Siemens WF723A FM Module: S7-400 Adapter DB and COM723 Reference

Overview

The Siemens WF723A (order number 6FM1723-3AA10) is an FM-class positioning module belonging to the Equipment for Special Machines family, designated WF 721 / WF 723 A. The module is designed to be plugged into a SIMATIC S7-400 FM adapter so that the S7-400 CPU can exchange process data, setpoints, and diagnostic information with the positioning firmware running on the WF module. The sibling part 6FM1721-3AA10 is the WF 721 A variant of the same family.

This reference consolidates the publicly documented module characteristics from the WF 721 / WF 723 A Positioning Equipment manual (PDF, 6FM1 721-3AA10) and combines them with the practical field context: configuring the module with COM723, mapping its data to the S7-400 via instance data blocks (DBs), and diagnosing Axis C homing anomalies that surface when the offline project lacks comments and the original programmer is no longer available.

Module Identification and Ordering Data

Before any debugging, verify the front plate of the physical module reads 6FM1723-3AA10. The -3AA10 suffix is the hardware release and must match the manual revision you are working from. Mixed revisions between the firmware on the module and the COM723 project can produce silent rejection of the download or, more commonly, an Axis C homing sequence that terminates at the wrong zero-mark relationship.

Hardware Architecture in the S7-400 Adapter

The WF 723 A is not a standard S7-400 signal module (SM) and is not a CPU. It is a function module (FM) intended to be slotted into an S7-400 FM adapter. The adapter presents the module to the S7-400 backplane as if it were a regular FM, which means:

• The CPU accesses the WF 723 A through standard SFC / SFB calls used for FMs (SFC 58 / SFC 59 read/write record, SFC 7 / SFC 13 event / alarm handling).
• Process data is mirrored into instance data blocks in the S7-400 user program. The DB numbers, offsets, and meanings of those data blocks are defined by the COM723 offline project.
• Configuration is downloaded from the PC running COM723 to the WF module, not directly into the CPU. The CPU is then handed the matching instance DBs to consume the cyclic data.

Programmable I/O per Axis

According to the WF 721 / WF 723 A manual, the module provides six programmable inputs and six programmable outputs per axis. When an incremental encoder is connected, one of those six inputs is consumed by the encoder interface for that axis, leaving five free inputs and six outputs for application use. The same rule applies symmetrically to Axes A, B, and C (and the additional axes supported by the WF 723 A variant).

This matters when a reported Axis C fault is in fact an I/O shortage. For example, if a field technician re-routed a reference cam signal into one of the encoder input pins to "free up" a digital input, the encoder channel will be silently corrupted, and homing will look as if Axis C is misbehaving while in reality the encoder is not being read at all.

COM723 Configuration Software

COM723 is the dedicated offline / online tool for the WF 721 / WF 723 A family. It is the only utility that can:

1. Edit axis machine data (encoder type, gear ratio, reference cam polarity, homing velocity, homing direction, zero-mark offset).
2. Compile the project into a binary download that is pushed into the WF module's flash.
3. Generate the S7-400 instance DBs (with comments when the source project contains them) that the CPU program must use to read process data and write setpoints.
4. Capture the online trace of the homing sequence, including cam events, zero-mark events, and module-internal state transitions.

COM723 was distributed as a separate installable package and is not part of STEP 7 classic. It is typically requested from Siemens Industrial Technical Support in the country of purchase; legacy installations sometimes still carry the disks and a hardware key. If your only source is a downloaded archive, the project files inside will have stripped comments because they were not produced by a developer who actively edited them — which is the exact condition described in the originating field case.

Data Block (DB) Layout in the S7-400

The WF 723 A communicates with the CPU through one or more instance DBs. The exact DB number, the start offset of Axis C, and the bit meanings are project-specific and only fully defined by the COM723 project that was downloaded to the module. The WF 721 / WF 723 A manual defines the canonical structure that COM723 maps onto the user DBs. The conventional layout follows the pattern below; verify every offset against your specific COM723 export before commissioning.

The offsets above are representative of the WF 723 A family and must be reconciled with the COM723 export. If your downloaded project lost its comments, open the matching .db source in STEP 7 and rebuild the symbolic table from the manual's data interface table.

Homing Sequence on Axis C

Axis C homing in the WF 723 A family follows a deterministic state machine. The module's firmware walks Axis C through the sequence below; observing the live STAT_AX_C byte in the S7-400 instance DB is the only reliable way to pinpoint where it stalls.

