Resolving STARTER Connection Failure After S7-300 CPU 319 PN/DP

1. Problem Summary

A Siemens S7-300 station running a CPU 319-3 PN/DP was migrated from MLFB 6ES7318-3EL00-0AB0 (firmware V2.8) to MLFB 6ES7318-3EL01-0AB0 (firmware V3.2). After the swap, HW Config was updated in SIMATIC Manager and the project re-downloaded to the rack. Subsequent attempts to go online with STARTER (the SINAMICS / SIMODRIVE commissioning tool) against a PROFINET-connected drive fail, even though the previous project file (now opened against the V3.2 CPU on the rack) connects successfully without changes.

Diagnostics reveal that NetPro shows "no PC assigned" for the S7 connection in both the old and the new project, which on its own would not normally be the blocking fault. The combination of (a) a new CPU MLFB, (b) PROFINET interface renumbering in NetPro, and (c) a missing PG/PC assignment on the routed S7 connection is what actually breaks the routing path between STARTER and the drive.

2. Affected Hardware and Firmware Mapping

The two MLFBs are mechanically and electrically compatible, but they differ in firmware features and in the way NetPro represents the PROFINET interfaces. That is the most common root cause of connection failures after the swap.

Field rule: Whenever STEP 7 reports the slot of the CPU differently between the old and the new project, NetPro will mark every S7 connection that terminates on the CPU as "connection to partner not assigned" or "no PC assigned", which is exactly the symptom in the source case.

3. Root Cause Analysis

STARTER can reach a SINAMICS drive on PROFINET by two paths:

1. Direct Ethernet path — STARTER talks to the drive's IP address over the same IP subnet. This does not require any S7 routing.
2. Routed S7 path — STARTER talks to the CPU 319-3 PN/DP using the S7 protocol, and the CPU forwards the request to the drive through its PROFINET IO controller. This is the standard path when the drive is on a private PROFINET segment behind the CPU.

When STARTER is configured for the routed S7 path, it relies on the S7 connection defined in NetPro. The CPU swap corrupts that mapping in three places:

The user report says the previous project file connects even with the V3.2 CPU on the rack. This is a strong indicator that the project artifact (NetPro PG/PC binding) is the culprit, not the rack hardware: the old project's S7 connection still points to the original interface index that the V3.2 CPU happens to expose in a compatible way, while the newly-updated project's S7 connection was re-bound to a new (or differently-numbered) interface index that STARTER cannot route through.

4. Network and Routing Topology

Figure 1 — STARTER-to-drive routing path through the S7-300 CPU 319-3 PN/DP. The PG/PC station must be assigned to the S7 connection in NetPro for STARTER's S7ONLINE path to traverse the CPU.

5. STEP 7 NetPro: Re-binding the PG/PC Station

The PG/PC station is a logical object in NetPro that represents the engineering workstation. It is the source end point of every S7 connection that STARTER uses for online access. If the object is not assigned to a physical network interface, the S7 connection has no end point on the PG side and STARTER cannot bind to it.

5.1 Procedure (STEP 7 V5.x / SIMATIC Manager)

1. Open the project in SIMATIC Manager and launch NetPro (Options → NetPro).
2. In the network view, find the PG/PC station. If none exists, insert it: Insert → PG/PC Station. Give it the same name as the Windows computer or use the default name.
3. Right-click the PG/PC station and choose Assign PG/PC.
4. In the dialog, set the Interface field to the industrial Ethernet adapter that physically connects to the CPU 319-3 PN/DP port (typically the X1 port of the CPU, subnet "PN/IO-1").
5. In the Assignment column, mark the subnet that contains the CPU 319-3 PN/DP. Click OK.
6. Open the S7 connection that STARTER uses (usually named "STARTER_online" or the connection associated with the SINAMICS drive). Verify that the Local end point is the PG/PC station and the Partner end point is the CPU 319-3 PN/DP.
7. If the partner end point shows "---" or is flagged in red, double-click the connection and re-select the CPU 319-3 PN/DP from the drop-down. Save and recompile NetPro (Network → Save and Compile).
8. Download the NetPro configuration to the CPU (PLC → Download to Station). Accept any warnings about connection aborts; STARTER must be offline during the download.

6. STARTER Access Point Configuration

NetPro binds the connection, but STARTER's runtime access point must point at the same S7ONLINE interface. Mismatch between the two is the second most common cause of "online disconnected" after a CPU swap.

1. Close STARTER.
2. Open Windows Set PG/PC Interface (Start → SIMATIC → STEP 7 → Set PG/PC Interface, or Control Panel → Set PG/PC Interface).
3. Select the access point S7ONLINE (STEP 7) in the left list.
4. In the right pane, choose the TCP/IP interface that is bound to the Ethernet adapter physically connected to the CPU 319-3 PN/DP. Do not select the softbus or any PROFIBUS entry unless the routing is intentionally via DP.
5. Click OK. If Windows prompts to confirm, accept.
6. Launch STARTER, open the drive project, and choose Online → Connect to target system. STARTER should now report the CPU and drive online.

7. Hardware Configuration Audit

After the CPU swap, the new PROFINET interface index in HW Config must match the index that NetPro binds the S7 connection to. Run this audit before touching NetPro again.

