S7-300 CP343-1 Ethernet Configuration in SIMATIC Manager

S7-300 CP343-1 Ethernet Configuration in SIMATIC Manager: Resolving Online/Offline Download Mismatches

Adding a Siemens CP 343-1 communications processor to an S7-300 station and bringing it online with SIMATIC Manager frequently produces an online/offline mismatch warning followed by a failed transfer. The root cause is almost always a misaligned project state between the STEP 7 project (offline), the real PLC hardware configuration (online), and the PG/PC interface assignment pushed by NetPro. This reference walks through the diagnostic steps and field-proven configuration sequence that produces a clean download the first time, using only official Siemens documentation and hardware catalog data.

1. Problem Description

After a SIMATIC S7-300 CPU is upgraded or extended with an Industrial Ethernet CP, the following failure mode is reported:

1. CPU is powered on without the new CP; CPU enters STOP with diagnostic buffer entry Module removed/inserted.
2. The CP (e.g., CP 343-1, MLFB 6GK7 343-1EX30-0XE0) is physically inserted in the configured rack slot (typically slot 7 or 8) and the system is powered again.
3. Accessible Nodes in SIMATIC Manager displays the SIMATIC 300 station by MAC address even though the CP has not been configured, indicating routing is already returning a discovery reply through the backplane.
4. Hardware configuration is opened, the CP is inserted into the correct slot, MAC address, IP address, subnet mask, and router are entered, and the project is saved and compiled.
5. The download is initiated. SIMATIC Manager first reports "The online configuration does not match the offline configuration" and lists modules that physically do not exist in the rack.
6. After acknowledging, the download terminates with an "Unable to download to module" or "Connection to target system has been interrupted" error.

This sequence is reliably reproducible when the PG/PC interface set in Set PG/PC Interface differs from the S7 subnet bound in NetPro, or when the programming device is not part of the STEP 7 project at all.

2. Root Cause Analysis

The S7-300 backplane uses the CPU as the central router. If the CPU has already been assigned an Industrial Ethernet interface (e.g., integrated PROFINET on a CPU 315-2 PN/DP) or the project carries a CP 343-1 from a previous revision, Accessible Nodes replies with the station even when the newly inserted module has not yet been configured. The discovery walks the MAC table of the CPU, not the CP being commissioned.

The download mismatch warning is generated by STEP 7's Online > Compare function, which compares the offline HW Config catalog entries against the diagnostic data the real station returns (slot 0 and slot 7/8). If the offline project contains placeholder modules from the catalog search, or if a previous CP entry was not deleted before the new CP was inserted, STEP 7 reports a structural delta and refuses to write a new configuration without explicit acknowledgement.

Two configuration planes exist and both must agree:

If the PG is part of the NetPro configuration, its Ethernet interface address is downloaded from the project and will replace whatever the engineer typed into Set PG/PC Interface. If the PG is not part of the project, then the PG must use S7ONLINE > TCP/IP > <NIC>, and the target station's IP must be reachable on that subnet.

3. Prerequisites

Verify the following before opening HW Config:

• CPU model and firmware. Identify the CPU MLFB (e.g., 6ES7 315-2EH14-0AB0 for CPU 315-2 PN/DP, firmware V3.2). Older CPUs do not support all CP 343-1 features; consult the Siemens Industry Online Support compatibility list for the specific CP firmware.
• CP 343-1 MLFB and firmware. The most common variants:
  • CP 343-1 Lean 6GK7 343-1CX10-0XE0 (1 × RJ45, 10/100 Mbit/s, 8 TCP connections).
  • CP 343-1 6GK7 343-1EX30-0XE0 (2 × RJ45 switch, 16 TCP connections, ISO-on-TCP).
  • CP 343-1 Advanced 6GK7 343-1GX31-0XE0 (2-port switch, IT functions, FTP, web server).
  • CP 343-1 ERPC 6GK7 343-1RX30-0XE0 (for S7-300 routing into PROFINET).
• STEP 7 version. SIMATIC Manager V5.5 SP2 or later; for newer S7-300 CPUs (firmware ≥ V3.3) STEP 7 V5.6 is recommended. TIA Portal V16+ may also be used but consumes a different project model.
• Physical slot. The CP must occupy a free slot in the same rack as the CPU (rack 0). Slot 4 is reserved for IM; slots 5–11 are valid signal module / CP positions.
• MAC address. Printed on the front of the CP and visible under PLC > Display Accessible Nodes. Capture before powering down.
• PG/PC network identity. The PG's IP address, subnet mask, and default gateway must be known and within the same subnet as the CP's planned IP. Allow ICMP and TCP port 102 (ISO-on-TCP/RFC1006) between PG and CP.

