Resolving WinAC RTX PROFIBUS-DP Communication Between Two PCs

Resolving WinAC RTX PROFIBUS-DP Communication Between Two PCs

Field engineers deploying WinAC RTX 2005 SP2 on two separate PCs (one equipped with a CP 5613 PCI card, the other with a CP 5611 A2) frequently hit a wall when attempting peer-to-peer PROFIBUS-DP data exchange through Step7 V5.4 SP3. The hardware can see each other on the wire, the WinLC RTX soft-PLC boots, and the CPs exchange diagnostic frames — yet NetPro shows "Partner not reachable," no S7 connection comes up, and the HW/NetPro download only commits the local station. This reference explains why the native DP-Slave path collapses in WinLC RTX, lays out the S7-Communication workaround on PROFIBUS, and provides a commissioning and verification matrix.

1. Problem Statement

The reference test bed consists of:

• PC 1: WinAC RTX 2005 SP2, CP 5613 (PCI PROFIBUS master card, order number 6GK1561-3AA01), Step7 V5.4 SP3.
• PC 2: WinAC RTX 2005 SP2, CP 5611 A2 (PCI/CPCI card, order number 6GK1561-1AA02), Step7 V5.4 SP3.
• Both stations wired into the same PROFIBUS-DP segment via NetPro.

Observed symptoms reported by the operator:

1. Step7 HW Configurator does not expose a dialog to map PROFIBUS I/O addresses for direct DP data exchange.
2. NetPro link status reports "Partner not reachable" even though the CP diagnostics tool can see both nodes on the bus.
3. Project download from PC 1 to PC 2 fails; only the local station portion of the project commits.
4. Switching to the standard Industrial Ethernet adapters of both PCs and configuring an S7 connection through the PC/PC interface succeeds — but the user requirement mandates a PROFIBUS link.

2. WinAC RTX 2005 Architecture Recap

WinAC RTX is a software PLC (WinLC RTX) running under the Venturcom RTX real-time extension of Windows. The architecture is:

• WinLC RTX – virtual CPU executing the STEP7 user program in a hard real-time thread.
• SIMATIC NET PC Software – provides the DP base services and CP driver interface.
• CP 5613 / CP 5611 A2 – physical PROFIBUS interface managed by the SIMATIC NET DP driver stack.
• Step7 V5.4 SP3 – engineering tool used for HW Config, NetPro, and program download.

The combination is described in the SIMATIC WinAC RTX 2005 Manual. WinLC RTX in its standard license behaves as a DP Class 1 master. DP-Slave operation is only available with the optional DP-Slave Add-On and a separate SIMATIC NET OPC DP-Slave configuration utility.

3. Root Cause — Why the Direct DP-Slave Path Fails

The HW Configurator dialog missing for PROFIBUS I/O addresses is the smoking gun. In a real S7-300/S7-400 station, Properties > PROFIBUS Interface > Operating Mode allows toggling between DP-Master and DP-Slave. In WinLC RTX, the PROFIBUS interface is owned by the SIMATIC NET CP driver and exposed to the soft-CPU as a slave-side slot — the operating-mode toggle is grayed out unless the DP-Slave add-on license and the corresponding SIMATIC NET OPC server variant are installed.

Consequences:

• No DP-Slave slot mapping in HW Config → no I/O address assignment for cyclic I/O transfer.
• No GSD-based slave description → NetPro cannot negotiate an S7 connection over the DP segment because the slave-side service access points (SAPs) are not bound.
• Download of NetPro from PC 1 only commits the local station because the partner station is unknown at the NetPro level (no reachable CPU signature).

4. Hardware Constraints — CP 5613 vs CP 5611 A2

For two-PC PROFIBUS coupling where both ends are soft-PLCs, the practical recommendation is to keep both CPs in DP-Master role and switch the data exchange mechanism from cyclic DP I/O to acyclic S7 Communication over the same PROFIBUS segment. S7 Communication does not require one side to be a DP-Slave; it only requires both stations to expose an S7 connection endpoint via the CP.

5. Solution Path 1 — S7 Communication over PROFIBUS via NetPro

S7 Communication works between two S7-400-compatible stations on a shared PROFIBUS segment using the S7-Connection service. Both ends must be configured in NetPro with the partner's PROFIBUS address, MPI/DP interface parameters, and connection resource (local TSAP / remote TSAP).

