Configuring Automatic Mode Switchover to Manual on Control System Fault (CSF) in PCS 7 APL PIDConL

In SIMATIC PCS 7 APL (V9.1 onwards), configuring the PID controller block PIDConL to switch automatically from Automatic to Manual mode upon a Control System Fault (CSF = 1) is accomplished by setting 'Feature.Bit4 = 1' ('Switch to manual mode on control system fault'). In PCS 7 versions prior to V9.1 where Feature.Bit4 is unavailable, this behavior is implemented by enabling link-controlled operating mode selection ('ModLiOp = 1') and interconnecting the inverted CSF signal to 'AutModLi' while connecting the raw CSF signal directly to 'ManModLi'.

Executive Summary

In continuous process control loops (such as distillation column temperatures, furnace pressures, and flow proportioning), an instrument or transmitter failure introduces dangerous runaway conditions if the PID controller remains in Automatic mode. If a process variable (PV) sensor cable breaks or a transmitter issues a bad quality code, the controller perceives an artificial process error and may drive the final control element (valve or VFD) to 100% or 0% output.

In SIMATIC PCS 7 Advanced Process Library (APL), an analog input driver block asserts Bad = 1, which propagates to the downstream PIDConL block as a Control System Fault (CSF = 1). However, by default, PIDConL does not automatically drop out of Automatic mode when CSF occurs; it issues an alarm while continuing to execute its control algorithm unless explicitly configured otherwise. This guide details how to implement automatic fail-safe manual switchover using native APL Feature.Bit4 in PCS 7 V9.1 and through standard CFC interlocking logic in earlier releases.

Architecture and Fundamental Mechanics

The operating mode transition logic of the APL PIDConL block governs automatic and manual transitions through priority arbitration:

Signal Flow from Field to Controller:
- The physical sensor signal is processed by an analog input channel driver (e.g., Pcs7AnIn).
- If the transmitter wire breaks or the ADC fails, Pcs7AnIn sets output Bad.Value = 1 and assigns Quality Code 16#00 (Bad).
- This signal connects to the CSF (Control System Fault) input of PIDConL.
The Native APL Feature.Bit4 Mechanism (PCS 7 V9.1+):
- In PCS 7 V9.1, Siemens introduced Feature.Bit4: 'Switch to manual mode on control system fault (CSF)'.
- When Feature.Bit4 = 1:
- Upon CSF transitioning from 0 to 1, PIDConL automatically forces the operating mode to Manual (ManAct = 1).
- The manipulated variable MV is frozen at its current value (bumpless hold), preventing dangerous valve travel.
- When CSF returns to 0, the controller remains safely in Manual mode until an operator or recipe sequence explicitly re-engages Auto.
The Link-Controlled Mode Architecture (PCS 7 V9.0 and Earlier):
- In earlier APL versions lacking Feature.Bit4, operating modes must be arbitrated using the linked inputs:
- Set ModLiOp.Value = 1 to enable external signal control over operating modes.
- Connect CSF to ManModLi (Manual mode linked command).
- Invert CSF and connect to AutModLi (Auto mode linked command).
- When a fault occurs, ManModLi forces manual mode instantly.

Step-by-Step Implementation Procedure

Follow these steps to configure automatic switchover to manual mode on CSF in PIDConL:

Method A: Native Configuration in PCS 7 V9.1+
1. Open CFC Chart and Select PIDConL Block:
- Open the CFC chart containing the PIDConL instance.
- Right-click the block and select 'Object Properties' (or double-click the block header).

Configure Feature Word:
- In the parameter table, locate the Feature input.
- Expand the bit structure or set the hexadecimal mask to enable Bit 4:
- Set Feature.Bit4 = TRUE (Switch to manual mode on control system fault).
- Ensure CSF input is interconnected to the Bad.Value output of the associated Pcs7AnIn channel block.

Method B: Interlocking Logic for PCS 7 V9.0 and Earlier
1. Enable Linked Mode Operation:
- On the PIDConL block, set input ModLiOp.Value = 1 (Switch operating mode via linked inputs).

Interconnect CSF to Mode Inputs:
- In CFC, connect Pcs7AnIn.Bad.Value to PIDConL.CSF.
- Connect Pcs7AnIn.Bad.Value directly to PIDConL.ManModLi.Value.
- Insert an inverter block (Not01 from the Logic library) or use an intermediate boolean flag:
- Connect Pcs7AnIn.Bad.Value to Not01.In.
- Connect Not01.Out to the automatic mode trigger (or interlock with recipe AutModLi).
Configure Bumpless Output Tracking:
- Verify that PIDConL.SP_TrkMan = 1 (Setpoint tracks process variable in Manual mode) to guarantee bumpless transfer when returning to Auto.
Compile and Download:
- Compile CFC charts ('Changes only').
- Download the delta program to the S7 controller in RUN mode.

Verification

Open the CFC chart in Test Mode (Online).
Place the PIDConL block into Automatic mode (AutoAct = 1) via its WinCC faceplate.
Simulate a sensor wire break: force Pcs7AnIn.Bad = 1 (or force PIDConL.CSF = 1).
Verify operating mode transition: observe that PIDConL immediately switches to Manual mode (ManAct = 1) and the WinCC faceplate updates to 'Manual'.
Verify output stability: confirm that the manipulated variable MV remains frozen at its last valid operating point.
Clear the fault (CSF = 0): verify that the controller remains safely in Manual mode and does not jump back to Auto unexpectedly.

Limitations and Operational Precautions

When using ModLiOp = 1, operators cannot switch the controller mode manually from the WinCC faceplate unless supervisory logic releases control. In contrast, native Feature.Bit4 allows full manual operator interaction while automatically handling the fault transition. Engineering packages and library versions should be verified through an authorized Siemens distributor or the Siemens Industry Mall.

Engineer FAQ

Which PCS 7 version introduced Feature.Bit4 for CSF manual switchover in PIDConL?

PCS 7 V9.1 (APL V9.1) natively introduced Feature.Bit4 for automatic switch to manual mode on CSF.

What happens to the controller output MV when switching to manual on CSF?

The manipulated variable MV holds its last valid calculated position, preventing erratic valve movement.

Does the controller automatically switch back to Auto when CSF clears?

No. For plant safety, once forced into Manual mode by CSF, the controller remains in Manual until deliberately restored by an operator or batch sequence.