Migrating Legacy PI / PA Channel Blocks to Standard Symbolic Drivers During PCS 7 V5.2 to V6.1 Upgrades

In legacy SIMATIC PCS 7 software updates (specifically migrating from PCS 7 V5.2 to V6.1), the reference to 'PI blocks' in documentation corresponds to PROFIBUS PA (Process Automation) channel blocks (often designated 'PA blocks' in German manuals). In PCS 7 V5.1 and earlier, these blocks utilized direct, absolute logical I/O addresses configured via the LADDR input parameter rather than symbolic process image interconnections. When upgrading to PCS 7 V6.x, Siemens modernized driver engineering to use symbolic interconnections (Interconnection to Operand...) mapped to process image partitions. Engineers must print or document their existing CFC charts prior to migration, configure symbolic names for all PA field devices in HW Config, replace the legacy PI/PA blocks with modern driver blocks (such as PA_AI or PA_DI), and reconnect them symbolically.

Historical Context: Legacy PI / PA Blocks in PCS 7 V5.x

During the era of SIMATIC PCS 7 V5.0 and V5.1, fieldbus integration with PROFIBUS PA instruments (transmitters, positioners, flowmeters connected via DP/PA Couplers or DP/PA Links) utilized first-generation driver blocks. In English documentation, these were sometimes translated as 'PI blocks' (Process Instrument blocks) or 'PA blocks' (PROFIBUS PA blocks).

These legacy blocks required engineers to enter the raw hexadecimal or decimal logical address (LADDR) of the field device's slot directly into the block input pin in CFC. While functional, this method lacked automatic hardware consistency checking: if an instrument address changed in HW Config, the CFC block continued polling the old address, leading to silent measurement errors or CPU read faults.

The Transition from Absolute LADDR to Symbolic Driver Interconnection

Starting in PCS 7 V6.0 and standardized in V6.1, Siemens completely overhauled fieldbus and peripheral I/O addressing:

Symbolic Addressing Paradigm: Direct hardware addresses (PIW 256, LADDR = 256) were superseded by symbolic tags (e.g., TT_101_PV) assigned directly within the Step 7 Symbol Table and HW Config.
Process Image Partitioning (PIP): Channel blocks were redesigned to read data cyclically from process image partitions updated deterministically in synchronism with the cyclic interrupt OB (e.g., OB 35).
Automatic Driver Generation (Generate Module Drivers): When compiling CFC charts, the driver generator automatically evaluates the symbolic links, instantiates diagnostic blocks (MOD_PA, DPPA_LNK), and connects quality codes (QC) without manual LADDR configuration.

Step-by-Step Procedure for Migrating PA Channel Blocks

To migrate charts containing legacy PI/PA blocks during a V5.2 to V6.1 software update:

Document Existing Charts: Before modifying the project, print all CFC charts containing legacy PI/PA blocks. Record the exact LADDR values, scaling parameters (VHRange, VLRange), and downstream interconnections.
Assign Symbols in HW Config: Open HW Config. Navigate to the PROFIBUS PA devices (under the DP/PA Link or Coupler). For each I/O slot (e.g., Analog Input profile), right-click the slot address and assign a clear, unique symbol name (e.g., PT_201_Out). Save and compile HW Config.
Replace Blocks in CFC: Open the CFC charts. Delete the obsolete V5.x PI/PA block. From the updated PCS 7 V6.1 library, insert the modern channel block (e.g., PA_AI or CH_AI).
Establish Symbolic Interconnection: Right-click the block's PV_In pin and select Interconnection to Operand.... Select the symbolic tag configured in HW Config.
Execute Driver Generation: Compile the CFC charts with Generate module drivers checked.

Handling PROFIBUS PA Device Profiles and Diagnostic Integration

Modern PA driver blocks in PCS 7 V6.x interpret the 5-byte PROFIBUS PA data structure (4 bytes IEEE 754 floating-point process value + 1 byte status/quality byte) natively:

Status Byte Decoding: The status byte conforms to PA Profile 3.0, communicating instrument health: 0x80 (Good), 0x40 (Uncertain), 0x00 (Bad), or 0x28 (Process Related Error).
Driver Failover: If the PA bus cable is severed or the DP/PA Link loses power, the modern driver block immediately substitutes the configured fail-safe value (SubsPV) and passes Bad status to downstream APL/Standard controllers, preventing erratic loop behavior.

Comparison of Legacy LADDR vs Modern Symbolic Channel Blocks

The following table compares legacy V5 PI/PA channel blocks with modern symbolic driver blocks:

| Architectural Feature | Legacy PI / PA Block (PCS 7 V5.x) | Modern Symbolic Driver Block (PCS 7 V6.x+) |
|---|---|---|---|
| Hardware Addressing | Absolute logical address (LADDR) | Symbolic tag name from HW Config Symbol Table |
| Diagnostic Hierarchy | Minimal; manual diagnostic evaluation | Fully automated via MOD_PA and DPPA_LNK |
| Address Change Handling | Manual reconfiguration required in CFC | Automated update upon CFC recompilation |
| Quality Code Handling | Proprietary or raw integer conversion | Standard IEEE/PA status byte decoding (QC) |
| Process Image Synchronization | Direct peripheral I/O access (PIW/PID) | Synchronized Process Image Partitions (PIP) |

Verification

Verify the successful migration of PA channel blocks:
1. In SIMATIC Manager, execute 'Chart > Compile > As Program' with 'Generate module drivers' enabled; verify zero errors.
2. Switch the AS controller to RUN. Open the CFC chart online in Test Mode.
3. Verify that the modern PA_AI channel block displays valid process values (PV_Out) matching calibrated field readings.
4. Verify that the quality code output (Quality Code or QC) displays 16#80 (Good) under normal operation.
5. Disconnect the PA bus wire on one transmitter; verify that the quality code transitions to 16#00 (Bad) and downstream alarm blocks assert channel fault alarms.

Limitations and Operational Precautions

Never mix legacy V5.x PI blocks and modern V6.x driver blocks on the same DP/PA Link subsystem; the PCS 7 driver generator cannot resolve conflicting diagnostic structures on a single master. When replacing blocks in CFC, do not execute a partial compile until all obsolete blocks have been deleted and replaced, otherwise unreferenced symbols can lock the Step 7 database. Always retain an unmigrated archive of the V5.x project for verification.

Engineer FAQ

Why does the English PCS 7 manual refer to 'PI blocks' instead of 'PA blocks'?

It was an early translation artifact in PCS 7 V5.x manuals, where 'Process Instruments' (PI) was used interchangeably with 'PROFIBUS PA' (Process Automation). German source documentation used 'PA-Bausteine'.

What does 'Interconnection to Operand' mean in CFC?

It means linking a CFC block pin directly to a symbolic name defined in the Step 7 Symbol Table (which points to an I/O address in HW Config), rather than connecting it to another CFC block output.

Do modern PCS 7 versions (V8/V9) still use PA_AI blocks?

In PCS 7 V8.x and V9.0 with the Advanced Process Library (APL), PA devices are handled using standard APL channel blocks like Pcs7AnIn with device-specific profile adapters, or specialized field device driver libraries.