WinCC: Compare Two Analog Values in Dynamic Dialog Expression

1. Problem Statement: Conditional Analog Comparison on a Runtime Screen

Burner control panels on combustion skids, reformer heaters and process furnaces routinely monitor two correlated analog variables: the primary flow tag and a derived back-pressure tag. Operators must react within seconds when these values drift out of the working envelope. A practical field case uses the following engineering limits:

The challenge is that both signals are read into the same controller and must be evaluated together against fixed thresholds before a runtime popup is raised. WinCC Dynamic Dialog accepts C-style expressions and can evaluate arithmetic and relational operators, but in practice the cleanest implementation routes the comparison through the controller and exposes the result as a Boolean tag, which the HMI then uses to drive Alarm Logging entries and Dynamic Dialog properties (color, flashing, visibility).

2. Architectural Decision: Compare in PLC or in HMI

WinCC Dynamic Dialog is a powerful runtime evaluator, but it is not a substitute for control logic. The Siemens documentation treats Dynamic Dialog as a presentation-layer tool: it controls the visual appearance of an object (background color, blinking, fill level, position, visibility). It can fire an event via a C-action or VB-script, but using it as the sole condition engine for safety-relevant alarms introduces four risks:

1. Scan independence. HMI scripts run on the HMI scan, not the PLC OB1 cycle. A PLC in OB35 (100 ms) can guarantee a deterministic reaction; the HMI may be busy with picture changes.
2. Connection loss behavior. If the S7 connection drops, the HMI stops evaluating. The PLC retains the last computed alarm bit, which can still drive an audible horn through a hardwired output.
3. Audit trail. Plant historians and FDA / GAMP reviews require the alarm bit to originate from the validated control layer.
4. Auditability of limits. The thresholds 3000, 10 and 2 should live as named constants in the PLC project so they appear in the validated source and the change log.

The recommended split is therefore:

• PLC: threshold comparison, hysteresis, first-up handling, latch bits.
• WinCC: alarm view, message text, dynamic dialog color/flashing, operator acknowledge.

3. Prerequisites

4. PLC Logic Implementation

4.1 S7-1500 STL Ladder (FBD view)

The classic two-network ladder using the IEC compare blocks. Each block evaluates one relational condition against the engineering tag and produces a BOOL result.

Network 1: BACK_PRESSURE_HIGH
      |  >     |
IN1---| O2_Flow |--OUT       AND
      |  3000 0 |
      |---------|---          |
      |  >     |             |
IN2---|O2_BackP |--OUT        |
      |  10.0  |             |
      |---------|-------------|
                              |
                              Q --- BackPressure_High_Alarm

Network 2: BACK_PRESSURE_LOW
      |  >     |
IN1---| O2_Flow |--OUT       AND
      |  3000 0 |
      |---------|
      |  <     |
IN2---|O2_BackP |--OUT
      |  2.0   |
      |---------|
                              Q --- BackPressure_Low_Alarm

Wire the constant block outputs to 3000.0 (REAL) and 10.0 / 2.0 (REAL). The compare blocks use the IEC 61131-3 standard operators ">" (GT) and "<" (LT) and are typed REAL so no implicit conversion occurs.

4.2 S7-1500 SCL (Structured Control Language)

SCL is preferred for complex comparisons because it is easier to maintain, supports hysteresis and one-shot latches inline, and reads as documentation. Drop the following into a function block named FB_O2_Burner_Alarm and call it from OB35 at 100 ms.

FUNCTION_BLOCK "FB_O2_Burner_Alarm"
VAR
    // Inputs
    O2_Flow           : REAL;          // Nm3/h
    O2_BackPressure   : REAL;          // bar
    Flow_Enable       : REAL := 3000.0; // Nm3/h threshold
    PressHigh_Limit   : REAL := 10.0;   // bar
    PressLow_Limit    : REAL := 2.0;    // bar
    Hysteresis        : REAL := 0.25;   // bar, prevents chattering
    // Outputs
    HighAlarm         : BOOL;
    LowAlarm          : BOOL;
    // Static
    HighAlarm_Latched : BOOL;
    LowAlarm_Latched  : BOOL;
END_VAR
BEGIN
    // High envelope: pressure above PressHigh_Limit + Hysteresis to set,
    // below PressHigh_Limit to clear (Schmitt-trigger behaviour)
    IF (O2_Flow > Flow_Enable) AND (O2_BackPressure > PressHigh_Limit + Hysteresis) THEN
        HighAlarm_Latched := TRUE;
    ELSIF (O2_BackPressure < PressHigh_Limit) THEN
        HighAlarm_Latched := FALSE;
    END_IF;

