Fix STEP 7 'Connection to Target Module' Error on S7-300 DP Slave

1. Problem Description

When commissioning a PROFIBUS-DP link between an S7-400 CPU 417-4 (master) and an S7-300 CPU 315-2 DP (slave) following Siemens Application Note 6518822 ("DP link between CPU 315-2DP (slave) and S7-400 (master) with STEP 7 V5.3"), the following sequence is reported:

1. The hardware configuration of the slave station (CPU 315-2 DP) is created, compiled, and saved without errors.
2. The user connects the programming device (PG) to the MPI/DP port of the S7-400 CPU.
3. The user triggers PLC → Download to Target System → CPU 315-2 DP.
4. STEP 7 raises the dialog: "Online: The connect to target module cannot be established".
5. PLC → Display Accessible Nodes returns an empty LiveList, or shows only the master.

The same failure appears when adding a second slave (CPU 313C-2 DP) to the DP master system of the CPU 417-4.

2. Root Cause Analysis

The error is raised by STEP 7 when no online connection can be opened to the target CPU over the configured PG/PC interface. On a CPU 31x-2 DP that sits behind a DP master, three architectural details decide whether the download succeeds; any one of them being wrong reproduces the symptom.

2.1 Interface architecture mismatch (primary cause)

The two CPU families use physically different interface assignments, and the user must understand what is wired to what:

The CPU 417-4 is the critical detail. Its X1 port is a combined MPI/DP interface; if the project has X1 set to DP master, the CPU has no MPI surface to connect to. In that case the PG must reach the CPU over PROFIBUS, not MPI. Configuring the PG/PC interface for MPI while the CPU 417-4 is in DP-master mode is the most common reason the LiveList comes back empty.

2.2 Target CPU not in STOP

HW Config download is a memory-erasing operation on the target CPU. The target must be in STOP. According to the official TIA Portal V20 documentation:

"A CPU must be set to STOP mode for the load operation to rule out possible damage to equipment or personal injury."

Source: Siemens TIA Portal V20 — Downloading a configuration to a device. The same STOP requirement applies to STEP 7 V5.x HW Config download.

2.3 Routing not enabled on the DP slave

When the PG is connected to the DP side of the S7-400 (or to the MPI/DP port while the master is in DP mode), the download is forwarded through the S7-400 master to the slave. STEP 7 can only forward a frame if the DP slave station has the checkbox "Test, commissioning, routing" enabled in DP slave properties → Operating Mode. With this checkbox cleared, the slave is not reachable across the router, and STEP 7 returns the same "connection cannot be established" message.

3. Prerequisites

• STEP 7 V5.3 or later, or TIA Portal V13 SP1 or later.
• A PROFIBUS-capable PG interface: CP 5611, CP 5613 A2, CP 5711, or USB PC Adapter (6ES7972-0CB20-0XA0). Ethernet interfaces (CP 343-1, CP 443-1) are also acceptable with routing.
• PG/PC interface driver installed and tested in Set PG/PC Interface.
• PROFIBUS DP cable type A, 120 Ω terminating resistors enabled at both physical ends of the segment.
• The S7-400 master and the S7-300 slave(s) mounted, wired, and powered.
• STEP 7 authorization on the PG.
• The slave HW Config compiled and stored inside the same STEP 7 project as the master, or imported as a separate station.

4. Pre-Download Checklist

5. Step-by-Step Solution

5.1 Set the PG/PC interface to PROFIBUS

1. Open Start → SIMATIC → STEP 7 → Set PG/PC Interface.
2. Select the access point S7ONLINE (STEP 7) → <your CP> (PROFIBUS).
3. Click Properties → PROFIBUS: set the PG address to an unused value (e.g., 1). Address 0 is reserved for diagnostic services on some masters and should be avoided.
4. Click Properties → Bus Parameters: choose Universal (DP/FMS); confirm the baud rate matches the DP master (default 1.5 Mbps; the project uses the value defined in the master station's DP properties).
5. Click OK → OK to apply.

5.2 Place the slave CPU in STOP

1. Flip the mode selector of the CPU 315-2 DP to STOP.
2. Confirm the STOP LED is on and the RUN LED is off.
3. Repeat for any additional slave (CPU 313C-2 DP, etc.).

5.3 Enable routing on the DP slave

1. In SIMATIC Manager, open the S7-300 slave station and double-click Hardware.
2. Select the DP interface row of the CPU 315-2 DP (X2).
3. Open Object Properties → Operating Mode.
4. Tick "Test, commissioning, routing". Tick "DP slave" if not already set.
5. Click OK, then Station → Save and Compile.

5.4 Download the HW Config

1. With the slave station open in HW Config, click PLC → Download to Target System → CPU 315-2 DP.
2. In Select Target Module, the slave CPU should appear with its DP address and rack/slot.
3. If the CPU is not listed, click Display Accessible Nodes (the third toolbar button with the networked boxes and "i" icon). The LiveList will scan the bus through the S7-400 master.
4. Select the target CPU and click OK.
5. Confirm the STOP / START dialog (STOP is mandatory; warm restart after load is optional but recommended for a clean state).
6. Wait for the transfer to complete; the CPU restarts automatically if Restart after download is ticked.

