Configuring SIMOTION D425 as PROFIBUS DP Slave via GSD

Overview

The SIMOTION D425 motion controller integrates a SIMOTION runtime with an S7-300-class CPU on a single platform and exposes a PROFIBUS DP interface (X136/P1, X136/P2) that can be configured as either a DP master or a DP slave. When a third-party PLC must act as the PROFIBUS master and the SIMOTION project engineer does not have access to that PLC's STEP 7 project, the only viable integration path is a GSD-based slave integration using the device master file SI0280AB.GSD. This reference covers that path end-to-end and documents the two most common commissioning failures — slot ordering on the slave side and the consistency attribute mismatch — both of which prevent DP datagram exchange even when the bus comes up without errors.

Three integration modes exist in the SIMOTION ecosystem:

1. Intelligent slave / I-slave — SIMOTION D425 is configured inside the same Scout project as a station-uploaded slave. The DP master and DP slave are programmed by the same engineer in Scout and STEP 7.
2. GSD-based slave — The DP master PLC is programmed by a third party. SIMOTION is inserted into the master project only via the GSD file SI0280AB.GSD. The SIMOTION engineer works only in Scout; the PLC engineer works only in STEP 7 / TIA Portal.
3. Direct cross-project coupling — Both projects are merged on the same engineering station. Rare in practice and not used when the master PLC is from another vendor or contractor.

Mode 2 is the focus of this article.

Prerequisites

Architecture and Topology

The GSD-based slave topology is asymmetric: two independent engineering stations, one shared PROFIBUS segment.

• Master side (PLC contractor): STEP 7 / TIA Portal project, SI0280AB.GSD installed, D425 dragged into the DP master system, slots filled with I/O modules.
• Slave side (SIMOTION engineer): Scout project, D425 hardware configured, DP1 interface set to DP Slave, process image partitions defined for the I/O slots.
• Physical layer: Shielded twisted pair, characteristic impedance 150 Ω, 220 Ω termination at both ends, baud rate selected to match the slowest device on the segment.

Baud-rate selection rule: at 1.5 Mbit/s the maximum stub length is 0.3 m and the maximum segment length is 200 m; at 12 Mbit/s the maximum segment length is 100 m and stubs must be eliminated. The D425 has no on-board repeater, so if the line is longer than the standard permits the bus must be split with a PROFIBUS repeater (e.g., 6GK1500-3AA10).

SIMOTION Scout Project Configuration

Before touching the PLC master, the SIMOTION D425 must be fully set up as a DP slave in Scout. The D425 hardware configuration determines which slots the master will see, and any change later requires a Scout Save & Compile and a hardware download to the D425.

1. In Scout, open the D425 project and double-click the device to enter HW Config.
2. On the D425 rack, locate the integrated DP1 interface (X136) and open its properties.
3. On the Operating Mode tab, select DP Slave. The interface will then expose a slot table that the master will populate via the GSD.
4. Open the slot editor and insert the I/O modules in the order they will appear on the master. For a 16-word I/O configuration, insert:
   • Slot 0: Empty (always required by the D425 as a placeholder)
   • Slot 1: 16-word input module (mapped to a process image of the SIMOTION I/O area)
   • Slot 2: 16-word output module
5. Assign each slot a symbolic name (e.g., ibDPSlaveIn, qbDPSlaveOut) and bind it to a process image partition if the application uses isochronous mode.
6. Set the DP slave address on the General tab. The address must match the address the PLC engineer will assign in HW Config of the master.
7. Save and compile. The D425 now has a compiled slave configuration but the bus will not run until HW Config is downloaded to the controller.

STEP 7 Master Configuration

The PLC engineer performs these steps in STEP 7 HW Config (S7-300/400) or TIA Portal (S7-1500). The official Siemens procedure for adding DP slaves to a master system is documented in the TIA Portal help under Adding DP slaves to the master system and configuring them (S7-300/S7-400/S7-1500).

1. Install the GSD: in STEP 7 V5.x, run Options → Install GSD File and select SI0280AB.GSD. In TIA Portal, use Options → Manage general station description files (GSD).
2. Open the DP master system in HW Config. The catalog now lists the D425 under PROFIBUS DP → Additional Field Devices → Drive → SIMOTION (exact path depends on GSD version).
3. Drag the D425 onto the master system. Assign a PROFIBUS address (default 3) and confirm.
4. Open the D425 slot table on the master side and insert modules in the same order as the Scout configuration. For a 16-word I/O configuration:
   • Slot 0: Empty (matches the Scout placeholder)
   • Slot 1: 16-word input word module
   • Slot 2: 16-word output word module
5. Double-click each slot and set the Consistency attribute. For a 16-word module select Total length (also called "All" in some TIA Portal versions) so that the entire 32-byte payload is transferred atomically.
6. Save and compile HW Config, then download to the PLC.

The Slot-Order Rule (Most Common Failure)

This is the root cause of approximately 70% of GSD-based SIMOTION slave commissioning failures. The PROFIBUS DP configuration telegram contains identifier bytes that specify the direction (input or output) and the length of each slot. The D425 firmware expects the slot order in the master configuration telegram to match the slot order declared in its own Scout configuration byte-for-byte, including the input/output direction of slot 1.

Scenario A — does not work:

• Scout slot 1: 16 input words (E, identifier 0x10 / 0x11 type)
• Master slot 1: First module is an output word (A, identifier 0x20 / 0x21 type)
• Result: D425 enters data exchange for one cycle, then drops to "parameterization fault" and the SF LED on the PLC and BF LED on the D425 both light steady.

