Troubleshooting CP 343-1 Ethernet Communication with WinCC V7.5

1. Problem Overview

An S7-300 station built around a CPU 315-DP and a CP 343-1 (6GK7343-1EX30) fails to establish a TCP/IP connection to WinCC V7.5 at runtime, while the same project downloads and runs cleanly under S7-PLCSIM. The engineering station can read the CP's IP address through the CP's MAC (online via the CP's front-panel MAC sticker and STEP 7 Accessible Nodes), so the physical LAN and link LED are confirmed healthy, yet monitored FCs return "no response from partner" and WinCC tags stay in bad quality / no connection. The same engineering station also fails to bind WinCC to the S7-300 over MPI using a PC-USB adapter, but succeeds against PLCSIM.

This pattern — PLCSIM works, real CPU does not, link LED is up — almost never points to the cable. It points to one of four configuration domains: (a) PG/PC interface assignment, (b) CP IP / subnet parameters, (c) NetPro connection resource binding, or (d) WinCC channel assignment. The remainder of this document isolates each domain and provides the verification step that proves which one is broken.

2. Hardware and Firmware Baseline

Confirm the exact catalog numbers and firmware states before changing anything. The article targets the configuration in the original report but the procedure generalizes to other variants in the CP 343-1 family.

The CP 343-1 / CP 343-1 EX20 manual defines the supported connection types and the maximum number of connection resources. EX30 supports up to 16 S7 connections and <>8 open TCP/ISO-on-TCP connections total; over-provisioning the CP is a silent failure cause that STEP 7 only surfaces when the connection is downloaded.

3. Root Cause Analysis

Four configurations independently cause the reported symptom. Each must be ruled out in order.

3.1 PG/PC Interface Access Point Mis-assignment

STEP 7 and WinCC resolve the physical NIC through Set PG/PC Interface (Control Panel → Set PG/PC Interface). If the access point used to compile/download the project differs from the access point used at runtime by WinCC, both will succeed offline and fail online — exactly the observed behavior.

• Access point for S7ONLINE: must point to the wired Ethernet NIC with TCP/IP (Auto) or TCP/IP <your NIC>.
• Access point used by WinCC: the WinCC channel driver for S7 uses the same S7ONLINE access point. If WinCC was installed before the NIC driver, or if a virtual NIC (Hyper-V, VMware, VPN TAP, Intel Thunderbolt adapter) was added later, S7ONLINE may bind to the wrong card.
• Symptom: Accessible Nodes shows the CP, but the FC monitor and WinCC both report no connection.

3.2 CP IP / Subnet Mismatch

The CP 343-1 boots with a factory default of 0.0.0.0 until STEP 7 downloads a project that contains a configured IP. Until then, only the MAC-based "Accessible Nodes" scan finds it. If the PC and the CP land in different subnets (or one has a subnet mask that excludes the other), ARP succeeds but TCP never connects.

• Confirm with ping <CP_IP> from an elevated command prompt.
• If ping fails but the CP shows up in Accessible Nodes, the IP you see in STEP 7 was not yet committed to the CP, or the CP has retained an older project.

3.3 NetPro Connection Resource Not Built into the Downloaded Project

The CP 343-1 only knows about a connection if NetPro contains an explicit S7 connection or TCP/ISO-on-TCP connection entry pointing at the CP and at the WinCC station (or at the unspecified partner, depending on protocol). If the project was assembled from blocks copied from another station, the connection resource often exists in the source but is not re-bound when the project is rebuilt.

• Open NetPro, right-click the CP 343-1, choose New Connection.
• Partner = (unspecified) for ISO-on-TCP, or partner = WinCC station with S7 connection.
• Save, Compile and Check All, download the connection configuration separately from the FC/FB download.

3.4 WinCC Channel Configuration

WinCC V7.5 uses the SIMATIC S7 Protocol Suite channel. Each logical connection inside the channel has its own Connection Properties dialog with a Station Address, Slot, Rack, and Connection Type. The IP address entered there is the one WinCC actually opens the socket against — it must match the CP IP exactly, not the CPU IP, and not the project name.

4. Required Information Collection

Before changing any setting, record the following. Skipping this step is the most common reason a fix appears to work in one session and silently regress.

1. CP IP address, subnet mask, gateway — read from Accessible Nodes → CP 343-1 → Properties and from the front-panel MAC label.
2. PC IP address, subnet mask, gateway, DNS — ipconfig /all on the engineering station.
3. Active access point — Control Panel → Set PG/PC Interface. Screenshot the dropdown.
4. Project connection table — NetPro, all connections anchored on the CP 343-1.
5. WinCC channel table — SIMATIC S7 Protocol Suite → MPI/TCP/ISO-on-TCP → connection names.
6. Firmware version of CP 343-1 — read via Accessible Nodes → Module Information; record the firmware banner.
7. STEP 7 version (V5.5 / V5.6 / V5.7) and WinCC version (V7.5 / V7.5 SP1 / V7.5 SP2).

5. Step-by-Step Resolution Procedure

Apply the checks in the order shown. Do not skip to step 6 — a wrong PG/PC interface setting will mask every other fix attempt.

Step 1 — Verify physical layer and IP reachability

1. Connect the engineering station and the CP 343-1 directly with a Cat5e/Cat6 cable (use a crossover if the switch does not support Auto-MDIX on a 10/100 link).
2. Confirm the CP's LINK LED is solid and the NIC LED shows link on the PC.
3. From the PC: ping <CP_IP> -t. A successful reply confirms L3 reachability.
4. If ping fails, temporarily assign the PC a static IP in the same /24 subnet (e.g., CP = 192.168.0.10/24, PC = 192.168.0.20/24) and retry.

