Interpreting AddFEM Module Operating States - Run Passive, RES, and DISO Status in Redundant Turbine Control

Observing solid illumination on the DISO, RES, and RUN LEDs on a Siemens AddFEM (Additional Front End Module) indicates the normal 'Run Passive' (Run status-reserve) operating state of the standby module in a redundant pair. In redundant turbine control configurations (such as steam or gas turbine governor and trip architectures in PCS 7 or SPPA-T3000), AddFEM modules operate in active/passive pairs. The active master module executes control loops and drives field outputs (RUN solid). The passive partner module executes identical firmware in hot standby (RUN solid, RES solid) while its digital and analog outputs are electronically isolated (DISO solid) to avoid output contention. This is 100% normal operation; there is no risk to turbine control, and the module does not require replacement.

AddFEM Module Architecture in Power Generation and Turbine Automation

In high-speed turbomachinery automation (steam turbines, gas turbines, turbo-compressors, and hydro turbines), standard PLC I/O update cycles (typically 10 ms to 100 ms) are insufficient for critical control functions such as electro-hydraulic actuator positioning, fast valving, speed governing, and overspeed trip monitoring. For these demanding applications, Siemens integrates the AddFEM (Additional Front End Module) into SIMATIC PCS 7 (or SPPA-T3000 / TELEPERM M / SIMADYN D).

The AddFEM is a specialized, intelligent frontend microcomputer module equipped with dedicated DSPs (Digital Signal Processors) capable of sub-millisecond (e.g., 1 ms) analog and binary signal acquisition, hardware frequency measurement, and autonomous valve position control. In power generation facilities, AddFEM modules are deployed in 1-out-of-2 (1oo2) or 2-out-of-3 (2oo3) redundant architectures to guarantee uninterrupted turbine availability.

Deciphering AddFEM Diagnostic LEDs: RUN, RES, and DISO

The front fascia of an AddFEM module features discrete LED status indicators that provide real-time diagnostic insight into internal processor and I/O channel states:

RUN (Green): Indicates that the internal microcontroller and firmware are actively executing application code without watchdog faults.
RES (Reserve - Yellow/Green): Indicates that the module is operating in Reserve (Standby) Mode within a redundant pair. It is synchronized with the active master module, continuously reading inputs and executing calculations in parallel, ready to assume active control within microseconds if the master falters.
DISO (Digital Outputs Isolated - Yellow): Indicates that the module's physical digital output switches and analog driver transistors are in high-impedance (tri-state / isolated) mode. In redundant configurations, field actuators are wired in parallel to both modules. If the standby module did not isolate its outputs (DISO), conflicting voltages or circulating currents would destabilize the turbine actuator circuits.

Therefore, the simultaneous solid illumination of RUN + RES + DISO precisely designates the intended 'Run Passive' standby state.

Redundant Master/Standby Coordination and Hot-Standby Synchronization

The redundancy mechanism between AddFEM modules operates autonomously via high-speed inter-module communication lines (optical or dedicated backplane/redundancy link):

Role Assignment: Upon power-up, negotiation logic establishes which module assumes the Active (Master) role and which assumes the Passive (Reserve) role.
Active Module State: The active module displays RUN solid, while RES and DISO remain OFF. Its output drivers are energized, actively governing turbine servo-valves.
Passive Module State: The reserve module mirrors all internal process states, controller integrator terms, and calibration data from the master. It displays RUN, RES, and DISO solid.
Bumpless Failover: If the master module experiences an internal hardware failure, power loss, or communication timeout, the reserve module instantaneously extinguishes DISO and RES, activates its output switches, and assumes turbine control without causing a speed dip or hydraulic pressure bump.

Risk Assessment: Distinguishing Normal Reserve from Redundancy Loss

When plant maintenance personnel observe an AddFEM in 'Run Passive' status, an engineering evaluation must determine whether redundancy is intact:

Normal Redundancy (Zero Risk): If Module A displays RUN (Active Master) and Module B displays RUN + RES + DISO (Standby Reserve), the system is in optimal health. Redundancy is 100% active. No corrective action or module replacement is warranted.
Loss of Redundancy (High Risk): If BOTH Module A and Module B display RUN + RES + DISO, both modules have dropped into passive mode due to a communication breakdown or missing master token. Turbine actuators will de-energize, risking an emergency turbine trip!
Single Standalone Module in Passive Mode: If an AddFEM installed in a non-redundant slot displays RES and DISO, the module has been incorrectly parameterized for redundancy in HW Config or has lost its fieldbus master connection.