1. Open HW Config and double-click the CPU 319-3 PN/DP.
2. On the Properties → PROFINET Interface tab, record:
   • IP address and subnet mask
   • PROFINET device name of the CPU
   • Port assignment (X1 / X2 / X3)
3. Open the IO devices tree. The SINAMICS drive must be listed as an IO device under the CPU's PROFINET interface. If it is missing, drag it from the catalog and re-assign its device name and IP.
4. For the drive, right-click → PROFINET device name → Assign name. The name must match the name that STARTER's project file references (e.g. sinamics-1).
5. Save and compile HW Config (Station → Save and Compile).

8. Drive-Side PROFINET Name Resolution

Even with a perfect S7 connection, STARTER will refuse to come online if the drive's PROFINET device name does not match. This is the single most-overlooked item after a CPU change, because the topology of the IO controller has been rebuilt.

9. Firmware Migration V2.8 → V3.2 Considerations

The V2.8 to V3.2 jump on the CPU 319-3 PN/DP is not a transparent change. Siemens republished the MLFB (6ES7318-3EL01-0AB0) for V3.x with internal changes that affect the way STEP 7 represents the PROFINET stack. The two issues that most often surface after migration are:

1. HSP required: STEP 7 V5.4 and earlier do not know the V3.2 firmware. Install the matching Hardware Support Package (HSP) from the Siemens HSP collection before opening the updated HW Config. Without it, STEP 7 may silently substitute a placeholder module and NetPro will generate broken S7 connections.
2. PN interface renumbering: V3.x exposes PROFINET interfaces with object identifiers that the S7 routing table treats as new. Existing S7 connections in the old project continue to work because they were bound to the old object identifier; the new project rebinds to the new identifier. Cross-loading a connection object from the old project to the new one is therefore not equivalent and is the root cause of the user's symptom.

10. Diagnostic Flowchart

Figure 2 — Decision path for STARTER connection failures after a CPU 319-3 PN/DP swap. The order matters: fix the access point first, then NetPro, then the drive name, then the firmware / HSP.

11. Verification Procedure

1. Accessible nodes check: In SIMATIC Manager, PLC → Accessible Nodes. Confirm the CPU 319-3 appears with its PROFINET IP. If it does, the S7ONLINE access point is correct.
2. NetPro compile check: In NetPro, Network → Save and Compile. The log must report zero errors and zero warnings about unbound S7 connections. Any warning on the STARTER-related connection must be cleared.
3. CPU diagnostic buffer: In SIMATIC Manager, PLC → Diagnostics/Settings → Diagnostic Buffer. Confirm no "connection aborted" or "routing error" entries since the last NetPro download.
4. STARTER accessible nodes: In STARTER, Project → Accessible nodes. The drive must appear with its PROFINET name and a green online indicator.
5. Online connect: Online → Connect to target system. STARTER must transition to the online view and read the drive parameters. If a fault persists, the diagnostic buffer of the drive (accessible via STARTER → Diagnostics) will identify whether the issue is at the PROFINET layer or at the S7 routing layer.
6. Project download test: Make a small parameter change in STARTER, save, and load to the drive (PG ↔ drive). A successful download confirms the full S7 route is healthy.

12. Edge Cases and Field-Proven Caveats

• Implicit vs explicit PG/PC assignment. Older STEP 7 projects sometimes compile successfully with "no PC assigned" because the runtime still finds a usable S7 route through the implicit default. The new project, with its rebinding, does not have that implicit default and the fault surfaces for the first time. Re-assigning the PG/PC explicitly is therefore a best practice after any CPU MLFB change.
• Subnets and routers. If the new CPU's PROFINET port sits on a different IP subnet from the engineering PG, the S7ONLINE path will ARP across the wrong subnet. Either re-IP the CPU to match the previous subnet, or add a static route on the engineering PC.
• Multiple PROFINET ports. The CPU 319-3 PN/DP exposes more than one PROFINET interface object. STARTER's S7 connection must be bound to the port that has a physical link to the drive, not to the port that faces the engineering network. Mis-binding here produces a "connection refused" that looks identical to a missing PG/PC assignment.
• PROFINET device name persistence. The drive retains its PROFINET device name across power cycles but not across a CPU module swap, because the topology discovery is initiated by the IO controller. Always re-issue the device name from HW Config after the swap.
• TIA Portal coexistence. If the project will eventually move to TIA Portal, capture the V3.2 HSP-supported STEP 7 baseline first, then migrate. Migrating from a V2.8 STEP 7 baseline directly to TIA Portal can lose S7 connection objects.
• Routing through multiple PLCs. If STARTER must route through more than one S7-300 station, every PLC in the chain must have a valid PG/PC binding and the S7 connection must be of type "S7 connection" (not "S7 connection, fault-tolerant") for STARTER's routing to traverse it.

13. Quick-Reference Parameter Table

14. Related Engineering Notes

• The "no PC assigned" indicator in NetPro is informational for connections that do not require a PG end point, but it is a hard fault for connections that STARTER will use. Treat it as an error until STARTER goes online successfully.
• STARTER versions older than V4.5 do not support some V3.2 CPU features in their topology discovery. If STARTER cannot enumerate the drive even with a healthy S7 route, upgrade STARTER to the latest SP for the SINAMICS family in use (SINAMICS S120, G120, S210, etc.).
• If the drive uses Safety Integrated over PROFINET (PROFIsafe), STARTER must use the F-host routing path, which adds an additional S7 connection that must be assigned in NetPro. This connection is independent of the standard STARTER connection and must be re-bound after every CPU swap.