Step 2 — Fix Set PG/PC Interface

1. Open Windows Control Panel → Set PG/PC Interface (the 32-bit version, not the App-based one).
2. For Access Point: S7ONLINE, set the interface parameterization to TCP/IP <your wired NIC>.TCPIP.1.
3. If multiple Ethernet cards exist (virtual + physical), select the physical one. Do not select TCP/IP (Auto) when more than one card is present — the OS will nondeterministically pick one.
4. Click OK, restart STEP 7 and WinCC Explorer.

Step 3 — Verify CP 343-1 IP configuration in STEP 7

1. Open HW Config, double-click the CP 343-1 (6GK7343-1EX30) slot.
2. Open Properties → Ethernet Interface. Verify:
   • IP address: matches the address that pings successfully.
   • Subnet mask: typically 255.255.255.0 for /24 networks.
   • Use router: unchecked unless traffic actually leaves the subnet.
3. If the project was inherited, the IP shown here may differ from the IP that was actually downloaded. Force a fresh download: PLC → Download → to Target Station → CP 343-1 only, then re-download the full station.

Step 4 — Build the connection in NetPro

1. In SIMATIC Manager, open NetPro.
2. Right-click the CP 343-1 → Insert New Connection.
3. Select S7 connection or TCP connection depending on the protocol used by WinCC.
4. For the partner:
   • S7 connection: partner = (unspecified), set local ID and the partner's IP manually in properties.
   • TCP / ISO-on-TCP: partner = (unspecified); define port and TSAP.
5. Save and compile: Network → Save and Compile. Watch the output log for warnings such as "Connection resources exceeded" or "Different project ID".
6. Download the network configuration: PLC → Download → Connections. This step is independent of the S7 program download.

Step 5 — Configure the WinCC V7.5 channel

1. In WinCC Explorer, open Tag Management → SIMATIC S7 Protocol Suite → TCP/IP (or ISO-on-TCP, matching NetPro).
2. Right-click TCP/IP → New Connection.
3. In Connection Properties:
   • IP address: CP 343-1 IP (e.g., 192.168.0.10).
   • Rack / Slot: 0 / 2 (CPU 315-DP occupies slot 2 in rack 0).
   • Connection type: S7 connection (default).
4. Add at least one diagnostic tag (e.g., a bit in MB0 or a counter) and verify it updates in Tag Simulator / Graphics Designer runtime.

Step 6 — Verify the connection from the field

1. On the CP 343-1, observe the TX/RX LEDs during a WinCC start. A flicker on RX confirms WinCC is reaching the CP.
2. In STEP 7 online, PLC → Diagnostic → Connection Overview. The connection should show status Established.
3. If the connection shows Not established, the CP returns the specific connection diagnostic in Accessible Nodes → CP 343-1 → Module Information → Connection Diagnostics. Common reasons:

6. Special Case: MPI to CPU 315-DP Failing Outside PLCSIM

The reported failure of MPI over PC Adapter USB A2 is a near-clone of the Ethernet fault. PLCSIM exposes a virtual MPI address; the real CPU uses the address set by STEP 7 (default 2) and the PC Adapter's own MPI address (default 0). The most common cause of the MPI variant failing is that the access point for the PC Adapter was never created.

1. Plug the PC Adapter into the S7-300 MPI port. Confirm the adapter's Active LED is solid.
2. Open Set PG/PC Interface. Create or select the access point S7ONLINE (MPI) → PC Adapter (MPI).
3. Set the adapter's MPI address to 0 and the highest MPI address to 31 (or 187 for PROFIBUS).
4. Transmission speed: 187.5 kbps matches the CPU's default MPI bus profile.

If the PC Adapter is recognized in Windows Device Manager but MPI still fails, verify the CPU's MPI address in HW Config → CPU → Properties → Interface → MPI. Two stations with the same MPI address on the same bus collide silently.

7. Why PLCSIM "Works" — The Boundary That Matters

S7-PLCSIM emulates the CPU's user-program execution but does not emulate the CP 343-1. Communication in PLCSIM goes through the simulated MPI/DP backplane path, bypassing TCP/IP, ISO-on-TCP, and the CP's firmware stack entirely. Anything that depends on the CP — IP, TSAP, port, MAC, SNMP, web diagnostics, NTP — is invisible to PLCSIM. Use PLCSIM to validate program logic; do not use it to validate network configuration.

8. Diagnostic Reference Table

9. Verification Procedure After the Fix

1. Stop WinCC Runtime.
2. Restart WinCC Runtime; observe that all tags transition to good quality within 5 seconds.
3. In STEP 7 online, confirm Connection Overview shows the CP 343-1 connection as Established.
4. Force a write from WinCC (e.g., set a tag mapped to DB1.DBX0.0 to TRUE). In STEP 7 online, monitor the DB and confirm the bit toggles.
5. Power-cycle the S7-300 rack and re-verify within 30 seconds — this catches keep-alive and DHCP lease issues.
6. Disconnect the engineering station from the CP and reconnect; tags must recover automatically.

10. Preventive Recommendations

• Always set CP 343-1 IPs to static values, never rely on DHCP for production HMIs.
• Reserve a documented IP range per CP; print the subnet mask on the cabinet label.
• Use one and only one wired Ethernet card on the WinCC station; disable Wi-Fi and virtual adapters.
• Document the access point binding in the project README; this is the single most common cause of "it worked yesterday" regressions.
• When replacing a CP, also re-download Connections from NetPro. Replacing hardware does not automatically re-bind connection resources.
• Keep the CP 343-1 firmware on the latest Siemens-released version. Older EX30 firmware has known issues with WinCC V7.5 SP2+ tag bursts.

11. Frequently Asked Questions