firmware and TIA Portal version in the compatibility matrix before commissioning.
• For PC-based SCADA (WinCC Runtime Professional), multiplexing is supported in the same way as on Unified; the same caveats about range re-evaluation of sliders apply.