    // Low envelope
    IF (O2_Flow > Flow_Enable) AND (O2_BackPressure < PressLow_Limit - Hysteresis) THEN
        LowAlarm_Latched := TRUE;
    ELSIF (O2_BackPressure > PressLow_Limit) THEN
        LowAlarm_Latched := FALSE;
    END_IF;

    HighAlarm := HighAlarm_Latched;
    LowAlarm  := LowAlarm_Latched;
END_FUNCTION_BLOCK

The hysteresis of 0.25 bar prevents the alarm from toggling every 100 ms when the back-pressure sits exactly on the threshold — a common nuisance on combustion rigs. Increase it for slow loops, decrease it for fast loops.

4.3 Classic STEP 7 (S7-300 / S7-400)

For older CPUs without SCL (CPU 312, 314, 315-2 AH01 etc.), use STL with the IEC compare instructions:

Network 1
      L     "O2_Flow"            // REAL in MD100
      L     3.000000e+003        // REAL constant
      >R                            // REAL greater-than, sets RLO
      JCN   NOHI                  // jump if RLO = 0
      L     "O2_BackPressure"    // REAL in MD104
      L     1.000000e+001        // 10.0 bar
      >R
      JCN   NOHI
      S     "BackPressure_High_Alarm"
      JU    ENDS1
NOHI: R     "BackPressure_High_Alarm"
ENDS1: NOP   0

The S7-300 instruction set does not include a single combined compare, so two REAL compares are chained. Use MD100/MD104 as scratch REALs declared in the symbol table; do not reuse them across FBs.

5. WinCC Tag Configuration

Once the PLC writes the two BOOL outputs, register them as WinCC tags under the same S7 connection that carries the analog tags.

1. Open WinCC Explorer and select Tag Management > SIMATIC S7 PROTOCOL SUITE > TCP/IP.
2. Right-click the existing connection (for example S7_VERB1) and choose New Tag.
3. Create four tags as listed:

Set the acquisition mode to Cyclic on change for the two BOOLs so the HMI never has to poll the bit when nothing has changed. For the analog tags, choose a 500 ms cycle: 250 ms burns CPU on the panel without adding value on slow combustion processes.

6. Alarm Logging: Triggering the Runtime Popup

WinCC Alarm Logging is the correct subsystem for raising operator messages with a popup. Dynamic Dialog is for visual style; Alarm Logging is for the message text, the acknowledge logic, and the audible horn.

1. Open Alarm Logging from the WinCC Explorer and create a new message class, for example Process alarms with priority 1 (highest).
2. Add a new single message with the following parameters:

3. Repeat for message 100002 bound to BackPressure_Low_Alarm with text BACK PRESSURE NOT MATCHING (LOW) - O2 flow @ %d s, Pressure @ %.2f bar.
4. Add a WinCC Alarm Control to the runtime picture, dock it to the right side, and bind the message filter to message numbers 100001 to 100002.
5. Enable the Popup Window property on the Alarm Control with a 5-second auto-close if the bit resets before operator acknowledgement.

The two format specifiers %d and %.2f are filled from process value blocks attached to the message. Add an output value field on each message and bind it to O2_Flow_Raw (format "@s%9d@") and O2_BackPressure_Raw (format "@s%9.2f@") respectively. WinCC will substitute the live value when the alarm is raised.

7. Dynamic Dialog Expressions for Visual Feedback

Now that the message is in Alarm Logging, layer Dynamic Dialog on top of the analog value object on the picture to give the operator a glanceable cue before the popup is even shown.

1. On the process picture, insert an I/O Field bound to O2_BackPressure_Raw.
2. Right-click the I/O field, choose Properties > Appearance > Background Color > Dynamic....
3. Select Dynamic Dialog as the trigger and enter the following ANSI C expression:

((GetTagFloat("O2_Flow_Raw") > 3000.0)
   && (GetTagFloat("O2_BackPressure_Raw") > 10.0))   ?  CO_RED
: ((GetTagFloat("O2_Flow_Raw") > 3000.0)
   && (GetTagFloat("O2_BackPressure_Raw") < 2.0))    ?  CO_YELLOW
:                                                    CO_GREEN

4. Apply the same Dynamic Dialog to the Flashing property with the boolean outputs to make the field blink when an alarm bit is set.

