Troubleshooting Frozen Timers in Minerals CEMAT Sequence Test Mode (OB1 vs OB35 Execution)

In SIMATIC PCS 7 running the Minerals CEMAT industry library, start-warning and sequence timers failing to count down during Sequence Test Mode with C_FB_PLC is caused by executing CEMAT technological blocks inside cyclic interrupt task OB35 (100ms) instead of the main cyclic task OB1. Unlike standard PCS 7 Advanced Process Library (APL) blocks that require fixed periodic cyclic interrupts, classic CEMAT group and drive blocks (C_GROUP, C_DRV_1D, C_VALVE, C_SIMOS) are architected to execute in Organization Block 1 (OB1), where internal cycle-time calculation algorithms track elapsed scan time. Moving CEMAT runtime groups to OB1 restores timer countdown in both sequence simulation and plant operation.

Task Scheduling Philosophy: Standard PCS 7 APL vs CEMAT Minerals

In standard SIMATIC PCS 7 engineering, continuous process control algorithms (such as APL blocks MotL, PIDConL, VlvL) must be placed in deterministic, periodic cyclic interrupt Organization Blocks—typically OB35 (100ms), OB34 (200ms), or OB32 (1000ms). This ensures precise numerical integration and predictable filter execution independent of total CPU scan time.

In contrast, the SIMATIC PCS 7 Minerals CEMAT Library (6ES7658-2BC58-0YA5) originates from the heavy cement, mining, and bulk materials handling industries where processing relies on high-speed interlocking sequences, conveyor line routing, and mill group control. To optimize CPU throughput across thousands of drives, CEMAT was historically designed around the continuous cyclic execution model of Organization Block 1 (OB1).

Why CEMAT Sequence Timers Stall in Cyclic Interrupt Tasks (OB35)

When an engineer tests CEMAT sequences using simulation tools (such as S7-PLCSIM and test block C_FB_PLC to simulate drive feedback):

Internal Elapsed Time Measurement: CEMAT blocks calculate timer durations (e.g. horn start-warning time, feedback fault monitoring times) by evaluating the CPU scan time elapsed between successive block calls, obtained from the OB1 start information or system time tick differences.
The OB35 Timer Stagnation: When an engineer mistakenly places CEMAT blocks into OB35 (as is standard for APL), the internal time delta calculation in blocks like C_GROUP (FB1010) and C_FB_PLC fails to increment or evaluates cycle deltas as zero. The start-warning timer sets its preset value but remains permanently frozen, halting sequence progression.
Hybrid Architecture: In a modern CEMAT project, peripheral channel drivers (Pcs7DiIn, Pcs7AnIn) reside in OB35, while all CEMAT technological blocks (C_GROUP, C_DRV_1D, C_VALVE) must reside in OB1.

Comparison of Organization Block Task Allocation in PCS 7 with CEMAT

The following table compares the mandatory task placement rules for PCS 7 libraries:

Library / Block GroupTypical BlocksTarget Organization BlockExecution RationaleCEMAT Group ControllersC_GROUP (FB1010), C_ROUTE (FB1011)OB1 (Main Task)Relies on OB1 scan calculation for group sequence timersCEMAT Drives & ValvesC_DRV_1D (FB1001), C_VALVE (FB1007)OB1 (Main Task)Standard CEMAT runtime group allocation rulesCEMAT Simulation BlockC_FB_PLC (Simulation helper)OB1 (Main Task)Synchronized with OB1 group sequence test modePCS 7 Channel DriversPcs7DiIn, Pcs7AnIn, MOD_1OB35 (100ms Cyclic Interrupt)Driver Wizard automatically assigns to OB35Standard APL ControlPIDConL (FB1874), MonAnL (FB1845)OB35 (100ms Cyclic Interrupt)Requires fixed periodic sampling for PID algorithmsSafety ProgramF_DI_24V, F_DO_24V, S7 F-SystemsOB35 (or dedicated F-Cycle)Strict deterministic fail-safe execution

Step-by-Step Procedure to Move CEMAT Runtime Groups to OB1

Follow this procedure in the CFC Editor to correct task scheduling for CEMAT blocks:

Open the CFC Runtime Editor:
- Open any CFC chart in the project.
- Click the Runtime Editor icon in the toolbar (or select menu Options -> Customize -> Runtime Groups...).
Locate CEMAT Runtime Groups:
- In the runtime editor tree, expand OB35.
- Locate the runtime groups containing CEMAT blocks (e.g. Grp_KilnFeed_C_GROUP, Drv_Belt01_C_DRV_1D).
Move Runtime Groups to OB1:
- Drag the CEMAT runtime groups from OB35 and drop them into the OB1 tree branch.
- Ensure the execution sequence respects hierarchy: group coordinator blocks execute after their child drive blocks, or as specified in the CEMAT engineering manual.
- Keep standard driver blocks (Pcs7DiIn, Pcs7AnIn, MOD_1) in OB35.
Compile and Download Changes:
- Select Chart -> Compile -> Changes only.
- Delta download to the S7-400 controller / S7-PLCSIM.
- Activate Sequence Test Mode via C_FB_PLC and initiate a group start command.

Verification

Verify sequence timer operation in PLCSIM and WinCC Runtime:
1. In WinCC Runtime, open the faceplate for C_GROUP and click the Start Group button.
2. Observe the start-warning acoustic horn timer: confirm that the timer actively decrements second-by-second (e.g. counting down from 15s to 0s).
3. Once the warning timer expires, verify that drive start commands trigger sequentially down the belt line.
4. In the CFC Editor online view, verify that C_FB_PLC automatically generates running feedback signals within the configured delay window, allowing the group to transition cleanly into healthy 'RUN' state.

Limitations and Operational Precautions

OB1 Cycle Time Growth: Because OB1 is non-interruptible by itself and runs at whatever speed the CPU processes all rungs, placing thousands of blocks in OB1 can increase cycle time to 50-100ms. Monitor the CPU scan time in Module Information.
Never Mix CEMAT and APL in Same Sub-Loop: Do not interconnect CEMAT drive commands directly into APL technological blocks across different OB execution cycles without considering single-cycle race conditions.
S7-PLCSIM Cycle Emulation: When running S7-PLCSIM, ensure the simulation slider is not set to minimum speed, which can artificially elongate OB1 scan times during sequence test mode.

Engineer FAQ

Why does CEMAT require OB1 while standard PCS 7 requires OB35?

Standard PCS 7 APL uses mathematical algorithms (integrators, derivatives) that depend on exact, constant periodic time intervals (100ms in OB35). CEMAT was developed for heavy discrete/sequence machinery where drive logic and group coordination rely on OB1 scan cycles for high-speed interlocking.

What does the C_FB_PLC block do in CEMAT?

C_FB_PLC is a simulation and sequence testing block in the CEMAT library. When sequence test mode is active, it automatically simulates motor running contactor feedbacks and limit switches so engineers can test group sequences without physical I/O.

Do channel driver blocks also need to be placed in OB1?

No. Standard PCS 7 peripheral channel driver blocks (Pcs7DiIn, Pcs7AnIn) and diagnostic blocks must remain in OB35 where the Driver Generator Wizard places them.