Siemens HMI Bool Tag: Operator Toggle via Input/Output Field

1. Overview

Boolean (BOOL) tags are the workhorse of operator interaction on Siemens SIMATIC HMI panels. A typical field requirement is to let an operator view the current state of a process flag in the PLC (TRUE / FALSE) and also force that flag from the HMI without leaving the screen. The correct way to do this on a Siemens OP (Operator Panel), KTP, TP, or Comfort Panel is to bind a screen object to the BOOL tag with read and write rights and to map the numeric representation (0/1) to a human-readable text using a Text List.

This reference covers the full configuration path on both TIA Portal (V15.1 and later, including V17 / V18) and the legacy SIMATIC WinCC Flexible (2008 SP5 and earlier), together with the required PLC tag properties, the operator interaction model, the verification steps, and a troubleshooting matrix for the most common field issues.

2. Prerequisites

3. Understanding Bool Tags in the Siemens HMI Model

A Siemens HMI tag is a pointer to a real memory location in the PLC. For a BOOL tag, the HMI transfers a single bit (0 or 1) on every configured acquisition cycle. The default data type mapping in TIA Portal uses Bool on the PLC side and Bool on the HMI side; the wire format is one bit packed into the smallest available byte. According to the SIMATIC TIA Portal Help - HMI Tags documentation, the HMI tag's Access mode must be set to read/write if the operator is to change the value from the panel.

4. Step-by-Step: TIA Portal Configuration

1. Create the HMI tag. In the project tree, open the HMI device → HMI tags → Show all tags. Add a new tag, give it a name (for example bOperatorEnable), and set the connection to the PLC.
2. Bind the PLC tag. In the tag's properties, click the browse button of PLC tag and select a BOOL tag from the PLC program (e.g. "Data_block_1".bEnable). Make sure the PLC tag itself is not marked as non-readable in the PLC's protection settings.
3. Set Access mode to <absolute access> read/write. This is the property that unlocks operator writes. TIA Portal will display a yellow warning icon if the connected PLC tag has read-only access; resolve that on the PLC side first.
4. Add a Text List. In the project tree, open HMI > Text and graphic lists > Text lists. Create a new text list with the following properties:

   Property	Value
   Name	tlBool_OnOff
   List range	0..1 (two entries)
   Default entry	0 (or whatever is process-safe)

5. Define the entries. Add value 0 → text OFF and value 1 → text ON. You can also use FALSE / TRUE, STOP / START, 0 / 1, or any bilingual pair by enabling the language switch.
6. Insert an Input/Output field on the screen. Open your screen, drag an IO field from the toolbox onto the canvas. In the properties:
   • Mode → Input/output (not Output only)
   • Process value → bind to bOperatorEnable
   • Display → set to Text
   • Text list → select tlBool_OnOff
   • Appearance → choose the desired field type, ideally Drop-down list or Toggle on Comfort panels
7. Assign operator authorization. Under Properties > Security, set the required operator level (e.g. User / Authorization 6). The operator will be prompted for login if the level is above their current session.
8. Compile and download both the PLC and HMI project. On Comfort Panels, use the "Software" load mode; on Basic Panels, delta load is sufficient when only the screen has changed.

5. Step-by-Step: WinCC Flexible Configuration

1. Open the project in SIMATIC WinCC Flexible and connect to the target OP.
2. Open Project > Communication > Tags, click New, and create a tag with the following properties:

   Property	Value
   Name	bOperatorEnable
   Connection	your S7 connection (e.g. PLC_1)
   Address	DB 1 DBX 0.0 (or M 0.0)
   Data type	Bool
   Acquisition mode	Cyclic continuous
   Acquisition cycle	1 s
   Length	1
   Access rights	Read/Write (operator level)

3. Open Project > Text Library (or directly on the IO field) to add the text list. The Wizard method described in the original engineering note is equivalent: create a text list with two entries:
   • Value 0 → FALSE
   • Value 1 → TRUE
4. Place an Input/Output field on the screen. In the configuration dialog, set:
   • Tag → bOperatorEnable
   • Type → Output is wrong; choose Input/Output or Input
   • Display → Text (decimal / binary will also work but is not human-friendly)
   • Text list → assign the two-entry list
5. Compile the project and transfer to the OP. The default operator interaction on the OP is: tap the field, the Shift indicator appears, then press the Cursor Up key (or the screen's up arrow) to toggle the value. On touch panels, the field acts as a direct toggle when configured with Display = Text and the dropdown list mode.