Advantages:

• No DP-Slave license required.
• Standard S7 instructions: PUT, GET (single-sided configurable) or BSEND/BRCV (both-sided configurable).
• Survives momentary bus glitches; connection supervision built into the CP firmware.

Limitations:

• PUT/GET payload limited to 160 bytes per call.
• BSEND/BRCV payload up to 32 KB but requires a both-sided configured connection.
• Connection establishment time on PROFIBUS at 1.5 Mbps is typically 2 s to 6 s; tune with the S7 connection watchdog.

6. Solution Path 2 — OPC DP-Slave with SIMATIC NET (Advanced)

If a true DP-Slave cyclic exchange is mandatory, deploy the SIMATIC NET OPC DP-Slave server on the second PC and configure the CP 5611 A2 to operate as a DP-Slave with a GSD file. The OPC server maps DP I/O to OPC tags, and a WinLC RTX OPC client block reads/writes the tags. This is supported by the SIMATIC NET OPC Server manual and is documented in the WinAC RTX DP-Slave application note.

Steps outline:

1. Install SIMATIC NET PC Software >= V6.4 SP1 with the OPC DP-Slave option on PC 2.
2. Configure CP 5611 A2 in the PC station as a DP-Slave, import the GSD for the WinLC DP-Slave profile.
3. Bind DP-Slave slots to OPC tags via Station Configuration Editor.
4. In WinLC RTX on PC 2, use OPC client FB (e.g., the SFB "OPC" wrapper from the WinAC samples) to expose tags to the user program.
5. On PC 1, configure CP 5613 as DP-Master and slot-map the WinLC DP-Slave on PC 2.

This path is heavier in licensing and configuration; only use it when cyclic I/O timing is required.

7. Step-by-Step — S7 Connection Commissioning on PROFIBUS

The remainder of this article focuses on Path 1 (S7 Communication), the most common remediation in the field.

7.1 Prerequisites

• Step7 V5.4 SP3 installed on both PCs.
• SIMATIC NET PC Software installed and "Set PG/PC Interface" configured with the appropriate CP access point (S7ONLINE (STEP7) -> CP5613.PROFIBUS.1 on PC 1, equivalent on PC 2).
• Both WinLC RTX stations booted; CP LEDs green and showing active token on PROFIBUS.
• A single Step7 project with two PC stations added in NetPro — not two separate projects.

7.2 Hardware Configuration

1. Open the project in Step7 V5.4 SP3.
2. Insert a SIMATIC PC Station for each physical PC. Use the HW Catalog entry SIMATIC PC Station > CP 5613 for PC 1 and SIMATIC PC Station > CP 5611 A2 for PC 2.
3. Insert a WinLC RTX (V3.x for RTX 2005) into each PC station, slot 2.
4. Set the PROFIBUS address on each CP: PC 1 = 2, PC 2 = 3 (avoid master address 1 and 2 for slaves where possible).
5. Set the bus profile (Transmission Speed) identically on both stations: 1.5 Mbps is a safe starting point. Verify the highest supported speed by both CPs.

7.3 NetPro Configuration

1. Switch to NetPro (click the NetPro icon in HW Config or via Options > NetPro).
2. Right-click the CP 5613 of PC 1 and choose Insert New Connection.
3. Select the partner as the WinLC RTX of PC 2, type S7 Connection.
4. Leave both sides configured (default). Set Active connection establishment ON at PC 1, OFF at PC 2 — this allows the connection to come up if only PC 1's project is downloaded; both-sided download still required for full functionality.
5. Verify the connection resource (TSAP) pair: local PC 1 = 01.01, remote PC 2 = 03.01 (the 03 reflects the PROFIBUS station address of the partner CP; WinLC RTX slot 1).
6. Save and compile NetPro.

7.4 Download Sequence

This is the most error-prone part in WinAC RTX deployments. The recommended sequence is:

1. Download HW Config + NetPro of PC 1 to PC 1 (target = CP5613.PROFIBUS.1).
2. Download HW Config + NetPro of PC 2 to PC 2 (target = CP5611A2.PROFIBUS.1).
3. Download the Step7 user program to each WinLC RTX using the PC Internal (MPI/DP) or CP5613/CP5611A2 access point, depending on what is licensed for programming access.

