Resolving Zero Value on APL Block Icon with Non-Zero Faceplate Display - Deadband Suppression Mechanics

When an APL analog monitoring block (such as MonAnL, PIDConL, or Pcs7AnIn) displays a process value of zero on its WinCC block icon while the faceplate, transmitter, and diagnostic zoom view clearly show a non-zero reading (such as 0.11 bar), the behavior is caused by the configured Deadband parameter (Deadband). To minimize network communication load between the automation station and WinCC OS servers, APL analog blocks suppress tag transmission until process value deviations exceed the configured deadband threshold. If the measurement changes from 0.0 to 0.11 while Deadband is set to 0.2, the block icon remains at 0.0. Setting Deadband to 0.0 or below the measurement threshold in CFC or the faceplate parameter view restores real-time icon updates.

Executive Summary

During commissioning and operational tuning in SIMATIC PCS 7 V9.0 SP1, instrumentation technicians frequently report a perplexing discrepancy on analog process measurements: a pressure or flow transmitter reads 0.11 bar on its local display, and opening the standard APL faceplate's detailed view confirms an active reading of 0.11 bar, yet the overview block icon on the plant graphic stubbornly displays 0.00. Operators frequently assume this is a software defect or WinCC tag update failure. In reality, this discrepancy is the deterministic outcome of the APL Process Value Deadband mechanism. This article explains how deadband filtering optimizes terminal bus bandwidth, details why faceplates bypass deadband limits, and provides step-by-step instructions to adjust deadband thresholds.

Architecture and Fundamental Mechanics

APL Communication Optimization Architecture and Deadband Mechanics:

Purpose of the Process Value Deadband in APL:
Large PCS 7 facilities can contain tens of thousands of continuous analog measurement points.
If every controller transmitted every analog reading on every cyclic OB execution (e.g. every 100 ms in OB35), the Plant Bus and WinCC OS tag logging engine would suffer catastrophic network congestion and SQL database bloat.
To optimize throughput, Siemens APL analog blocks (MonAnL, PIDConL, Pcs7AnIn, TotalL) incorporate the Deadband parameter (Deadband, default typically 0.0 or configured per engineering standards).
The block tracks the last transmitted process value (PV_Out). As long as the absolute difference between current PV_In and last transmitted PV_Out is less than Deadband (|PV_In - PV_Out| < Deadband), the block suppresses updating the OS tag, leaving the block icon display unchanged.
Why the Faceplate Displays the Live Value (0.11):
WinCC block icons evaluate cyclic process tags broadcast across the communication channel.
When an operator opens an APL faceplate and clicks the magnifying glass (Diagnostic / Detailed View), WinCC initializes a direct, high-priority read of the internal block parameters (PV_In or raw channel inputs).
This direct query bypasses the deadband transmission filter, exposing the true raw measurement (0.11 bar) while the background block icon remains suppressed at 0.00.
Setting Guidelines:
For high-precision or safety-monitored variables (toxic gas, reactor pressure), Deadband should be set to 0.0.
For noisy variables (slurry flow, level in agitated tanks), Deadband is set to 0.5% - 1.0% of the span to eliminate graphical flicker and unnecessary archive entries.

Step-by-Step Implementation Procedure

Inspect the Deadband Parameter in CFC Test Mode:
* In SIMATIC Manager, open the CFC chart containing the MonAnL (or Pcs7AnIn) block.
* Switch CFC to Test Mode (Test -> Test Mode).
* Locate input pin Deadband on the block.
* Inspect its current value: If Deadband is configured to 0.2 (or higher) while the transmitter is reading 0.11, the transmission suppression condition is confirmed.
Modify Deadband via the Standard WinCC Faceplate (Runtime):
* In WinCC Runtime, click the block icon to open the standard APL faceplate.
* Navigate to the Parameter view (key icon or sliders tab; requires operator authorization level 5 or 6).
* Locate the parameter field labeled Deadband.
* Change the value from 0.2 to 0.0 (or a smaller value, e.g. 0.01).
* Click Apply / OK to write the new threshold to the controller.
Alternative: Adjust Deadband in CFC and Compile Changes:
* If the parameter should be permanently fixed in the offline engineering database:
* In the CFC editor, double-click the Deadband pin on the block.
* Enter 0.0 (or desired engineering limit).
* Save the chart and execute Chart -> Compile -> Changes Only.
* Download changes to the CPU in RUN mode.
Verify Signal Smoothing and Noise Limits:
* If setting Deadband := 0.0 introduces excessive graphical jitter due to sensor electrical noise, configure first-order low-pass filtering via parameter SmoothTI (smoothing filter time in seconds) on the channel driver instead of using an aggressive deadband.
Validate Block Icon Display in WinCC Runtime:
* Observe the block icon on the plant graphic screen.
* Confirm that the block icon immediately updates to display 0.11 matching the transmitter and faceplate.

Verification

Verify Process Value Synchronization and Tag Transmission:
1. In WinCC Runtime, confirm that the block icon numerical display updates dynamically with minor process changes.
2. Compare the reading on the block icon, the standard faceplate, and the detailed diagnostic zoom view; verify that all three displays show identical values.
3. In Tag Logging trend view, confirm that analog values record smoothly without artificial stairstep plateauing.

Limitations and Operational Precautions

Operational Precautions and Network Load Balancing:
- Setting Deadband := 0.0 across thousands of rapidly fluctuating analog tags can increase WinCC Tag Logging archive storage growth; configure deadband values proportionally based on process dynamics.
- A deadband only suppresses transmission of values within the band; if an analog signal crosses an alarm limit (PV_AH_Lim, PV_AL_Lim), the block overrides deadband suppression and raises the alarm immediately.

Engineer FAQ

Why does my block icon show 0.00 while the faceplate shows 0.11?

The parameter Deadband is configured higher than the current measurement change (e.g. Deadband=0.2 vs value=0.11). The block suppresses OS tag updates until the deviation exceeds the deadband.

Why does the faceplate show the real value while the icon is suppressed?

Opening the detailed view in an APL faceplate triggers a direct, real-time read of the controller memory, bypassing the cyclic deadband transmission filter.

What is the default value of Deadband on APL analog blocks?

The default value in standard APL blocks is typically 0.0, but engineering templates or mass parameterization passes often set default deadbands (e.g. 0.1 to 0.5) to manage archive growth.

Can operators adjust Deadband from the faceplate?

Yes, authorized operators (authorization level 5 or 6) can modify the Deadband setting directly in the Parameter view of the faceplate without downloading CFCs.

Does deadband suppression prevent alarm generation?

No. If a process variable violates an alarm limit, the block immediately forces an update and transmits the alarm event to WinCC Alarm Logging regardless of deadband settings.