Configuring APL Channel Driver Pcs7AnIn (FB1869) Bipolar Measuring Ranges and Substitute Values with Feature Bit 29

In SIMATIC PCS 7 Advanced Process Library (APL), if the channel driver block Pcs7AnIn (FB1869) outputs an unexpected value (such as -50.0) at PV_Out without an active input at PV_In or SimPV_In, the cause is an unconfigured bipolar measuring range (+/- 10V) or an active substitute value triggered by bad signal quality. For bipolar inputs, raw integer counts range from -27648 to +27648. If the channel detects open-circuit underflow or invalid quality, Feature.Bit29 determines output behavior: when Feature.Bit29 = 1, Pcs7AnIn outputs the substitute value configured at parameter SubsPV_In (defaulting to 0.0 or a configured offset like -50.0). To resolve this, align HW Config measuring range with Pcs7AnIn Scale parameters, verify module driver generation (Mode and DataXchg), and configure Feature.Bit29 according to plant safety requirements.

Executive Summary

Process engineers configuring analog input channel drivers in SIMATIC PCS 7 V9.0 SP2 with APL 9.0 occasionally observe anomalous static outputs (e.g., PV_Out = -50.0) on newly instantiated Pcs7AnIn blocks (FB1869), despite having no active input wired to PV_In and simulation disabled. This behavior typically occurs on distributed ET 200 stations configured with bipolar analog inputs (such as +/- 10 V or +/- 20 mA). When physical inputs are disconnected or unpowered, raw A/D converters float to open-circuit limits, triggering wire-break or underflow diagnostics. This guide clarifies the mathematical relationship between S7 raw counts, APL scaling, and Feature Bit 29 substitute value handling.

Architecture and Fundamental Mechanics

The APL analog channel driver Pcs7AnIn (FB1869) processes raw analog inputs through three sequential computational stages:
1. Raw Value Acquisition & Quality Evaluation: The block reads the peripheral input address (PV_In) and interrogates the module driver connection (Mode, DataXchg). Standard unipolar signals (0..20 mA, 4..20 mA, 0..10 V) map physical signals to 0..27648 counts. Bipolar signals (+/- 10 V, +/- 20 mA) map negative full-scale to positive full-scale across -27648 to +27648 counts.
2. Normalization and Scaling: The block converts raw counts into engineering units according to the configured Scale structure:
$$PV_Out = \left( \frac{Raw - RawMin}{RawMax - RawMin} \right) \times (Scale.High - Scale.Low) + Scale.Low$$
If Scale.Low = -100.0 and Scale.High = 100.0, an intermediate raw count of $-13824$ (-5V) yields exactly $-50.0$.
3. Substitute Value Processing (Feature.Bit29):
When the module detects an underflow, open circuit, or invalid hardware state, the signal status degrades to Bad (ST = 16#00 or 16#28).
- If Feature.Bit29 = 0 (default): The block outputs the calculated raw value (or last valid value if configured) accompanied by a Bad quality code.
- If Feature.Bit29 = 1 ('Output substitute value if raw value is invalid'): The block discards the live input and outputs the value configured at input parameter SubsPV_In (with quality code ST = 16#60 or 16#28). If SubsPV_In was set to -50.0, PV_Out.Value outputs -50.0 continuously until the field channel recovers.

Step-by-Step Implementation Procedure

Verify Module Measuring Range in HW Config:
a. Open STEP 7 HW Config, locate the ET 200 analog input module, and open its Object Properties.
b. Check the configured 'Measuring type' and 'Measuring range' (e.g., Voltage +/- 10 V vs. Current 4..20 mA).
c. Note whether wire-break diagnostics and open-circuit monitoring are enabled.
d. Save and compile HW Config.
Configure Pcs7AnIn Scale and Mode in CFC:
a. In the CFC chart, open the Pcs7AnIn block properties.
b. Ensure the Scale parameter reflects the physical process span (e.g., Scale.Low = -100.0 and Scale.High = 100.0 for +/- 10 V transmitters).
c. Verify that the input parameter Mode is automatically assigned by module driver generation. For +/- 10 V, Mode typically carries format 16#0203.
Configure Feature Bit 29 and Substitute Value:
a. Inspect input parameter Feature on Pcs7AnIn.
b. To enable substitute value output upon signal failure, set Feature.Bit29 = 1.
c. Configure the desired fail-safe default value at SubsPV_In (e.g., 0.0 or a predefined safe state).
d. If substitute output is undesirable, set Feature.Bit29 = 0 so that downstream blocks (MonAnL) can process the raw signal failure and initiate technological interlocking.
Recompile CFC Charts with Driver Generation:
a. In CFC Editor, execute Chart -> Compile -> Chart as Program....
b. Ensure 'Generate module drivers' is checked.
c. Download changes to the AS controller in RUN mode.

Verification

In CFC test mode, monitor Pcs7AnIn: apply a known reference voltage (e.g., 0 V to a +/- 10 V channel) and confirm PV_Out.Value reads 0.0 with ST = 16#80 (Good).
Apply -5 V: verify PV_Out.Value scales linearly to -50.0.
Disconnect the input terminal to simulate a field break: verify that PV_Out.ST changes to 16#00 or 16#28 (Bad).
If Feature.Bit29 = 1, confirm that PV_Out.Value transitions to SubsPV_In and downstream faceplates display the maintenance fault icon.

Limitations and Operational Precautions

Feature Bit 29 alters the output value of the channel driver during field faults; safety-critical interlocks must evaluate the signal status byte (PV_Out.ST) or Bad output (Bad = 1), rather than relying solely on the numerical PV_Out.Value, to prevent an unexpected substitute value from masking an active process trip condition.

Engineer FAQ

What does Feature Bit 29 do on Pcs7AnIn?

It enables the output of a pre-configured substitute value (SubsPV_In) at PV_Out whenever the raw analog input detects bad quality, wire break, or measuring range violation.

Why did PV_Out display -50.0 when no input was connected?

On a +/- 10 V module with -100 to +100 scaling, an open-circuit floating voltage or a configured substitute value of -50.0 caused PV_Out to report -50.0.

What raw count corresponds to 0V on a +/- 10V S7 analog module?

Zero counts (0) represents 0 V; -27648 represents -10 V, and +27648 represents +10 V.

Can I use Feature Bit 29 to hold the last valid value?

Yes, in combination with Feature Bit 28, the block can be configured to hold the last valid value instead of outputting SubsPV_In.

Does Pcs7AnIn require manual module driver programming for bipolar cards?

No. When 'Generate module drivers' is compiled, the compiler automatically determines card type and writes the appropriate hexadecimal Mode code to the channel driver.