C135 PCS 7 (PCS7)
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Resolving Zero Output and QBAD Faults on PCS 7 Analog Output Channel Drivers

Resolving Zero Output and QBAD Faults on PCS 7 Analog Output Channel Drivers

When a PCS 7 analog output channel driver block (such as CH_AO / Pcs7AnOu) receives a valid input value (e.g. 50.0%) but outputs 0 to physical hardware while asserting fault output QBAD = 1, the failure is caused by an unconfigured or missing input parameter MODE. In PCS 7, MODE defines the module's measuring/output type and range combined with the signal status quality code. If module drivers are compiled automatically via the CFC Driver Generator, MODE is connected to diagnostic block MOD_D1. If manual engineering is used or module drivers are bypassed, you must manually enter the hex mode code into parameter MODE (for ET 200S module 6ES7135-4GB01-0AB0 in 4..20 mA mode, enter 16#80000203).

Table of Contents

Executive Summary

In SIMATIC PCS 7 control applications, converting calculated controller manipulated variables (e.g. 0.0 to 100.0% from a PIDConL block) into physical 4..20 mA or 0..10 kV electrical signals requires analog output channel driver blocks (such as CH_AO in classic libraries or Pcs7AnOu in the Advanced Process Library). A perplexing commissioning failure occurs when the block receives a valid numerical setpoint at input U or PV_In, yet the physical module outputs 0 mA, and the block's diagnostic pin QBAD asserts TRUE (1). This condition occurs because the channel driver refuses to write to peripheral memory unless parameter MODE confirms valid hardware range configuration and good quality status. This technical guide explains the structure of the MODE parameter and provides the resolution procedure.

Analog Output Channel Driver Architecture: Value, MODE, and QBAD

The analog output driver block acts as the software encapsulation of the physical D/A converter module:

  • Input U / PV_In: Engineering unit floating-point command (e.g. 0.0 to 100.0%).
  • Parameter MODE: A 32-bit DWORD combining the Quality Code (upper byte) with the hardware measuring/output type (lower word).
  • Output QBAD / Bad: Safety status flag. If QBAD = 1, the driver detects an invalid mode, unparameterized module, or wire break, immediately forcing the physical output to a predefined safe fallback state (typically 0 mA).

Functional Mechanics of the MODE Parameter (Quality Code + Range)

The 32-bit MODE parameter is structured into two distinct segments:

  • Bits 24-31 (Quality Code): 16#80 indicates valid, healthy signal quality. If the upper byte is 16#00, the block treats the channel as defective and forces QBAD = 1.
  • Bits 0-15 (Measuring Range & Type): Identifies electrical characteristics (current vs voltage, 0..20 mA vs 4..20 mA, bipolar vs unipolar).

Combining Quality Code 16#80 with the 4..20 mA current mode 16#0203 produces the exact composite hex word: 16#80000203.

Automated Module Driver Generation vs Manual Parameterization

In standard PCS 7 engineering, the engineer never manually types hex values into MODE. When the AS program is compiled with Generate module drivers enabled, the Driver Generator automatically instantiates diagnostic block MOD_D1 and creates a dynamic interconnection to Pcs7AnOu.MODE.

If an engineer disables the Driver Wizard, places the module from the STEP 7 Standard Profile, or manually types an invalid peripheral address, MODE remains unlinked (16#00000000), triggering persistent QBAD = 1.

ET 200S Analog Output Range and MODE Coding Matrix

The following table provides verified hex values for parameter MODE for the ET 200S 2 AO U/I module (6ES7135-4GB01-0AB0):

Output Electrical RangeModule Measuring Type CodeQuality Code ByteComposite Hex MODE ParameterExpected Output BehaviorCurrent: 4 to 20 mA16#020316#80 (Good)16#80000203Scales 0..100% to 4..20 mA (Standard process).Current: 0 to 20 mA16#020116#80 (Good)16#80000201Scales 0..100% to 0..20 mA.Voltage: ±10 V16#010116#80 (Good)16#80000101Scales -100..+100% to -10..+10 V.Voltage: 1 to 5 V16#010316#80 (Good)16#80000103Scales 0..100% to 1..5 V.Unconfigured / Default16#000016#00 (Bad)16#00000000Block faults; QBAD = 1; output forced to 0.

Step-by-Step Troubleshooting and Parameterization Procedure

Method 1: Enabling Module Driver Generation (Recommended Standard)

  1. Open HW Config in SIMATIC Manager.
  2. Ensure the ET 200S interface module and 2 AO U/I module (6ES7135-4GB01-0AB0) are placed from the PCS 7 Profile.
  3. In module properties, set the output channel measuring type to 4..20 mA.
  4. Note the output address assigned (e.g. PQW 512).
  5. In CFC, ensure input ADDR of Pcs7AnOu (or CH_AO) is set to 512.
  6. Select Chart > Compile > Charts as Program, check Generate module drivers, and execute compilation.
  7. Download changes to the controller. Confirm that MODE is automatically interconnected to MOD_D1 and QBAD clears to 0.

Method 2: Manual Parameterization for Standalone Testing

  1. If testing on a test bench without full module drivers, open the CFC chart.
  2. Sever any broken interconnections on input MODE.
  3. Double-click parameter MODE and enter the static value: 16#80000203.
  4. Compile changes and download to the AS.
  5. Observe that QBAD switches to 0 and physical current flows from the module.

Verification

To verify that the analog output driver functions correctly:

  1. CFC Test Mode: Switch CFC to Test Mode. Confirm that QBAD evaluates to FALSE (0).
  2. Physical Current Measurement: Connect a digital multimeter (in mA mode) across the physical output terminals of the ET 200S module.
  3. Span Test: Set input PV_In = 0.0% -> verify multimeter reads 4.00 mA. Set PV_In = 50.0% -> verify 12.00 mA. Set PV_In = 100.0% -> verify 20.00 mA.

Limitations and Operational Precautions

  • Module Backplane Power: ET 200S electronic output modules require 24 VDC auxiliary power supplied via the upstream Power Module (PM-E). If the PM-E power fails, the module asserts hardware fault regardless of software MODE.
  • Load Resistance Limits: Ensure total loop resistance (wiring + positioner impedance) does not exceed the module's maximum allowable load impedance (typically 500 Ω for 4..20 mA outputs).
  • Delta Download Caution: Modifying module driver interconnections manually can cause compilation mismatches during subsequent automated compiles.

Engineer FAQ

What does QBAD = 1 mean on a PCS 7 channel driver?

QBAD indicates that the driver output is bad due to invalid MODE parameterization, hardware diagnostic interrupt, or broken communication to the peripheral rack.

Why did my manually entered MODE value get overwritten after compiling CFC?

When 'Generate module drivers' is enabled during compilation, the Driver Generator overwrites manual MODE values with dynamic interconnections from diagnostic blocks.

How do I configure the channel driver to hold the last valid value on fault?

Configure parameter Feature.Bit28 = 1 on APL Pcs7AnOu to retain the last valid output upon sensor or communication failure.

Technical References

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