Dynamic Scale Limit Configuration on APL Analog Input Channel Drivers (Pcs7AnIn) and Monitoring Blocks (MonAnL)

In SIMATIC PCS 7 Advanced Process Library (APL), configuring dynamically variable high and low scale limits for analog values cannot be done directly on the Pcs7AnIn channel driver's Scale parameter if module driver generation is active, as the compiler automatically overwrites Scale from HW Config. To achieve dynamic or operator-adjustable scale limits in runtime, engineers must apply the variable limits to the PV_OpScale parameter on the downstream monitoring block MonAnL (FB1845). Connecting dynamic calculation blocks or operator setpoint blocks (such as OpAnL) to MonAnL.PV_OpScale.High and MonAnL.PV_OpScale.Low dynamically updates faceplate bar graphs, trend boundaries, and alarm limits without driver overwrite.

Executive Summary

Process automation applications often require variable measuring spans—such as multi-product batch vessels where liquid density or recipe volume shifts the meaningful 0–100% scale, or specialized analytical instruments with switchable ppm/ppb ranges. Engineers frequently attempt to connect variable REAL tags to Scale.High and Scale.Low on the Pcs7AnIn (FB1869) channel driver block in CFC. However, executing 'Generate module drivers' during compilation immediately severs or overwrites these interconnections with static hardware settings from HW Config. This guide details the architectural boundary between hardware channel scaling and technological visualization scaling, providing the standard method to achieve dynamic scaling using MonAnL.PV_OpScale.

Architecture and Fundamental Mechanics

The APL analog processing pipeline divides physical conversion from technological presentation:
1. The Role of Pcs7AnIn (Hardware Normalization):
Pcs7AnIn converts raw integer counts (-27648 to +27648 or 0 to 27648) into a physical floating-point engineering value (PV_Out.Value). When 'Generate module drivers' compiles, the driver generator interrogates HW Config module properties (e.g., 4–20 mA = 0.0 to 100.0 bar) and writes these fixed constants directly into Pcs7AnIn.Scale and Mode. Overriding this input risks breaking raw A/D conversion mathematics.
2. The Role of MonAnL (Technological & Operator Scaling):
The downstream monitoring block MonAnL receives PV_Out from Pcs7AnIn. MonAnL possesses its own independent scaling structure:
- PV_OpScale: An input structure (High, Low REAL) that defines the upper and lower limits of the WinCC faceplate bar graph, trend display coordinate axes, and normalized percentage calculations.
- By default, MonAnL.PV_OpScale is interconnected to Pcs7AnIn.ScaleOut. However, because MonAnL is not a hardware driver, its inputs are never overwritten by module driver generation.
3. The Dynamic Scaling Solution:
Disconnecting MonAnL.PV_OpScale from Pcs7AnIn.ScaleOut and connecting it to dynamic CFC logic (or an OpAnL operator control block) allows dynamic, runtime modification of scale boundaries on WinCC faceplates while preserving stable hardware A/D conversion on the channel driver.

Step-by-Step Implementation Procedure

Maintain Fixed Physical Scaling on Pcs7AnIn:
a. Open the CFC chart containing Pcs7AnIn (FB1869).
b. Allow Pcs7AnIn.Scale to reflect the physical transmitter span configured in HW Config (e.g., Scale.Low = 0.0, Scale.High = 100.0 mA or bar).
c. Confirm that module driver generation compiles cleanly without warnings.
Configure Dynamic Scaling on MonAnL:
a. In the same CFC chart, locate the connected MonAnL (FB1845) block.
b. Break the default CFC interconnection between Pcs7AnIn.ScaleOut and MonAnL.PV_OpScale.
c. To allow operator adjustment from the OS faceplate, insert an OpAnL (FB1865) block for High Limit and Low Limit, or use technological calculation blocks.
d. Connect the dynamic calculation outputs to MonAnL.PV_OpScale.High and MonAnL.PV_OpScale.Low.
Adjust Alarm Limits Dynamically (Optional):
a. If alarm thresholds (HH_Lim, H_Lim, L_Lim, LL_Lim) must scale proportionally with the dynamic scale range, interconnect ratio calculation blocks to update limit inputs dynamically.
b. Ensure that Feature.Bit23 ('Limits are evaluated dynamically') is configured if required.
Compile Charts and Download Changes:
a. In SIMATIC Manager, execute Chart -> Compile -> Changes only with 'Generate module drivers' checked.
b. Verify that the compiler does not alter your custom MonAnL.PV_OpScale interconnections.
c. Download changes to the AS controller in RUN mode.

Verification

In CFC Test Mode, modify the dynamic calculation driving MonAnL.PV_OpScale.High (e.g., change from 100.0 to 250.0).
Open WinCC Runtime and click the analog indicator to open the MonAnL faceplate.
Verify that the bar graph top limit immediately updates from 100 to 250.
Confirm that the normalized bar fill accurately tracks the process variable relative to the new dynamic range.

Limitations and Operational Precautions

Changing PV_OpScale on MonAnL alters only the visual presentation and percentage display on the OS; it does not change the physical A/D conversion span on Pcs7AnIn. If physical field transmitter ranges change, HW Config must be re-parameterized.

Engineer FAQ

Why does the compiler overwrite my Scale input on Pcs7AnIn?

Because 'Generate module drivers' automatically synchronizes channel driver Scale inputs with the measuring range configured in STEP 7 HW Config during every compile.

What parameter on MonAnL controls the faceplate bar graph limits?

The PV_OpScale input structure (PV_OpScale.High and PV_OpScale.Low).

Can I connect an operator faceplate block to change scale limits at runtime?

Yes. Connecting an OpAnL (FB1865) block allows operators to enter custom scale limits from the WinCC faceplate.

Does changing PV_OpScale affect MonAnL alarm trips?

No. Absolute alarm limits (HH_Lim, H_Lim) operate in engineering units independently of PV_OpScale unless explicitly programmed with mathematical ratios.