6. Alternative Connection Methods

6.1 Direct connection to the slave's MPI port (X1)

When the S7-300 slave is wired standalone and not yet physically connected to the S7-400 segment, the PG can be connected directly to the MPI port (X1) of the CPU 315-2 DP. Set the PG/PC interface to MPI, set the PG address to a unique value, place the CPU in STOP, and run PLC → Download to Target System. This is the simplest path during initial commissioning and is the recommended first step for a brand-new CPU.

6.2 Ethernet download via S7 routing

If the S7-400 master is fitted with an Ethernet CP (e.g., CP 443-1, 6GK7443-1EX11-0XE0), the PG can connect over TCP/IP. STEP 7 then uses S7 routing to reach the DP slave. Configure an Ethernet PG/PC interface, enter the IP of the CP 443-1 as the gateway, and run PLC → Download to Target System; the frame is forwarded by the CPU 417-4 to the slave over PROFIBUS. The same Test, commissioning, routing checkbox on the slave is required.

6.3 Adding a second DP slave (CPU 313C-2 DP)

The CPU 313C-2 DP supports DP slave operation on its second interface in the same way as the CPU 315-2 DP. Procedure:

1. Add a new S7-300 station to the STEP 7 project.
2. Insert the CPU 313C-2 DP from the hardware catalog. Order numbers include 6ES7313-6CF00-0AB0 (firmware V2.6 or later) and 6ES7313-6BG00-0AB0.
3. In module properties, configure the DP port as DP slave, set the slave PROFIBUS address, and enable Test, commissioning, routing.
4. In the S7-400 master HW Config, drag the new CPU 313C-2 DP station to the DP master system.
5. Save, compile, and download the master HW Config to the CPU 417-4 first; then download the slave HW Config to the CPU 313C-2 DP using the procedure of Section 5.4.

7. Verification

1. Open PLC → Display Accessible Nodes. Both the CPU 417-4 and the CPU 315-2 DP (and the CPU 313C-2 DP, if installed) should appear with their DP addresses.
2. Open the slave HW Config and click PLC → Online → Module Information to confirm the configured slot and the actual slot match.
3. On the master HW Config, open the online view of the DP master system. The slave should be reported as OK.
4. Watch the slave CPU's SF / BF LEDs. A steady BF (Bus Fault) means a DP communication problem — usually a duplicate address or a baud-rate mismatch.
5. Toggle a few I/O points and verify them in the online monitor.

8. PROFIBUS DP Bus Parameter Reference

When the PG is connected to the segment, the bus parameters it uses must match the master segment, or STEP 7 will refuse to enumerate. The default values of a typical S7-400 / S7-300 DP master at 1.5 Mbps are:

If the PG/PC interface dialog reports "No bus parameters found", the most likely cause is that the master is not cyclic on the bus, or the PG address is the same as the master address.

9. LED Status Reference for the S7-300 DP Slave

A steady BF immediately after a download is the most common secondary symptom and is almost always caused by a duplicate address, a wrong baud rate, or the routing checkbox being off.

10. Troubleshooting Matrix

11. STEP 7 V5.x vs TIA Portal Equivalents

Engineers migrating a project from STEP 7 V5.x to TIA Portal will find the same physical network and the same slave CPU, but the menu paths differ. The following table maps the actions of this article to the TIA Portal V20 path.

12. Field-Proven Caveats

• The CPU 317-2 DP and CPU 319-3 PN/DP behave the same as the CPU 315-2 DP for the purposes of this procedure; the DP port is the second interface (X2 or X2/X3 depending on the variant).
• If the DP segment is longer than 100 m, verify cable type A and termination. 1.5 Mbps supports up to 200 m, 500 kbps up to 400 m, 187.5 kbps up to 1000 m. Repeaters extend this further but each repeater segment must be re-terminated.
• PG address 0 can collide with some master diagnostic blocks; use 1 or 2 to be safe.
• When migrating to TIA Portal, the same physical network is reused, but the slave must be re-imported from a V5.x archive, or re-inserted from the TIA hardware catalog.
• Online accessibility should be tested after every change. A successful master download does not imply a successful slave download.
• Some PROFIBUS connectors (e.g., 6ES7972-0BA12-0XA0) have an additional switch for the terminating resistor. Leaving the resistor enabled on a mid-segment connector is a frequent cause of intermittent BF faults.
• The mode selector position RUN-P allows the CPU to be placed in STOP by STEP 7 for an HW Config download. RUN does not.
• Some older CPU 31x-2 DP variants ship in DP master mode by default. Use the hardware catalog to switch the port to DP slave mode before the first download, otherwise the slave will refuse the master's parameter assignment.

13. Safety and Operational Notes

FAQ