Cyclic I/O Data Exchange and Acyclic Parameterization for PROFIBUS PA Devices via AFDiSD in SIMATIC PCS 7

In SIMATIC PCS 7 V8.2 with SIMATIC PDM V9.0 SP1, integrating up to 30 PROFIBUS PA field instruments (such as temperature transmitters, level switches, and control valve positioners) behind AFDiSD couplers operates via two decoupled communication mechanisms: (1) Cyclic I/O Process Data Exchange running automatically over the controller's Process Image Input/Output (PII/PIQ) tables using standard PCS 7 channel blocks (Pcs7AnIn for transmitters, Pcs7DiIn for level switches, and Pcs7AnOu for valve positioners), requiring no manual read/write commands from the user; and (2) Acyclic DPV1 Parameterization executing on-demand via SIMATIC PDM for calibration, configuration, and maintenance data without consuming cyclic controller bandwidth.

Architectural Separation: Cyclic Process Image vs Acyclic DPV1 Messaging

A foundational strength of the PROFIBUS PA protocol (IEC 61158-2) is its ability to separate mission-critical deterministic real-time control from diagnostic and maintenance communications:

Cyclic Process Data (MS0 Connection): Transmitted continuously on every single bus scan. The controller reads measurement values from transmitters and writes setpoints to actuators directly into the CPU's Process Image Partitions (PIP). No S7 communication blocks or handshakes are required.
Acyclic Data (MS1 / MS2 Connection): Transmitted during the spare token rotation time between cyclic frames. Used by SIMATIC PDM or advanced asset management systems to read device parameters, calibration curves, and error histories on demand.

Cyclic Data Ingestion for Process Transmitters and Discrete Switches

In SIMATIC PCS 7 CFC, handling inputs from PROFIBUS PA devices follows standardized driver engineering:

Analog Transmitters (Temperature, Pressure, Flow): The PA Profile 3.0 specifies a 5-byte data telegram: 4 bytes IEEE 754 floating-point process value + 1 byte status/quality byte. Connect the symbolic input operand directly to the PV_In input of Pcs7AnIn (FB1869). The block continuously extracts the engineering value and parses the status byte into the APL .Value and .ST attributes.
Discrete Level Switches and Limit Contacts: A PA discrete input device outputs 2 bytes: 1 byte binary state (0 or 1) + 1 byte quality code. Connect the operand directly to the PV_In pin of Pcs7DiIn (FB1865).

Writing Actuator Setpoints to PROFIBUS PA Valve Positioners

For control valves equipped with PROFIBUS PA positioners (such as SIPART PS2 or third-party positioners):

Telegram Structure: The positioner accepts a 5-byte setpoint telegram (4 bytes REAL setpoint 0.0 to 100.0% + 1 byte setpoint status byte) and returns a 5-byte actual position feedback telegram.
CFC Block Mapping: Connect the control output from the continuous valve block (VlvAnL.MV) to the PV_In input of the analog output driver Pcs7AnOu (FB1870).
Process Image Output: Pcs7AnOu writes the floating-point setpoint cyclically into the Process Image Output (PQW / %Q) assigned in HW Config, updating the physical valve stroke deterministically on every cyclic OB interrupt.

Acyclic Diagnostic and Calibration Access via SIMATIC PDM V9.0

When an engineer needs to access secondary variables not mapped into the cyclic process image (e.g., total operating hours, internal temperature, valve travel histogram, or sensor zero-point calibration):

Do NOT Write SCL Logic: Do not write custom SFB 52 RDREC / SFB 53 WRREC logic in CFC to poll these variables cyclically, as this wastes CPU communication channels and slows down bus turnaround times.
Use SIMATIC PDM: Launch SIMATIC PDM V9.0 SP1 from the Process Device Network View. PDM opens an acyclic Layer 2/Layer 7 channel through the DP/PA Link to the field device, allowing full parameter editing, diagnostic evaluation, and maintenance logging.

Comparison of PROFIBUS PA Device Profiles and Channel Block Mapping

The following table maps common PROFIBUS PA field device profiles to their corresponding PCS 7 APL channel blocks and process image telegrams:

Field Device TypePA Profile StandardCyclic Telegram SizePCS 7 APL BlockCyclic Signal FunctionAnalog Transmitter (Pressure/Temp)AI Profile (Analog Input)5 Bytes Input (4B Real + 1B QC)Pcs7AnIn (FB1869)Continuous measurement readingDiscrete Level SwitchDI Profile (Digital Input)2 Bytes Input (1B Bool + 1B QC)Pcs7DiIn (FB1865)High/Low alarm contact stateModulating Valve PositionerAO Profile (Analog Output)5B Input / 5B OutputPcs7AnOu + VlvAnLValve setpoint command and stroke feedbackFlowmeter with Dual TotalizersMulti-Variable Profile15 Bytes Input (3x AI)3x Pcs7AnInFlow Rate, Forward Total, Reverse TotalMulti-Channel Temperature Multiplexer4x AI Profile20 Bytes Input (4x AI)4x Pcs7AnInFour independent RTD temperature sensors

Verification

Verify cyclic and acyclic communication on the PA bus:
1. In HW Config, verify that all 30 PA field devices show 'Configured and Active' with valid assigned DP addresses (e.g., addresses 10 to 39).
2. Open CFC in test mode; verify that process values on Pcs7AnIn blocks update continuously with status code 16#80 (Good).
3. Change the valve setpoint in WinCC Runtime; verify that Pcs7AnOu updates PQW and that the field valve physically travels to the commanded percentage.
4. In SIMATIC PDM, execute 'Upload to PG/PC' for a temperature transmitter; verify that calibration parameters are retrieved successfully over the acyclic channel.

Limitations and Operational Precautions

When designing a PROFIBUS PA segment with 30 devices connected via AFDiSD distributors, carefully calculate the total bus cycle time in HW Config. While PROFIBUS PA baud rate is fixed at 31.25 kbit/s, polling 30 devices with multiple cyclic variables can extend segment cycle times to between 300 ms and 600 ms. If faster cycle times (<100 ms) are required for high-speed pressure loops, divide the devices across two separate DP/PA Couplers or use standard 4-20 mA HART I/O modules.

Engineer FAQ

Do I need to trigger a read command in CFC to get measurement values from a PA transmitter?

No! PROFIBUS PA process measurements are transferred cyclically and continuously into the CPU's Process Image Input (PII) table. The Pcs7AnIn channel block reads this data automatically on every execution cycle.

What is the purpose of the 5th byte in a PA analog input telegram?

The 5th byte is the PA Profile Quality Code (Status Byte). It conveys instrument health conforming to NAMUR NE 107 (e.g., 0x80 = Good, 0x00 = Bad, 0x40 = Uncertain, 0x28 = Process Related Error).

Can S7-1500 controllers communicate directly with PROFIBUS PA?

S7-1500 controllers do not support native PA interface cards. They communicate with PROFIBUS PA via a DP/PA Link connected to an S7-1500 PROFIBUS DP master module (such as CM 1542-5).