C135 PCS 7 (PCS7)
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Filtering Transient Wire-Break Alarms on Pneumatic Positioner Analog Outputs Using DelTiBad

Filtering Transient Wire-Break Alarms on Pneumatic Positioner Analog Outputs Using DelTiBad

To eliminate nuisance wire-break ('Drahtbruch') diagnostic alarms caused by pneumatic valve positioners (such as ABB TZIDC) momentarily drawing down loop current during large setpoint steps on analog output modules (6ES7332-5HD01-0AB0), configure the 'DelTiBad' (Delay Time for Bad Signal Status) parameter on the APL channel driver block Pcs7AnOu (FB1870). Introduced in PCS 7 V9.1 SP1, setting DelTiBad to a non-zero value (e.g., 1.0 to 1.5 seconds) delays the propagation of transient hardware diagnostic interrupts, filtering out momentary sub-second wire-break glitches before they trigger 'black' system diagnostic alarms in WinCC. In earlier PCS 7 versions lacking DelTiBad, configure setpoint rate limiting (SP_RateUp / SP_RateDn) on upstream control blocks or install loop-powered signal isolators.

Table of Contents

Executive Summary

In industrial process control plants utilizing pneumatic control valves, smart positioners (such as the ABB TZIDC series) modulate valve position based on a 4–20 mA setpoint received from PCS 7 analog output modules (e.g., 6ES7332-5HD01-0AB0). A common and disruptive phenomenon occurs when significant setpoint changes are commanded: during large setpoint steps (e.g., stepping from 0% to 60%), the internal I/P converter and electronics of the positioner momentarily cause loop impedance fluctuations, depressing loop current below the module's wire-break detection threshold for several hundred milliseconds.

While the valve actuates smoothly and the loop recovers immediately, this sub-second voltage sag generates a hardware diagnostic alarm (OB 82 diagnostic interrupt). In WinCC, this manifests as a recurring 'black' system alarm (wire-break / channel fault). Over time, constant false alarms cause alarm fatigue, leading operators to ignore genuine hardware failures. While older PCS 7 versions lacked driver-level diagnostic filtering, PCS 7 V9.1 SP1 introduced the dedicated DelTiBad parameter on channel drivers. This guide analyzes the electrical mechanism behind positioner loop dips, details the implementation of DelTiBad in CFC charts, and outlines alternative mitigation strategies for legacy PCS 7 versions.

Architecture and Fundamental Mechanics

The interaction between the analog output module, channel driver block, and field positioner involves both electrical and software processing loops:
1. Electrical Loop Dynamics: The 6ES7332-5HD01-0AB0 analog output module continuously monitors loop integrity. When configured for 4–20 mA with wire-break diagnostics enabled, internal comparators detect an open circuit if the loop resistance exceeds maximum permissible load impedance (e.g., > 600 ohms) or if loop current cannot be driven to setpoint. During rapid step changes, positioner capacitive loading and piezo-valve firing can transiently starve the output circuit, triggering an instantaneous open-circuit condition lasting 50 ms to 300 ms.
2. Diagnostic Pipeline: The module issues a hardware diagnostic interrupt (OB 82). The PCS 7 Basis Library diagnostic block (MOD_D1) decodes the interrupt frame and forwards the channel fault flag to the channel driver (Pcs7AnOu, FB1870).
3. Channel Driver Evaluation (DelTiBad):
* In PCS 7 versions prior to V9.1 SP1, Pcs7AnOu immediately evaluated the diagnostic flag, setting Bad = 1 and transmitting an alarm to WinCC within the same CPU execution cycle.
* Starting in PCS 7 V9.1 SP1, Siemens enhanced the channel driver blocks with the input parameter DelTiBad (Delay Time for Bad Signal Status, in seconds).
* When DelTiBad > 0, the block starts an internal timer upon receiving a channel error. If the error condition clears before DelTiBad expires (as is the case with 300 ms positioner step dips), the error is suppressed: Bad remains 0, and no WinCC system alarm is generated.
* If the wire break persists longer than DelTiBad (such as a physical severed field wire), Bad transitions to 1 and the alarm triggers normally.