4. Hardware Configuration Order of Operations

STEP 7 enforces a strict order; deviating from it produces the mismatch described above.

1. Open SIMATIC Manager > Project > Open the existing S7-300 station.
2. Select the station and double-click Hardware to open HW Config.
3. If an older CP occupies the target slot, right-click the slot row and select Delete (do not just type a different MLFB — STEP 7 carries configuration data forward).
4. Open the hardware catalog, navigate to SIMATIC 300 > CP 300 > Industrial Ethernet > CP 343-1, and drag the exact MLFB into the target slot.
5. Double-click the inserted CP to open Properties > Ethernet Interface.
6. Set the IP address, subnet mask, and (if required) router to the project-defined values. Do not enable "Set MAC address" unless the firmware on this CP was lost; setting an artificial MAC disables the CP.
7. Save and compile (Station > Save and Compile). Note warnings; correct any error W123456789 style issues reported against the slot.

5. Configuring NetPro and the PG as a Project Node

NetPro is the most common source of failure because it overrides the operating-system PG adapter setting. Two valid arrangements exist.

5.1 PG as a project node (recommended for engineered sites)

1. Open NetPro via SIMATIC Manager > Options > NetPro.
2. From the catalog, drag SIMATIC PC Station > User Application > SW V7.x onto the Ethernet subnet that carries the CP.
3. Open Properties > Interface on the PC station's SW application and select S7ONLINE as the access point. The access point is mapped to a real NIC by Set PG/PC Interface.
4. Save and compile. The download now provides the PG with project-blessed addressing.

5.2 PG not in project (flexible / commissioning scenario)

1. Confirm Set PG/PC Interface > S7ONLINE > TCP/IP > <NIC> is set to the correct physical adapter.
2. Verify the engineering station's IP belongs to the same subnet, e.g., CP: 192.168.0.10 / 255.255.255.0, PG: 192.168.0.5 / 255.255.255.0.
3. Disable firewalls between PG and CP. Windows Defender Firewall blocks TCP 102 by default on a fresh install.

In both cases, the connection test that follows must target the CP's IP, not the CPU's PROFINET IP (if the CPU has one). The CP fronts the Industrial Ethernet side; the CPU may have a different PROFINET name and IP.

6. Online/Offline Mismatch Resolution Procedure

STEP 7 may refuse to write HW Config when the online diagnostic image disagrees with the offline build. The standard reconciliation is:

1. Open HW Config online: PLC > Download to Target System > Configure Network first, then Download to Target System.
2. When the warning "Online configuration differs from the offline configuration" appears, click Details… and log the listed slots.
3. Cancel the download. Run PLC > Display Accessible Nodes and confirm the displayed MAC matches the CP's sticker MAC.
4. Open Hardware > Catalog in HW Config. Right-click the CP, choose Object Properties > Interfaces, and click Read MAC from target. Accept the read MAC.
5. Save, compile, and reissue the download. STEP 7 now compares identical MACs and clears the mismatch warning.
6. If STEP 7 still reports a slot mismatch (typically slot 0 or 2), perform PLC > Clear/Reset > Reset memory from the online menu. This places the CPU in STOP, erases the SDBs, and re-enables a clean download.

