Resolving QBAD=1 Bad Quality in Siemens PCS 7 CFC Drivers

Overview

The QBAD output of a Siemens PCS 7 CFC channel driver block indicates that the quality code returned for the associated process value is bad. When QBAD equals 1, the operator station (OS) renders the faceplate tag greyed out, displays a yellow maintenance hand icon, and freezes any derived control values. This behavior is mandated by the IEC 62443 / NAMUR NE 107 quality model used throughout PCS 7 AS-OS communication and is the same flag carried inside the S7-1500 / S7-400 user data word for analog, digital, and driver blocks in the Advanced Process Library (APL).

QBAD=1 is not a fault of the controller, the field wiring, or the I/O module by itself — it is a status bit set by the CFC runtime when the driver cannot guarantee a valid process value. The most frequent observed causes in operating plants are:

• The MODE parameter of the channel driver block is incorrect for the connected signal type (e.g., left at default while the field device is 4-wire 4–20 mA).
• Module drivers (CH_DRV) have not been generated or were generated against a hardware configuration that no longer matches the rack.
• The instance DB of the channel driver was edited manually, or the OS compilation has not been performed after a CFC change.
• The block is held in simulation mode (yellow hand) by an active SIM_ON interconnect with no valid substitute value source.
• The S7 connection between the AS and the affected OS partner is degraded (SIMATIC NET / S7-1200 OPC routing broken).

This guide provides a deterministic, engineering-grade procedure for isolating the offending block, validating MODE, regenerating module drivers, recompiling the OS, and clearing the QBAD=1 latched condition.

Understanding the QBAD Status Bit

Every APL channel driver block (e.g., CH_AI, CH_AO, CH_DI, CH_DO, P_IN, P_OUT) exposes the following standard outputs:

When QBAD = 1, the OS graphic logic masks the PV (renders grey) and forces QC_SUM = 1 downstream. Interlocks and block icons downstream of a QBAD=1 source must be reviewed for fail-safe direction; by default PCS 7 driver blocks route SUBS_V (substitute value) to the PV output when QBAD rises, but only if SUBS_ON is enabled by the application.

Channel Driver Block Architecture

PCS 7 uses two cooperating block layers:

1. CH_DRV (Channel Driver Generator instances) – automatically created when module drivers are generated. Each physical channel has a CH_DRV instance that knows the slot, channel number, and signal type from HW Config.
2. CH_AI / CH_AO / CH_DI / CH_DO (Application Channel Drivers) – placed manually in the CFC and connected to one CH_DRV. These blocks are what the OS faceplates reference.

The MODE parameter lives on the application channel driver (CH_AI / CH_AO / CH_DI / CH_DO). The CH_DRV receives its configuration from the hardware, and the runtime hand-shake between CH_DRV and CH_AI is what produces the QBAD bit.

MODE Parameter Structure and Encoding

MODE is a DWORD whose bit fields specify the signal type, scaling, simulation permission, and substitute-value behavior. The exact encoding is documented in the APL online help for each driver block; the most commonly referenced values are:

The high-order bit (0x80000000) enables simulation. If this bit is missing on an analog input where the field engineer expects to test with a hand-held calibrator, the block will refuse to enter simulation and may latch QBAD=1 when the physical signal is removed.

Common Root Causes of QBAD=1

Diagnostic Procedure

Use the following ordered sequence before modifying the MODE parameter or regenerating drivers. This protects against unnecessary AS downloads.

1. Identify the affected tag: In the OS, right-click the greyed tag → Picture → Picture Tree and note the CFC chart and block instance name (e.g., SCL101/MOT_PV.CH_AI1).
2. Open the CFC chart: In SIMATIC Manager / PCS 7, navigate to the corresponding S7 program and double-click the chart. The block icon shows a yellow hand when QSIM=1.
3. Inspect block I/O: Right-click CH_AI → Block Properties → tab I/Os. Note QBAD, QSIM, MODE, CH_DRV connection state.
4. Cross-check CH_DRV: Open the CH_DRV instance (auto-generated, located in the master data library of the AS). Verify its MODE value and that it points to the correct slot/channel.
5. Compare with HW Config: Open HW Config → locate the AI/AO/DI/DO module. Confirm signal type (e.g., 4DMU vs 2-wire vs 4-wire) matches the MODE of CH_AI.
6. Check CFC compile status: CFC → Chart → Compile. Look for warnings such as Instance DB number collision or Lost interconnection.
7. Check OS compile status: OS → Compile OS. The compile log must show No errors. Stale OS compilation is the single most common cause of phantom QBAD=1 after a CFC change.

