Why Classic Driver Blocks CH_AI and CH_DI Are Missing in PCS 7 V8.1 and How to Migrate to APL Channel Drivers

In SIMATIC PCS 7 V8.1, legacy channel driver blocks such as CH_AI, CH_DI, CH_AO, and CH_DO do not appear in the default CFC block catalog because they belong to the deprecated 'Classic' PCS 7 Library (V7.1 SP3). Starting with PCS 7 V7.1 and mandatory for all new projects in V8.x, Siemens replaced the Classic library with the Advanced Process Library (APL). In APL, channel drivers are standardized under the PCS 7 Basis Library as Pcs7AnIn (analog input), Pcs7DiIn (digital input), Pcs7AnOu (analog output), and Pcs7DiOu (digital output). These modern drivers utilize structured signal types containing 8-bit signal status (ST) for automated diagnostic monitoring.

Evolution of PCS 7 Driver Libraries: Classic Library vs APL

When engineers begin developing in SIMATIC PCS 7 V8.1, they often search the block libraries for familiar driver names:
* The Classic Driver Architecture (PCS 7 V6.x / V7.0):
* Provided basic channel drivers: CH_AI (FC275), CH_DI (FC276), CH_AO (FC277), and CH_DO (FC278).
* Passed raw values and primitive quality bits (QUALITY).
* The Advanced Process Library (APL) Paradigm:
* Introduced in PCS 7 V7.1 to establish a unified, object-oriented process automation framework.
* Channel drivers were decoupled into the PCS 7 Basis Library to automatically interface between physical I/O modules and APL technological blocks (MonAnL, MotL, PIDConL).
* Greenfield projects must exclusively deploy APL drivers to ensure compatibility with modern PROFINET, ET 200SP, and ET 200M hardware.

Direct Functional Mapping: Classic Drivers to APL Channel Drivers

The table below outlines the 1-to-1 functional replacement mapping from Classic to APL channel drivers:

Analog Input:
Classic: CH_AI (FB105 / FC275)
APL: Pcs7AnIn (FB1869) - Processes 4-20 mA, RTD, thermocouple, and HART variables with Namur NE43 live zero tracking.
Digital Input:
Classic: CH_DI (FB106 / FC276)
APL: Pcs7DiIn (FB1871) - Processes 24 VDC / 120 VAC digital states, flutter suppression, and pulse stretching.
Analog Output:
Classic: CH_AO (FB107 / FC277)
APL: Pcs7AnOu (FB1870) - Converts floating-point setpoints to 4-20 mA DAC registers.
Digital Output:
Classic: CH_DO (FB108 / FC278)
APL: Pcs7DiOu (FB1873) - Drives digital output relays and solenoid channels.

Structured Data Types and Quality Codes (Value + ST)

The fundamental technological advance of APL channel drivers is the transition from scalar primitives to Structured Signal Types:

Structure Definition:
pascal TYPE Struct_AnalogReal : STRUCT Value : REAL; // Process measurement value ST : BYTE; // Signal Status / Quality Code (IEC 61158) END_STRUCT; END_TYPE
Signal Status Hierarchy (ST):
16#80: Good - Channel valid, process measurement reliable.
16#60: Simulated - Forced via CFC Test Mode or channel driver simulation.
16#00: Bad - Wire break, short circuit, or module failure.
Downstream APL blocks inspect .ST automatically. If a wire snaps, Pcs7AnIn outputs ST = 16#00, causing PIDConL to shed from Auto to Manual and triggering a bad-quality alarm on WinCC faceplates without writing custom logic.

Installing Legacy Classic Drivers in PCS 7 V8.1 for Brownfield Migrations

If an engineer must maintain a legacy migrated project without rewriting logic:

Locate Classic Library Installation Media:
* Insert the PCS 7 V8.1 DVD 2 (or download package Entry ID 109480136).
* Navigate to folder: Additional_Products\PCS7_Library_V713\.
* Run Setup.exe to install the PCS 7 Library V7.1 SP3.
Re-Run Basis Library Setup (Critical):
* Installing the old library can overwrite CFC driver generator templates.
* Immediately re-run Setup.exe for PCS 7 Basis Library V8.1 in Repair mode to preserve modern compiler rules.
Library Coexistence:
* SIMATIC Manager will now display both PCS 7 Library V7.1 and PCS 7 Basis Library V8.1 in the library catalog.

Comparison of Classic vs Modern APL Channel Drivers

The table below contrasts the technical characteristics of Classic and APL channel drivers.

Feature / CapabilityClassic Drivers (CH_AI / CH_DI)APL Basis Drivers (Pcs7AnIn / Pcs7DiIn)Data Type OutputScalar REAL / BOOLStructured Struct (Value + ST)Diagnostic Code ComplianceProprietary QUALITY byteStandardized IEC 61158 / Namur NE43Module Driver AutomationBasic ET 200M supportFull support for ET 200SP, ET 200M, PROFINETSimulation HandlingManual CFC logic overrideBuilt-in SimOn and SimValue parametersFlutter SuppressionRequires separate filter blocksIntegrated digital flutter filterRecommended ApplicationMigrated brownfield plants onlyAll new PCS 7 projects (Mandatory standard)

Verification

To confirm that APL channel drivers are operating properly:
1. In CFC, insert Pcs7AnIn into OB35 and interconnect input PV_In to an analog input peripheral address (e.g., IW 512).
2. Interconnect Pcs7AnIn.Out to MonAnL.PV.
3. Execute Chart > Compile > Entire Program with Generate Module Drivers checked.
4. Verify that the compiler automatically instantiates diagnostic driver blocks (MOD_D1, OR_M_16) without errors.
5. In Test Mode, verify that Pcs7AnIn.Out.Value reports the live sensor measurement and Out.ST displays 16#80 (Good).
6. Disconnect the sensor wire: confirm that Out.ST drops to 16#00 (Bad) and the WinCC faceplate immediately displays the bad quality indicator.

Limitations and Operational Precautions

Never mix Classic drivers and APL technological blocks in the same signal path; APL blocks expect structured signals (Struct), requiring complex manual conversion if fed from classic drivers.
Avoid deploying Classic driver blocks on modern PROFINET or ET 200SP remote I/O stations; legacy driver generators lack the XML templates required to decode PROFINET diagnostics.
In new installations, always engineer with the Advanced Process Library to ensure long-term support and lifecycle compatibility.
Always validate final software licensing and system documentation against the applicable Siemens catalog and authorized distributor quotation.

Engineer FAQ

Can I convert CH_AI to Pcs7AnIn automatically?

No direct conversion macro exists. You must replace the block in CFC and reconnect the input address to PV_In and the output to the APL consumer block.

Why did the CFC compiler fail with driver generation error after adding CH_AI?

Because the driver generator could not find diagnostic rules linking CH_AI to modern hardware modules configured in HW Config. Deploy Pcs7AnIn instead.