Step-by-Step Implementation Procedure

Step 1: Verify PCS 7 and APL Version Support

  1. In SIMATIC Manager, check the installed PCS 7 version: Help > About SIMATIC Manager.
  2. Confirm that PCS 7 V9.1 SP1 or higher is installed with the matching PCS 7 Basis Library / APL version.
  3. Open the target CFC chart and inspect the Pcs7AnOu (FB1870) block instance: verify that parameter DelTiBad is present in the block interface.

Step 2: Configure DelTiBad on the Analog Output Channel Driver

  1. Locate the Pcs7AnOu block connected to the problematic valve positioner.
  2. Set the input parameter DelTiBad to 1.0 or 1.5 (seconds).
    * A value of 1.0 to 1.5 seconds reliably filters transient positioner inrush dips while maintaining rapid detection for genuine physical cable breaks.
  3. Ensure that the channel driver parameter Feature.Bit28 (Discrepancy time / diagnostic handling) is configured per standard project rules.

Step 3: Mitigation for Legacy PCS 7 Versions (V9.0 and Earlier)

If running on a PCS 7 version earlier than V9.1 SP1 where DelTiBad is unavailable, apply one of the following alternatives:
1. Configure Slew Rate Limiting in Upstream Control Block: In the upstream PIDConL or VlvAnL block, configure SP_RateUp and SP_RateDn (e.g., ramping setpoint at 10% per second rather than a step). This prevents violent positioner current draws without modifying controller tuning.
2. Hardware Loop Conditioning: Install a DIN-rail loop-powered signal isolator or active repeater between the 6ES7332 module and the ABB TZIDC positioner. The isolator presents a stable, constant load impedance to the Siemens module, isolating positioner current transients from the PLC diagnostics.

Step 4: Compile and Download Changes to the AS Controller

  1. In SIMATIC Manager, select the Charts folder.
  2. Select Chart > Compile > Changes only and ensure compilation completes successfully.
  3. Download changes to the AS controller (Target System > Download > Changes).

Verification

  1. In WinCC Runtime, open the valve faceplate or PID controller associated with the ABB TZIDC positioner.
  2. Command a large setpoint step change: transition the valve output from 0.0% to 60.0% in Manual mode.
  3. Observe the field positioner: verify that the valve actuates smoothly to the 60% position.
  4. Inspect the WinCC Alarm Logging window: verify that NO wire-break or channel fault alarms trigger during the setpoint transition.
  5. Validate physical fault detection: temporarily disconnect one leg of the 4–20 mA field wire at the terminal block.
  6. Verify that after exactly 1.5 seconds (the configured DelTiBad window), the channel driver asserts Bad = 1 and a legitimate wire-break diagnostic alarm triggers in WinCC.

Limitations and Operational Precautions

  1. DelTiBad delays genuine wire-break detection by the configured duration; do not set excessive delay times (> 5.0 s) on safety-critical or high-speed process loops.
  2. Setting DelTiBad too low (e.g., 0.1 s) may fail to filter longer electrical transients caused by aging positioner coils.
  3. Do not disable module diagnostic alarms globally in HW Config, as this removes wire-break and short-circuit protection across all channels on the card.
  4. Verify that field loop resistance including cable length does not exceed the maximum load rating (600 ohms) of the 6ES7332-5HD01-0AB0 module.

Engineer FAQ

Why does the ABB TZIDC positioner trigger wire break on step changes?

During rapid setpoint steps, the positioner internal I/P stage and micro-valve firing transiently lower loop voltage and increase apparent impedance, dropping the loop current below the module open-circuit detection threshold for several hundred milliseconds.

Can I disable wire-break detection for only a single channel on 6ES7332-5HD01-0AB0?

No. In HW Config, diagnostic interrupt and wire-break detection settings apply across channel groups on the module. Channel-selective filtering must be performed in software via DelTiBad.

In which PCS 7 version was DelTiBad introduced?

The DelTiBad parameter was officially introduced in PCS 7 V9.1 SP1 within the APL and Basis Library channel driver blocks.

What is the recommended value for DelTiBad?

A value between 1.0 s and 1.5 s is recommended for pneumatic positioners; this effectively filters transient electrical dips while ensuring rapid detection of actual wiring failures.

Does using DelTiBad require updating the WinCC OS project?

No. DelTiBad is an internal AS channel driver timing parameter. Modifying it only requires a CFC compile and AS program download ('Changes only').

Technical References

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Related Tags

  • Pcs7AnOu
  • DelTiBad
  • Wire Break
  • Analog Output
  • ABB TZIDC
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