7. Step-by-Step Download Procedure

With addressing fixed and the online diagnostic image aligned, perform the actual transfer:

1. In HW Config, click Download to Target System (the toolbar icon).
2. When prompted, select the target station by MAC (the CP's sticker MAC). Do not select the CPU's PROFINET MAC.
3. Confirm the connection dialog reports "Connection established to ..." with the CP's IP.
4. STEP 7 writes the new SDBs to the CPU. The CPU will perform an initial run-up cycle that can take 30–90 s for a CP 343-1 EX30 firmware V3.0+.
5. Open the CPU's online diagnostic buffer (PLC > Module Information > Diagnostic Buffer) and confirm the latest event is "Module inserted: CP 343-1 in slot 7".
6. Place the CPU in RUN via PLC > Run. Observe that RUN LED holds steady green and the CP's LINK and ACT LEDs pulse with traffic.

8. Verification

Adopt a verification rubric that protects against the most common commissioning regressions:

• Ping: From the PG command line, ping <CP-IP> should reply within 1 ms on a switched LAN, <5 ms on a routed site. Failure to reply means the IP/MAC link between PG and CP is broken (cable, VLAN, ACL, firewall) and not a STEP 7 problem.
• Accessible Nodes: The CP must be listed under its own IP, not only by MAC.
• Diagnostic buffer: No SF LED on CP; no events with ID 0x13B1 or 0x13B2 (CP firmware diagnostic events for SDB mismatch).
• NetPro download: After the hardware download, also download NetPro > Download to Target System to push the connection table. Some "Online/Offline mismatch" errors re-appear after a hardware-only download because the connections live in a different SDB set.
• Cyclic OB1: Place a one-line SET in OB1, download it, and observe the output module's green LEDs. This confirms end-to-end S7-online path PG → CP → CPU → I/O.

9. Troubleshooting Matrix

10. Field-Proven Caveats

• Routed discovery is not free. If the rack contains a CPU with PROFINET and a CP with Industrial Ethernet (two distinct subnets), STEP 7 will still show the station in Accessible Nodes only on the configured interface. Always check that the bubble appears on the NIC the PG is bound to, not on the wrong one.
• MAC writes are sticky. Once a CP's MAC is overwritten by a downloaded SDB, recovering the original MAC is impossible without the sticker — read it before commissioning. Lost MACs have caused unplanned CP replacements in the field.
• NetPro trumps Set PG/PC Interface. The most common cause of "Accessible Nodes shows the device but download times out" is a PG/PC adapter selection that disagrees with the project's PG node binding.
• Cable vs switch matters. Auto-crossover on CP 343-1 EX30/Lean: supported on EX30 firmware V1.1+; older C-Plug variants may require a crossover or a crossover-capable switch. Always use a managed switch for commissioning because it preserves the IGMP/Bridge flows used by SIMATIC discovery.
• Firewall rules. Windows firewall rules in SIMATIC Net are installed by the SIMATIC PC station installer; manual installations of STEP 7 on Windows Server may not include the rules. Add explicit inbound TCP/102 and UDP/161-162 (SNMP) if CP 343-1 Advanced is in use and OPC UA or SNMP is enabled.
• Firmware coherence. CP firmware must match the catalog entry. Mixing a CP 343-1 EX30 with firmware V3.0 against a STEP 7 V5.4 SP3 catalog entry for V1.0 produces an SDB-rejection diagnostic buffer entry.
• Reset before reuse. When a CP is moved to another rack, always run PLC > Clear/Reset on the source CPU before reuse. Old SDBs survive MRES on some firmware versions.

11. Quick Reference — Status LED Decoding for CP 343-1

12. Related Siemens Documentation

• Configuring Hardware and Communication Connections STEP 7 — Siemens Industry Online Support application example.
• Configuring Connections Using the SIMATIC Manager — third-party vendor documentation mirrored from Siemens official procedures (referenced for configuration principles).
• Siemens TCP/IP Server (Unsol) Ethernet Driver Reference — access path and NIC binding conventions.
• Siemens Ethernet ISO over TCP/IP S7-300/400 Setup — IP/subnet and port assignment reference.