Comparative State Matrix: AddFEM Operating Modes and LED Combinations

The following table details the LED combinations and corresponding operational states for Siemens AddFEM modules:

RUN LEDRES LEDDISO LEDSF / BF LEDOperating ModeRedundancy StatusAction RequiredSolid ONOFFOFFOFFRun Active (Master)Normal (Active Partner)None (Normal Operation)Solid ONSolid ONSolid ONOFFRun Passive (Reserve)Normal (Standby Partner)None (100% Normal Status)OFFOFFOFFSolid ONStop / Fatal FaultRedundancy DegradedInspect diagnostic buffer / Replace moduleFlashingOFFOFFFlashingFirmware Download / InitInitializingWait for startup sequence to concludeSolid ONSolid ONSolid ONSolid ON (Both)Redundancy BreakdownCritical (Outputs Isolated)Re-establish inter-module link immediately

Diagnostic Procedure to Verify Redundant AddFEM Health in PCS 7 / SPPA-T3000

Follow this procedure to confirm that the redundant AddFEM pair is operating flawlessly:

Physical Visual Inspection:
- Locate the redundant pair in the turbine control cabinet.
- Verify that one module displays RUN only, and the partner module displays RUN + RES + DISO.
- Confirm that System Fault (SF) and Bus Fault (BF) LEDs are dark on both units.
Inspect HW Config / Module Information in SIMATIC Manager:
- In SIMATIC Manager, open Hardware Configuration (HW Config).
- Go online (Target System > Open Online Station).
- Double-click the AddFEM module to open Module Information (Ctrl+D).
- Check the Diagnostic Buffer tab. Confirm entries state: 'Module operating state: Redundant standby (Reserve) active'.
Check APL / AddFEM Driver Blocks in CFC:
- In CFC, locate the AddFEM channel driver blocks (e.g., AddFEM_DI, AddFEM_AO).
- Verify that output pin QBAD = 0 (Quality Good) and QREDUND = 1 (Redundancy Active).
Controlled Redundancy Switchover Test (During Outage Only):
- During a scheduled unit outage, disconnect the active module's front connector.
- Verify that the passive module instantly extinguishes DISO and RES, assumes the active role, and maintains actuator output voltage.

Verification

Verify normal system health through the following diagnostic checks:

Check the turbine governor faceplate on the WinCC operator station. Verify that valve positions (e.g., HP Control Valves) track setpoints smoothly.
Confirm that no system alarm appears in the WinCC alarm journal regarding 'AddFEM partner failure' or 'Redundancy loss'.
Verify that the physical redundancy link cable between the two AddFEM modules is securely fastened and free of mechanical strain.
Document the LED status in the plant shift log as verified normal hot-standby operation.

Limitations and Operational Precautions

Hot Swapping Precautions: AddFEM modules support hot-swapping in redundant configurations. However, never extract a module while its partner is faulted or showing SF/BF alarms, as doing so will cause an immediate turbine trip.
Output Ground Isolation: The DISO function electronically decouples the output stages. Ensure that external 24 VDC / 48 VDC power supplies powering the actuator circuits share a common plant reference ground as specified in the AddFEM installation manual.
Firmware Parity: Both modules in a redundant AddFEM pair must run the identical firmware revision. Mismatched firmware prevents redundancy synchronization, locking the standby module in passive isolation.

Engineer FAQ

Why is the DISO light on when the module is not broken?

DISO stands for 'Digital Outputs Isolated'. When a module is in standby reserve, its output circuits are deliberately tri-stated and isolated so they do not conflict with the active module currently driving the turbine actuators.

Can I force the reserve AddFEM module to become the active module?

Yes. In software via the diagnostic faceplate or by momentarily switching the active module to STOP, the reserve module will instantly take over as master.

What happens if the communication cable between redundant AddFEM modules breaks?

If the inter-module link is severed, both modules attempt to determine bus mastership. Depending on parameterization, one remains active and the other reports a redundancy failure alarm without tripping the turbine.

Is AddFEM used only in PCS 7?

No. AddFEM is widely used in Siemens SPPA-T3000, TELEPERM M, and SIMADYN D platforms for gas and steam turbine governing and generator excitation.

Does the AddFEM module require periodic recalibration?

No. AddFEM modules feature factory-calibrated precision analog converters with internal drift compensation, eliminating routine manual potentiometer calibration.