8. Program-Side — PUT/GET Implementation

Minimal STL sample for PC 1 (active partner) sending 20 bytes from DB10 to DB20 on PC 2:

// One-shot trigger (positive edge on M0.0)
AN    M 0.0
=     L 0.0
A     M 0.0
L     S5T#2S            // retry timer
SD    T 1
NOP   0
NOP   0
A     T 1
JCN   NO1

CALL  "PUT" (FC / SFB15 style)
     REQ   := L0.0
     ID    := W#16#0001            // connection ID from NetPro
     DONE  := M10.0
     ERROR := M10.1
     STATUS:= MW12
     ADDR_1:= P#DB20.DBX0.0 BYTE20 // remote DB (PC 2)
     SD_1  := P#DB10.DBX0.0 BYTE20 // local  DB (PC 1)
NO1: NOP 0

PC 2 side runs GET (SFB16) to mirror PC 1's DB10 into its own DB10, or alternatively use USEND/URCV with the same connection if the direction requires it. For >160 byte blocks, replace PUT/GET with BSEND/BRCV and ensure the connection is both-sided configured with the larger TSAP resource.

9. Diagnostic Tools

• CP Diagnosis (Start > SIMATIC > SIMATIC NET > CP Diagnosis) — shows bus nodes, addresses, retries, and lost frames per node.
• NetPro > Connection Status — online view of S7 connection state (established / not established / aborted with cause).
• WinLC RTX Diagnostic — integrated in Step7 via PLC > Module Information; useful for CPU status, but PROFIBUS layer is owned by the CP.
• SIMATIC NET Commissioning Tool — for diagnosing OPC DP-Slave setup if Path 2 is used.

10. Troubleshooting Matrix

11. Verification Procedure

1. Physical layer: Both CP PROFIBUS LEDs green; CP Diagnosis lists both addresses.
2. Logical layer: In NetPro, right-click the S7 connection > Connection Status shows "Established."
3. Data layer: Trigger a PUT/GET with a known pattern (e.g., incrementing counter in DB). Monitor both DBs in two separate Step7 online sessions; values mirror each other within 200 ms–500 ms at 1.5 Mbps.
4. Soak test: Leave the link up for 24 h under load and verify that STATUS counters in CP Diagnosis show no retries > 3% of total frames.
5. Failure mode test: Disconnect one station from the bus. Within the configured watchdog time (default 30 s), the connection should transition to "Aborted" with reason; reconnection should happen automatically once the bus returns.

12. Field-Proven Caveats

• Step7 V5.4 SP3 + WinAC RTX 2005 SP2 requires the RTX 5.x service to be running; check Control Panel > RTX Runtime > State = Started. If RTX is stopped, WinLC RTX boots but cannot drive the CP.
• The CP 5611 A2 is a legacy card with limited firmware update options. If CP Diagnosis is unstable, replace it with a CP 5622 (PCIe) for a more robust setup; see Siemens CP selection guide.
• PROFIBUS at 12 Mbps over a long cable path with two soft-PLCs frequently runs into retry storms; defaulting to 1.5 Mbps or 6 Mbps is recommended.
• S7 Connection over PROFIBUS uses one of the CP's connection resources. CP 5613 supports up to 50 connections; CP 5611 A2 supports up to 50 as well, but eight concurrent BSEND/BRCV pairs at full payload is the practical limit.
• When migrating to TIA Portal in the future, the same hardware is still supported via PC station with WinLC RTX; the S7 connection setup moves to Devices & Networks > Networks view > S7 Connection.

13. Standards and References

• IEC 61158 / IEC 61784 – PROFIBUS DP and PROFIBUS PA profile definitions. IEC web site.
• PROFIBUS Nutzerorganisation e.V. – installation guidelines for RS485 segments, termination, and repeater rules. PI web site.
• SIMATIC WinAC RTX 2005 Manual – see Siemens Support entry 21139608.
• SIMATIC NET OPC Server – see Siemens Support entry 27013510.