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How to Pack and Unpack Double Words (DWORD) in SIMATIC PCS 7 CFC

How to Pack and Unpack Double Words (DWORD) in SIMATIC PCS 7 CFC

To pack individual boolean signals into a 32-bit double word (DWORD) or unpack a DWORD into separate boolean variables in SIMATIC PCS 7 Continuous Function Charts (CFC), you must deploy standard conversion blocks from the CFC Elementary Library. In process control engineering, packed double words are frequently used to communicate status bitmasks, fault matrices, and command words between controllers and third-party subsystems (such as VFDs, packaged skids, and RTUs). The CFC library provides standard conversion blocks—including BO_DW (Boolean to Double Word), DW_BO (Double Word to Boolean), W_DW, and DW_W—that execute bit shifting and masking in CPU microcode, ensuring deterministic execution without requiring custom SCL scripts.

Table of Contents

Why Bitmasking and DWORD Packing Are Essential in PCS 7

In industrial process automation, controllers exchange extensive operational status and command data with external equipment:
* The Communication Bandwidth Challenge: Transmitting 32 individual boolean tags across fieldbuses (PROFIBUS DP, PROFINET, or Modbus TCP) generates significant protocol overhead and consumes multiple communication tags.
* The Packed Solution: Packing 32 boolean states into a single 32-bit DWORD condenses the entire status vector into 4 consecutive bytes.
* APL Status Words: In the SIMATIC PCS 7 Advanced Process Library (APL), blocks use 32-bit status words (e.g., Status1, Status2, Status3, OS_Perm) to transmit operating modes, interlock statuses, and alarm permissions to WinCC faceplates in a single atomic memory transfer.

Standard Conversion Blocks in the CFC Elementary Library

The CFC editor includes pre-tested conversion blocks located in the library catalog under CFC Library > Blocks > CONVERT:

Block NameBlock NumberInput Data TypeOutput Data TypeTypical Engineering ApplicationBO_DWFC17632 x BOOL (IN00–IN31)1 x DWORD (OUT)Aggregating plant interlocks or motor alarms into a 32-bit SCADA status word.DW_BOFC1771 x DWORD (IN)32 x BOOL (OUT00–OUT31)Unpacking third-party drive status words into individual run/fault signals.W_DWFC1802 x WORD (IN1, IN2)1 x DWORD (OUT)Combining two 16-bit fieldbus registers into a 32-bit double word.DW_WFC1811 x DWORD (IN)2 x WORD (OUT1, OUT2)Splitting a 32-bit counter or accumulator into high/low words for Modbus.

These blocks execute cleanly in all cyclic interrupt tasks (OB30 through OB38) and incur negligible CPU processing overhead.

Bit-Ordering and Endianness in SIMATIC S7 Architectures

When packing or unpacking data between Siemens S7-400 controllers and third-party devices, engineers must account for bit-ordering conventions:
* Big-Endian Architecture: SIMATIC S7-300, S7-400, and AS 410 CPUs utilize Big-Endian byte ordering (most significant byte stored at the lowest memory address).
* Bit Assignment in BO_DW / DW_BO:
- IN00 corresponds to Bit 0 (Least Significant Bit - LSB).
- IN07 corresponds to Bit 7 (end of the lowest byte).
- IN31 corresponds to Bit 31 (Most Significant Bit - MSB).
* Third-Party Inversion: Many third-party Modbus and Ethernet/IP devices use Little-Endian byte or word arrangements. If unpacked bits appear swapped across 16-bit boundaries, deploy SWAP blocks or re-map word inputs (W_DW with transposed pins) to match the external device's memory model.

Method: Step-by-Step Implementation in CFC

Follow this workflow to unpack a 32-bit status word from a variable frequency drive into discrete signals:

  1. In SIMATIC Manager, open the target CFC chart.
  2. Open the block catalog and navigate to Libraries > CFC Library > Blocks > CONVERT.
  3. Drag the block DW_BO (FC177) into the chart.
  4. Position the block in cyclic interrupt task OB35 (or the task matching your drive update rate) using the Run Sequence editor.
  5. Connect the input:
    - Interconnect parameter IN of DW_BO to the 32-bit peripheral input tag (e.g., PID 512 or the output of a Modbus read block).
  6. Interconnect the output bits:
    - Connect OUT00 (Ready for Operation) to technological drive input Permit.
    - Connect OUT01 (Running Forward) to MotL.FbkRun.
    - Connect OUT03 (Fault Present) to MotL.Trip.
    - Connect remaining unused bits to local chart connectors or leave unassigned.
  7. Save and compile the CFC chart.

Verification

To verify that double word conversion operates correctly:

  1. Switch the CFC editor to Test > Test Mode.
  2. Double-click the IN parameter of DW_BO (or write a test hexadecimal value to the source peripheral address):
    - Enter 16#00000001. Confirm that output OUT00 transitions to TRUE and all other outputs remain FALSE.
    - Enter 16#00000002. Confirm that output OUT01 transitions to TRUE.
    - Enter 16#80000000. Confirm that output OUT31 transitions to TRUE.
  3. In WinCC Runtime, open the equipment faceplate linked to the downstream block. Verify that motor status transitions from Stopped to Running when the corresponding bit is set.

Limitations and Operational Precautions

  • Atomic Execution: BO_DW and DW_BO evaluate all 32 bits simultaneously in a single CPU execution cycle. However, if individual input signals are interconnected from different cyclic tasks running at different frequencies (e.g., some from OB35 and others from OB32), the packed double word may reflect asynchronous values during transition cycles. Always pack signals within the same execution group.
  • Process Object Impact: Standard CFC elementary conversion blocks (FC176, FC177) do not consume PCS 7 Process Object (PO) licenses. They are lightweight system utility functions.
  • Avoid Nested Bit Conversions: Chaining multiple bit-to-byte, byte-to-word, and word-to-dword blocks clutters CFC charts and increases execution time. Use BO_DW directly to bridge booleans to double words in a single step.

Engineer FAQ

Can I use BO_DW to pack an operator permission mask for APL faceplates?
Yes. APL blocks feature parameters such as OS_Perm that evaluate 32-bit double words to grant or deny operator button access. BO_DW is the standard tool used to construct custom OS_Perm masks in CFC.

What happens if I leave unused input pins unassigned on BO_DW?
Unassigned pins default to FALSE (0). They do not generate compile warnings and resolve cleanly to 0 in the packed output double word.

Can I convert a DWORD to a 32-bit floating-point number (REAL) using standard blocks?
Yes. Use the type converter block DW_R (Double Word to Real) from the CFC Elementary Library to interpret the 32-bit bit pattern as an IEEE 754 float without mathematical transformation.

Technical References

  • Siemens Industry Online Support Entry ID 109742323: SIMATIC Process Control System PCS 7 CFC for S7 Manual
  • CFC Elementary Function Blocks Reference Guide (Document No. A5E00278782)
  • Siemens Industry Online Support Entry ID 109809264: APL Structure and Status Word Processing Guidelines

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