Module Driver Regeneration

If the CH_DRV layer was never generated or is out of sync with HW Config, you must regenerate the driver instances:

1. In the S7 program, open the Charts folder and select Options → Chart Insertion.
2. Choose the rack or station that contains the affected signal.
3. Click Generate Module Drivers. PCS 7 will iterate over every configured channel and create or update the CH_DRV instances in the master data library.
4. After generation, recompile the CFC charts and download the program to the AS.

Regeneration is non-destructive to the CH_AI / CH_AO application blocks, but it will renumber instance DBs when slots are added or removed. An OS compile must always follow.

OS Compile and Download Resolution

The most reported "fix" in operating plants for QBAD=1 after a CFC edit is the following sequence:

1. CFC → Charts → Compile All. Resolve every warning. Pay special attention to Address areas are different and Interface change in instance DB.
2. Download the entire S7 program to the AS (or run Download → Changes Only if the running AS already contains compatible code).
3. OS project editor → Compile OS with Range of compilation = Entire OS. This regenerates the picture tree, alarm logging, and tag connections.
4. Transfer / download the OS server project. On a redundant pair, perform a complete download to both servers and let the redundancy sync complete.
5. On the OS client, perform Update Picture Tree or restart the WinCC runtime if the picture tree still references the old DB.

If QBAD=1 persists after this sequence, the runtime value is latched and the driver block has not refreshed. Cycle the block's ACK_REQ input or momentarily drop the SIM_ON interconnect to force the driver to re-evaluate the upstream channel state.

Simulation Mode Recovery

A persistent yellow hand on the faceplate with greyed PV almost always indicates the block was placed in simulation by an earlier test session and never released. Inspect the following inputs on the CH_AI / CH_AO block:

To safely release simulation from the OS:

1. Open the faceplate → Maintenance → Simulation.
2. If Operator Enable is greyed out, check the OP_ENABLE input and the MOD_LOC (mode lock) interconnect.
3. Toggle Simulation Off and confirm the hand icon disappears.
4. Within one cycle of the OB1 (typically 100 ms), QBAD should return to 0 if the underlying channel is healthy.

Field I/O and Wiring Cross-Check

When the CFC and OS layers are clean, but QBAD still latches, the fault moves one layer down to the I/O. The following checks should be performed with the I/O channel placed in Substitute / Disable state to avoid driving a live actuator:

• Verify the marshalling cable (e.g., from the field junction box to the front connector of the AI module) for continuity, shield grounding, and reversed polarity on 4–20 mA loops.
• Confirm the channel is configured for the correct measurement range in HW Config (4DMU vs 2-wire vs 4-wire RTD). A wrong range will produce a constant over-range flag that propagates to QBAD.
• Use the module's diagnostic interrupts (SF / BF LEDs on SM 331 / SM 332) to read the diagnostic buffer via the AS hardware diagnostic faceplate.
• For HART channels, confirm that the HART modem is enabled on the channel if the connected device is HART-only and the CH_AI expects HART variables.
• Check for open-circuit detection: PCS 7 channel drivers in 4–20 mA mode flag QBAD if the loop current drops below 3.6 mA (NAMUR NE 43 lower limit).

Verification and Acceptance Test

Once the corrective action is applied, perform the following verification:

1. Block-level verification: On the AS, force the CH_DRV output via a test interconnect and observe that CH_AI QBAD falls to 0 within one OB1 cycle.
2. Faceplate verification: At the OS, the tag should display its live PV, the maintenance hand should be absent, and the QC_SUM lamp should be green.
3. Alarm verification: Trigger a process alarm at the field end and confirm the alarm is logged with the correct priority and that the QBAD alarm (if any) does not re-appear.
4. Trend verification: Open the tag in Process Historian and confirm the value is being archived with quality code 0xC0 (good) instead of 0x00 (bad).
5. Cross-AS consistency: If the plant uses a redundant AS pair, perform a fail-over and verify the tag transitions to QBAD=0 on the standby AS within the configured redundancy switchover time.

Engineering Workflow Summary

Documentation and Standards Anchors

The following official references govern the quality model, MODE encoding, and driver behavior described in this article:

• Siemens — PCS 7 Process Control System Service Manual (current edition)
• Siemens — APL Driver Blocks CH_AI / CH_AO Online Help (V9.0)
• Siemens — PCS 7 CFC / SFC Programming and Operating Manual
• Siemens — PCS 7 OS Configuration Manual
• NAMUR NE 107 — Status Categories for Field Devices
• NAMUR NE 43 — Standardization of the Signal Level for the Failure Information of Digital Transmitters

Frequently Asked Questions