The three-way ternary returns a color constant from the WinCC palette (CO_RED = 255, CO_YELLOW = 65535, CO_GREEN = 65280). At runtime, every change event on either analog tag re-evaluates the expression in the HMI scan and repaints the field.

8. WinCC Professional (TIA Portal) Variant

If the runtime is WinCC Professional V17/V18 inside TIA Portal, there is no Dynamic Dialog dialog box; the equivalent is the Animations editor and the PLC code-driven look-and-feel configured in the HMI tag properties.

1. Open the HMI device, navigate to Screen > IO field > Properties > Animations > Appearance.
2. Add a new animation of type Dynamic filling / color, bind the trigger to BackPressure_High_Alarm OR BackPressure_Low_Alarm using a script tag, and configure the appearance ranges:

3. For the popup itself, use the HMI Alarm Control element and bind it to the configured alarm class "Process".
4. On the PLC side, ensure that the alarm bits are routed through Program_Alarm source so the message text is generated from the PLC code (TIA V17+). This eliminates the duplication between PLC code and HMI message text and is the recommended approach for GAMP 5 projects.

The Program_Alarm instruction in SCL looks like:

IF (O2_Flow > 3000.0) AND (O2_BackPressure > 10.0) THEN
    "Program_Alarm_High" := TRUE;   // raises the alarm and sends text to HMI
END_IF;

9. Runtime Verification Checklist

1. Download the project to the S7-1500 and put it in RUN.
2. Start WinCC Runtime and open the burner process picture.
3. Force O2_Flow = 3500 Nm3/h and O2_BackPressure = 12 bar in the PLC watch table.
4. Verify that BackPressure_High_Alarm becomes TRUE within one OB35 cycle (100 ms).
5. Verify that the IO field turns red, starts flashing, and that message 100001 appears in the Alarm Control popup within 1 second.
6. Acknowledge the message; the popup auto-closes; the field remains red until the bit clears.
7. Force O2_BackPressure = 1 bar. Verify that BackPressure_High_Alarm stays latched (hysteresis) until the value crosses 9.75 bar (10.0 - 0.25).
8. Force O2_BackPressure = 0.5 bar. Verify that BackPressure_Low_Alarm goes TRUE and message 100002 fires.
9. Reset both forctes; verify both alarm bits clear and the field turns green.
10. Pull the PROFINET cable to simulate a network drop. The PLC should keep both alarm bits at their last state. Reconnect the cable; the HMI should resynchronize within 5 seconds without operator intervention.

10. Troubleshooting Matrix

11. Performance and Best Practices

On a Panel PC running WinCC Runtime Professional with 50 000 tags and 30 screens, the Dynamic Dialog engine evaluates at picture-change time and again on every configured change event. Keep the following heuristics in mind for plants with hundreds of analog points:

• Limit Dynamic Dialog to visible elements. Hidden I/O fields still evaluate unless you tick "Disable dynamic when hidden" in the configuration dialog.
• Prefer BOOL tags over re-evaluation. A single Boolean tag "O2_Burner_OutOfEnvelope" computed in the PLC drives color, flashing and visibility with one expression. This reduces the HMI scan load.
• Group related alarms under one message class. Avoid spamming the Alarm Logging archive with one message per tag.
• Use the Watchdog concept. Add a heartbeat tag toggled every cycle from the PLC; Alarm Logging raises a "Communication failure" message if the heartbeat is missing for more than 3 seconds.
• Validate the thresholds against P&ID. The numbers 3000 Nm3/h, 10 bar and 2 bar must trace back to the HAZOP and the burner OEM datasheet. Keep them in a single constants block (FC_O2_Limits) so a calibration update is one edit, not a grep-and-replace.
• Document the hysteresis. Operators will ask why a value above the high limit does not raise the alarm immediately; a comment in the FB is not enough. Add the hysteresis value as a configurable HMI tag visible from the engineering screen.

Frequently Asked Questions