6. PLC Program Considerations

The HMI only writes a value when the operator confirms. The PLC program must therefore tolerate an asynchronous write and must not treat a brief transition as an event.

// SCL (S7-1500, TIA Portal V18)
FUNCTION_BLOCK fbOperatorEnable
VAR
    bEnable        : BOOL;   // written by HMI, read by logic
    bEnableAck     : BOOL;   // optional echo for the HMI
    bOperatorReq   : BOOL;   // one-shot from the HMI request
END_VAR

// Latch the operator's command on a rising edge of the request
IF bOperatorReq AND NOT bEnable THEN
    bEnable := TRUE;
    bEnableAck := TRUE;
ELSIF bOperatorReq AND bEnable THEN
    bEnable := FALSE;
    bEnableAck := FALSE;
END_IF;

// ... rest of process logic uses bEnable as a permissive

For non-retentive memory (M bits), the value is reset on STOP → RUN. Place operator-controlled bools in a retentive DB if the process must survive a warm restart.

7. Verification

1. Go online with the PLC. Watch the BOOL tag in the watch table. Force a manual change on the HMI and confirm the value flips in TIA Portal / STEP 7 within one acquisition cycle (default 1 s).
2. Toggle in the opposite direction and verify the text list entry swaps from ON → OFF on the OP.
3. Switch operator to a non-authorized user; the field should display but be unchangeable, and the OP should raise the standard "Insufficient user rights" message.
4. Power-cycle the panel (or simulate a re-connect) and confirm the OP reads the current PLC state on the first scan — the screen must not show a stale value.
5. On Comfort Panels, enable the Diagnosis Viewer and check the connection status of the HMI tag. A red X indicates the tag is unbound or the PLC is in STOP.

8. Security and Access Control

The standard SIMATIC authorization model is documented in the TIA Portal Help - User Administration page. Operator-controlled bools typically belong to authorization level 6 ("Operator"). Higher-risk flags — e.g. a remote reset of a safety permissive — should require a Supervisor (level 4) or Administrator (level 1) login, or better still, be implemented through a separate confirmation screen.

9. Troubleshooting Matrix

10. Alternative Methods for the Same Goal

11. Cross-Platform Notes (OP 73 / OP 77A-B / OP 177B / KTP / Comfort)

• OP 73 / OP 77A / OP 77B (text-based, monochrome): use the IO field with the DEC display and a two-line text list. The toggle is via SHIFT + Cursor Up as described in the original engineering note.
• OP 177B (touch, color): identical configuration but the screen is touch-driven, so a Drop-down list IO field is friendlier.
• KTP400 Basic / KTP700 Basic / KTP1200 Basic (WinCC Basic): the configuration dialog hides some advanced properties; the IO field + Text List recipe still applies.
• TP Comfort / TM Comfort (WinCC Comfort / Advanced): full drop-down list, toggle button, and confirmation dialog available. Use these for new projects whenever possible — they are the current generation and benefit from active Siemens firmware updates.

12. Field Commissioning Checklist

• PLC tag is a BOOL in a reachable DB or memory area.
• HMI tag is configured read/write with the correct acquisition cycle.
• IO field mode = Input/Output (not Output only).
• Display = Text and the text list has two entries (0 → OFF, 1 → ON).
• Operator authorization matches the user group policy.
• PLC program does not unconditionally overwrite the bit.
• Watch table confirms the bit flips on every operator change.
• Power-cycle test passed.
• Functional-safety boundary respected (the bit is not a safety I/O).