Scenario B — works:

• Scout slot 1: 16 input words (E)
• Master slot 1: First module is an input word (E)
• Result: Bus start-up succeeds, cyclic data exchange begins within 1–2 seconds.

When the source engineer observed that "the first slot I configured in the DP1 interface of D425 is E (16 input word) and I have to insert as first slot at the PLC side a Master A Slave E slot" he had already identified the rule from the bus fault diagnostics. The fix is to reorder the slots on the master side so the first non-empty data slot matches the first data slot on the slave side, or to reorder the slots on the slave side and re-download HW Config to the D425.

The Consistency Attribute

The consistency attribute defines whether the S7 firmware guarantees that an entire slot's data is transferred atomically across the backplane bus to the application. Three values are available in STEP 7 / TIA Portal:

If the consistency setting on the master and slave sides differs, the D425 logs a configuration mismatch in its diagnostic buffer and the bus enters "clear" state. The Scout slot properties and the STEP 7 slot properties must match exactly. For a 16-word (32-byte) slot, set both to Total length.

Hardware Config Download to the D425

A common mistake is to download the Scout project to the SIMOTION runtime but not the HW Config to the D425 CPU portion. The two downloads are separate operations:

• Target system → Download → To target device → Hardware — pushes the rack and DP slave configuration to the D425 CPU. Requires the D425 to be in STOP or the download to be issued with "Continue" mode.
• Target system → Download → To target device → Software (or "Project") — pushes the SIMOTION program and configuration data to the runtime.

If the HW Config is not downloaded, the D425 boots with a default empty DP configuration, the master's check-config fails, and the slave does not enter data exchange. The Scout message window shows "Configuration has not been downloaded to the target" on the relevant hardware item.

Verification Procedure

1. Bus start-up: With both the PLC and D425 powered, the BF (bus fault) LED on the D425 should extinguish within 2 seconds. If the BF LED blinks at 2 Hz, the D425 is receiving the parameterization telegram but rejecting it (slot order or consistency fault).
2. Master diagnostic buffer: In STEP 7, open PLC → Module Information → Diagnostic Buffer on the DP master. A successful start-up shows "DP slave entered data exchange". A failure shows "Configuration error" with a slot index and the offending identifier byte.
3. Slave diagnostic buffer: In Scout, open Target system → Diagnostics → Diagnostic Buffer on the D425. Look for entries of class Configuration error (event ID 0x150) with the slot number in the parameter.
4. Online I/O test: In Scout's I/O monitor, force a value on a slave input bit and read the corresponding byte in the STEP 7 variable table on the master side. Force a value in the STEP 7 variable table and verify it on the D425 I/O monitor.
5. PROFIBUS diagnostics: Use a PROFIBUS analyzer (e.g., Softing PROFIusb or Intrepid ValueCAN 4 PROFIBUS) to capture the start-up sequence: parameterization, check-config, and data-exchange telegrams. The check-config response should be a positive acknowledgment (byte 0xFF) before the master switches to data exchange.

Common Fault Codes and Diagnostics

Troubleshooting Matrix

Alternative Configuration: Intelligent Slave / I-Slave

If the third-party PLC contractor is willing to share the STEP 7 project or the engineering is done on a common station, the intelligent slave / I-slave approach is more robust because both projects share a common configuration source. The D425 is added to the STEP 7 master project as a station-uploaded slave (i.e., the STEP 7 project includes a symbolic representation of the D425 I/O image), and the Scout project is downloaded separately. Slot order, consistency, and identifier bytes are then defined once in the shared project and cannot diverge. This approach is documented in the SIMOTION Communication manual and removes the most common failure mode described in this article.

Edge Cases and Field-Proven Caveats

• Empty slot 0: The D425 requires slot 0 to be empty (module type "none"). If the master inserts a module in slot 0, the D425 rejects the configuration.
• Station status byte: Many S7 master projects disable the station status byte (SSB) by default. SIMOTION slaves expect SSB = 1 in the parameterization telegram. Set this in the DP master properties under Bus parameters → Profile or in the slot 0 properties.
• Watchdog: A DP watchdog shorter than 100 ms causes spurious drop-outs on a D425 booting from a flash card. Use 1 s as the default and tighten only after the application is stable.
• Hot restart of the D425: When the D425 reboots, the master logs a station failure (OB86) and the inputs freeze. Programs that use SFC14/SFC15 to read DP data must handle the busy/return code of W#16#80A0 (DP error) until the master restores the data exchange.
• Isochronous mode: If the application requires isochronous PROFIBUS, the Equidistance checkbox must be enabled on both sides and the Ti/To times must match. The D425 supports equidistant DP only on the integrated interface (X136), not on the optional PROFIBUS extension module.
• GSD file revision drift: SI0280AB.GSD ships with each Scout release. Using a GSD from Scout V4.4 against a D425 running V4.5 firmware is permitted; using a GSD from Scout V5.4 against a D425 running V4.2 firmware is not. Match GSD revision to firmware, not to Scout version.

Safety and Commissioning Notes

Before any field deployment, perform a documented FAT (factory acceptance test) covering: bus start-up under power-on, bus start-up under hot-restart of the D425, bus start-up under hot-restart of the PLC, response to cable break, response to D425 power loss, and response to PLC stop. Each test must record the time to re-enter data exchange, the value of OB86 / OB82, and the value of the diagnostic buffer on both sides.

Frequently Asked Questions