1. Enable. CPU sets CTRL_AX_C.Bit0 = 1 (drive enable). Module acknowledges by setting STAT_AX_C.Bit0 = 1.
2. Reference cam search. CPU sets CTRL_AX_C.Bit2 = 1 (start homing). Module drives Axis C in the configured homing direction at homing velocity (machine data MD-C.04x) until the reference-cam input transitions.
3. Cam exit / direction reversal. Once the cam is found, Axis C continues to the next encoder zero mark, then reverses direction by a configured overrun distance and re-approaches the cam for precise referencing.
4. Zero-mark capture. Module captures the encoder zero mark and offsets it by the configured home position (machine data MD-C.05x).
5. Referenced. Module sets STAT_AX_C.Bit0 = 1 (referenced) and clears STAT_AX_C.Bit2 (homing complete). Axis C is now in a known absolute coordinate frame.

Axis C Homing — Diagnosis Flowchart

When Axis C does not complete the homing sequence, walk this tree in order. Each branch corresponds to a measurable condition in the live DB or in the COM723 machine data.

1. Check STAT_AX_C.Bit0. If it never becomes 1, drive enable is not reaching the module. Verify the enable wiring on the FM adapter and the control bit in the user program.
2. Check STAT_AX_C.Bit2. If it never goes high after CTRL_AX_C.Bit2 is set, the module rejected the homing command. Inspect the COM723 machine data for a missing or zero homing velocity.
3. Watch the reference-cam input. Use a scope or COM723 trace on the cam input. A floating, bouncing, or polarity-inverted cam signal will cause the module to "see" a cam event at startup, skip the cam-search phase, and look for a zero mark immediately, often at the wrong shaft position.
4. Verify incremental encoder wiring. The WF 723 A consumes one of the six digital inputs per axis when an encoder is connected. If the encoder channel was inadvertently re-routed to a digital function in the project, Axis C will never see a zero mark.
5. Compare Axis A / B behaviour. If only Axis C fails, the fault is in the Axis C branch (machine data, encoder cable, or servo amplifier on that axis). If all axes fail, the fault is in the global project (download integrity, adapter addressing, or COM723 version mismatch).
6. Check Axis C machine data integrity. In COM723, export the project and inspect the Axis C machine data block. Look for zero, default, or out-of-range values in the homing velocity, homing direction, reference-cam polarity, and zero-mark offset.

Reconstructing the DB When the Original Project Is Lost

This is the situation the source field case describes: the original system integrator is no longer available, the COM723 project on disk has no comments, and the S7-400 user program references DBs that are otherwise opaque. Use the following procedure to recover the DB layout.

1. Open the surviving COM723 project. Even if the comments are stripped, the binary download file inside it is valid.
2. From COM723, export the DB interface report (the tool produces a structured text or CSV listing of every DB byte and bit that is exposed to the CPU). This report is the authoritative map of the instance DB.
3. In STEP 7, open the S7-400 program and cross-reference the DB numbers in the user code. Replace the missing DB symbols with the names from the COM723 report.
4. If the COM723 project is also lost, reconstruct the DB layout from the data interface table in the WF 721 / WF 723 A manual. The manual defines the canonical structure; COM723 only reorders and re-symbols the bytes according to the project.
5. Re-download the COM723 project to the WF module, then reload the S7-400 user program. Verify that the live STAT_AX_C byte transitions match the expected state machine before re-attempting homing.

Troubleshooting Matrix

Manual and Documentation Sources

The primary public reference for the WF 723 A family is the legacy PDF manual indexed in the Siemens support attachments area:

• WF 721 / WF 723 A Positioning Equipment — Equipment for Special Machines (PDF, manual ID 39820631)

Additional sources to consult, in order of preference:

1. The COM723 online help — the help file is bundled with the COM723 install and contains the parameter-by-parameter description of every machine data word.
2. Siemens SiePortal — search the order number 6FM1723-3AA10. SiePortal may surface application notes and firmware readme files that are not in the public attachments area.
3. Your local Siemens Industrial Technical Support office. They can issue COM723 installation media and access the legacy application database for older module families.

Field-Commissioning Checklist

1. Verify the front-plate order number reads 6FM1723-3AA10 and matches the COM723 project.
2. Confirm the COM723 version supports the firmware revision on the module.
3. Download the COM723 project to the WF module and observe a successful checksum acknowledgement.
4. Re-load the S7-400 user program and verify the instance DB numbers match the COM723 export.
5. Bring up each axis in jog mode, confirm direction, then run a controlled homing on Axis A, B, and finally C.
6. Capture a COM723 trace of the Axis C homing sequence and archive it with the project.

FAQ