Decoding Error Output QGR_ERR on Legacy Siemens Motor Block FB66 (MOTOR_LS)

On the legacy SIMATIC PCS 7 Standard Library motor block FB66 (MOTOR_LS / MOTOR 'Motor with control signals'), output QGR_ERR represents a latched Group Error (Gruppenfehler). It activates (QGR_ERR = 1) whenever any monitoring or operational fault occurs, including runtime monitoring timeout (QMON_ERR), motor protection switch trip (MSS = 0), interlock trip (L_STOP = 0), or external error (EXT_ERR = 1). To clear QGR_ERR, verify that field feedback (ERM_ON), motor protection (MSS), and interlock signals are healthy, and apply a rising-edge pulse to the RESET input (RESET = 1) to clear the internal fault flip-flop.

Functional Overview of Legacy Standard Library Motor Block FB66

In legacy SIMATIC PCS 7 installations (PCS 7 V6.x, V7.x, and legacy migrations in V8.x/V9.x), FB 66 represents the technological function block MOTOR_LS (Motor with control signals / Motor mit Steuersignal), derived from the classic PCS 7 Standard Library.

FB 66 coordinates unidirectional motor actuation, processing manual/automatic commands, feedback monitoring, interlocks, and alarm generation via Alarm_8P. The output pin QGR_ERR serves as the primary summary fault flag for plant interlocks and HMI group displays.

Root Causes Triggering Group Error Output QGR_ERR

The group error output QGR_ERR is the internal logical OR combination of four distinct operational faults:

$$\text{QGR_ERR} = \text{QMON_ERR} \lor \lnot \text{MSS} \lor \lnot \text{L_STOP} \lor \text{EXT_ERR}$$

Runtime Monitoring Fault (QMON_ERR = 1): When a start command (QSTART) is issued, the feedback signal (ERM_ON) must arrive within the monitoring time parameter (TIME_MON, e.g. 5.0 s). If feedback fails or drops while running, QMON_ERR trips and latches.
Motor Protection Switch Trip (MSS = 0): The motor circuit breaker or thermal overload contact is wired to MSS. Because MSS is active-high for healthy operation, an open contact (MSS = 0) triggers an immediate trip.
Interlock Trip (L_STOP = 0): Interlock input for process stops. An active-low state (0) stops the motor and asserts QGR_ERR.
External Fault (EXT_ERR = 1): A generic error input for third-party alarms or drive ready faults.

Detailed Analysis of Input Parameters Affecting QGR_ERR

To diagnose which input is tripping FB 66, monitor the block online in CFC and inspect these pins:

MSS (Motor Protection Switch): Must be 1 (healthy). If 0, check the field thermal overload relay.
ERM_ON (Feedback Run): Must transition to 1 following QSTART = 1. If ERM_ON is delayed past TIME_MON, increase TIME_MON.
L_STOP / L_OFF (Interlock Stop): Must be 1. If 0, trace upstream interlocking logic in the CFC chart.
EXT_ERR (External Error): Must be 0. If 1, check external trip signals connected to this terminal.
RESET (Reset Fault): Input used to unlatch the error flip-flop after healthy conditions are restored.

Reset and Fault Clearance Procedure in CFC and Faceplates

Because FB 66 contains an internal latched SR flip-flop, simply restoring healthy field inputs will NOT clear QGR_ERR:

Verify that MSS = 1, L_STOP = 1, and EXT_ERR = 0 online in CFC.
In WinCC OS Runtime, open the motor faceplate and click the Reset button.
If operating without an OS faceplate (e.g. testing in CFC), right-click the RESET input in CFC, toggle value to 1, execute a CPU cycle, and toggle back to 0.
Confirm that QGR_ERR transitions to 0 and QMON_ERR transitions to 0.

Comparison: Classic FB66 MOTOR_LS vs Modern APL MotL

The table below contrasts the legacy FB66 block with the modern APL equivalent:

| Block Parameter | Classic FB66 (MOTOR_LS) | Modern APL MotL (FB 1850) |
|---|---|---|---|
| Group Error Output | QGR_ERR (Boolean) | GrErr (Structure: Value + Status) |
| Feedback Input | ERM_ON (Single bit) | FbkRun (Structure: Value + Status) |
| Motor Protection | MSS (Active-high bit) | Trip (Active-low bit) |
| Monitoring Time | TIME_MON (Seconds) | MonTiDynamic (Seconds) |
| Fault Reset | RESET (Pulse input) | RstOp (Faceplate) / RstLi (Logic) |
| Faceplate Template | @PG_MOTOR.pdl | @PG_MotL.pdl |

Verification

Verify fault clearance and motor operation in CFC:

Open the CFC chart online with FB 66 in test mode (Test > Watch).
Confirm that MSS = 1, L_STOP = 1, and EXT_ERR = 0.
Apply a pulse to RESET = 1 then 0.
Verify that QGR_ERR changes from 1 to 0.
Issue a start command (AUT_ON = 1 or manual start); confirm QSTART = 1 and ensure ERM_ON = 1 arrives within TIME_MON without re-triggering QGR_ERR.

Limitations and Operational Precautions

Review the following legacy library precautions:

Migration Planning: The PCS 7 Standard Library (FB 66) is legacy software. For new projects or plant expansions, Siemens strongly recommends migrating to the PCS 7 Advanced Process Library (MotL FB 1850).
Inverted Logic Checks: Pay close attention to active-low pins (MSS, L_STOP); a disconnected or uninitialized pin evaluates as 0, immediately tripping the block.
Monitoring Time Margin: Ensure TIME_MON accounts for physical contactor pull-in times and auxiliary contact bounce (typically 2 to 5 seconds).

Engineer FAQ

What does QGR_ERR stand for on FB 66?

QGR_ERR stands for 'Quittierbarer Gruppenfehler' (Acknowledgeable Group Error). It indicates a latched summary fault on the motor block.

Why doesn't QGR_ERR clear when the motor protection switch (MSS) is closed?

Because FB 66 latches fault states into an internal flip-flop. Even after MSS returns to 1, an explicit rising edge on the RESET input is required to clear the error.