Why Pcs7AnIn Outputs Uncertain Status 16#68 Instead of Bad on ET 200SP HA Rack Failure

The Pcs7AnIn channel block outputs an uncertain signal status of 16#68 (Device-related, maintenance demanded) rather than Bad (16#00) when an ET 200SP HA station loses power because PCS 7 APL evaluates station-level communication loss as an upstream rack fault (QRACKF = 1) rather than a local sensor channel failure. By default, APL only sets the Bad output bit to 1 for definitive sensor faults unless Feature bit 28 or Feature bit 30 is enabled to evaluate uncertain process values as bad, or downstream logic evaluates the signal status byte directly against good quality (16#80).

NAMUR NE107 and APL Quality Code Evaluation Architecture

In SIMATIC PCS 7 Advanced Process Library (APL) operating on redundant S7-410 systems with PROFINET field devices such as ET 200SP HA, signal quality is structured according to the PROFIBUS/PROFINET PA profile and NAMUR NE107 standard.

Each analog process value transmitted through Pcs7AnIn is encapsulated in a structured data type (PV_Out) comprising:
* Value: 32-bit floating point real number (REAL).
* ST: 8-bit signal status byte (BYTE).

Under standard operation:
* 16#80: Good (normal measurement).
* 16#00: Bad (non-device related, hardware fault).
* 16#28 to 16#68: Uncertain (process or device-related diagnostic degradation).

The binary output pin Bad on Pcs7AnIn is specifically designed to indicate definitive channel or measurement invalidity. However, when an entire remote I/O head station loses power over a single PROFINET link, the upstream diagnostic driver MOD_D16_PN detects a rack failure and asserts QRACKF = 1. Under APL driver conventions, this condition maps to status byte 16#68 (Uncertain, device-related), which by default does not assert Bad = 1.

Analysis of Quality Code 16#68 and QRACKF Behavior

Engineers migrating from legacy STEP 7 driver blocks or PROFIBUS configurations often expect total head station power loss to immediately force Bad = 1 and quality 16#00. In ET 200SP HA PROFINET architectures:

Driver Separation: The MOD_D16_PN driver block manages PROFINET device communication, while Pcs7AnIn processes the localized I/O signal. When power is lost, PROFINET IO supervision triggers OB 86 (rack failure), setting QRACKF = 1 on the rack driver.
APL Quality Mapping: APL interprets an upstream network or rack fault as an interruption in device communication rather than an internal sensor wire break or ADC hardware defect. Therefore, the signal status byte is clamped to 16#68.
Downstream Impact: If downstream interlocking blocks or custom converter blocks (StrAnOu) check only the binary Bad pin without inspecting the signal status ST, the interlock fails to trip because Bad remains 0.

Configuration Methods to Trip Bad Logic on Uncertain Status

To ensure that downstream plant interlocks react safely when an ET 200SP HA station drops offline, apply one of the following methods:

Method 1: Configure APL Feature Bits on Pcs7AnIn

APL provides built-in Feature word settings that govern how uncertain process values are handled:
1. In CFC, open the properties of the Pcs7AnIn block instance (or modify its Control Module Type).
2. Set Feature.Bit28 (Discharging a process value in case of device-related event) or Feature.Bit30 (Set output Bad on uncertain process value) to TRUE (1).
3. When enabled, any signal status of 16#68 automatically forces Bad = 1 and substitutes the configured fail-safe value (SubsPV or last valid value).

Method 2: Evaluate Signal Status Directly via STIn or Logic Comparison

If modifying Feature bits across hundreds of existing CMT instances is impractical:
1. Interconnect PV_Out.ST to the standard APL block STIn (Signal Status In).
2. Inspect output ST_80 (Good signal). If ST_80 = 0, the signal is either Bad or Uncertain.
3. Invert ST_80 to generate a master plant fault interlock, ensuring that both 16#00 and 16#68 immediately trigger trip logic.

Verification

In SIMATIC CFC, open the target Pcs7AnIn chart in dynamic test mode.
Simulate an ET 200SP HA power interruption or disconnect the PROFINET bus cable.
Confirm that MOD_D16_PN.QRACKF transitions to 1.
Confirm that Pcs7AnIn.PV_Out.ST displays 16#68.
Verify that with Feature bit 28 or 30 active, Pcs7AnIn.Bad switches to TRUE (1) and downstream safety interlocks trip as engineered.

Limitations and Operational Precautions

Enabling Feature bit 30 treats all uncertain states as bad, including non-critical maintenance requests or self-calibration cycles. Evaluate process hazards before applying globally.
Do not rely solely on the binary Bad pin in safety-critical interlocks without evaluating signal status ST in APL architectures.
In redundant PROFINET ring topologies (MRP / S2 redundancy), a single link break does not trigger QRACKF = 1 unless both redundant paths fail.

Engineer FAQ

What does status code 16#68 specifically mean?
In NAMUR NE107 / PROFIBUS PA terminology, 16#68 indicates "Uncertain - Device related - Maintenance demanded/alarm", signifying that the value cannot be guaranteed due to an upstream hardware or communication anomaly.
Why does StrAnOu fail to create Bad = 1 from 16#68?
The StrAnOu structure-to-analog converter maps only the lowest quality band (16#00 to 16#27) to its Bad output pin by design. Uncertain signals (16#28 to 16#7F) are passed through with their numerical values intact.
Can this behavior be changed centrally in the project?
Yes, if your project utilizes Control Module Types (CMT) or Process Tag Types (PTT). Modifying the Feature word in the master type and executing "Synchronize with CMT" applies the behavior to all instances.

Technical References

SIMATIC PCS 7 Advanced Process Library (APL) Function Manual (A5E43360408-AA).
SIMATIC PCS 7 Channel Blocks and Diagnostic Blocks Manual (A5E41356241-AA).
NAMUR NE107: Self-Monitoring and Diagnosis of Field Devices.

Related Tags

PCS 7, APL, ET 200SP HA, Pcs7AnIn, Signal Status, MOD_D16_PN, PROFINET, S7-410

Why Pcs7AnIn Outputs Uncertain Status 16#68 Instead of Bad on ET 200SP HA Rack Failure