CP 343-1 Lean Send/Receive STOP: NetPro Connection Download Fix

Problem Overview

The CP 343-1 Lean communications processor (Siemens order number 6GK7 343-1CX10-0XE0) is a compact Ethernet interface for the S7-300 family used to attach an S7-300 station to Industrial Ethernet, LAN, or intranet networks. Field-proven behavior: a project that is fully tested in a development environment can be transferred to a customer site, downloaded into the CPU, and start up with the diagnostics reporting "Module is OK" and "S7 Communication is OK" while Send/Receive Communication is STOP. The user program calls FC5 AG_SEND and FC6 AG_RECV as designed, the Ethernet cable passes continuity tests, ping replies are received, and yet no payload ever crosses the wire. HMI polls time out with send/receive errors and no application data is returned by the CP.

This article documents the root cause that is observed most often in the field, the exact step to eliminate it, and the surrounding diagnostic and verification procedures that close out the case.

Affected Hardware and Firmware

Refer to the official Description of the CP 343-1 Lean manual (Siemens Support, PDF) for the precise connection limits and firmware behaviour of your specific hardware release.

Symptom Summary

• Project runs correctly in the engineering office / test rack.
• Project is downloaded (full download) into the customer's CPU and CP 343-1 Lean.
• After restart, the CP's online diagnostics report:
  • Module: OK
  • S7 Communication: OK
  • Send/Receive Communication: STOP
• The non-Siemens HMI / OPC server initiates connection attempts and reports send/receive errors; no application data is exchanged.
• ICMP ping between the HMI server and the CP is successful.
• The CP-to-CPU backplane (S7 communication channel) is operational - OB1/FC/communication blocks load and run without errors.

Root Cause

When the user performs a full project download from STEP 7 to the CPU, the hardware configuration (HW Config) and the user program blocks (OB, FB, FC, DB) are transferred, but the connection configuration stored in NetPro is not always downloaded with the project. The CP 343-1 Lean therefore starts up with its previously stored connection table - which is empty in a brand-new or freshly commissioned module. The result is that the Send/Receive functions FC5 AG_SEND and FC6 AG_RECV are called with valid parameters but have no partner connection to use, and the diagnostic state of the CP moves to Send/Receive Communication STOP.

This behaviour is well documented in the Siemens S7-300 / C7 / M7 programming and operating manuals: connection data is a separate configuration object from HW Config and must be downloaded through the NetPro editor (or, in STEP 7 V5.x, via PLC > Download > Connections to Target System).

Solution: Download the Connection Configuration from NetPro

1. Open the STEP 7 project in SIMATIC Manager.
2. Select the S7-300 station in the project tree.
3. Open NetPro (button on the toolbar, or menu Options > NetPro).
4. In the network view, select the CP 343-1 Lean of the target station.
5. Open the connection table for the CP (right-click > Object Properties, or double-click the connection row).
6. Confirm that the configured Send/Receive connections (ISO-on-TCP, TCP, UDP, or ISO Transport) are present in the table and that the local and partner endpoints are correct.
7. With the target CP 343-1 Lean selected, choose PLC > Download > Connections to Target System (or right-click and select Download Connections).
8. STEP 7 will prompt for the target module's IP address. Enter the IP of the CP 343-1 Lean (or the CPU's MPI/TCP path).
9. Confirm the download. The online window will report "Download of connection configuration to CP 343-1 Lean completed".
10. Perform a CP restart (STOP/RUN on the CPU, or power cycle the CP) so the new connection table is loaded into runtime.

Verification

After the connection download and CP restart, perform the following checks in order:

1. Open SIMATIC Manager > Station > Open Online on the CP 343-1 Lean.
2. Open the CP's Diagnostics page. Confirm that Send/Receive Communication is now in state RUN rather than STOP.
3. From the partner HMI / server, send a single poll. The first call to FC5 AG_SEND should report DONE=1, ERROR=0, STATUS=0000H. The matching FC6 AG_RECV should return the partner's reply.
4. Monitor the CP's connection list (NetPro online view, or CP Web diagnostics if enabled) - the Send/Receive connection should appear in state established.
5. Check the CPU diagnostic buffer for any new communication-error events. After a successful fix, no new events should appear.

Send/Receive Programming Reference (FC5 / FC6)

The CP 343-1 Lean uses the legacy AG_SEND / AG_RECV interface to exchange raw data over ISO-on-TCP (RFC1006), TCP, ISO Transport, or UDP. The function blocks are:

A working template (suitable for ISO-on-TCP) looks like this in STL:

      CALL  FC    5
       ACT   :=TRUE              // always send when called
       ID    :=1                 // connection ID from NetPro
       LADDR :=W#16#0100         // logical base address of CP (HW Config)
       SEND  :=P#DB10.DBX0.0 BYTE 128  // 128 bytes of payload
       LEN   :=128
       DONE  :=M10.0
       ERROR :=M10.1
       STATUS:=MW12

      CALL  FC    6
       ID    :=1
       LADDR :=W#16#0100
       RECV  :=P#DB11.DBX0.0 BYTE 200  // 200-byte receive buffer
       LEN   :=200
       NDR   :=M10.2
       ERROR :=M10.3
       STATUS:=MW14

Why the HMI Did Not Talk Even After the NetPro Fix

After the Send/Receive connection was loaded into the CP 343-1 Lean, the underlying transport came up. Any remaining HMI-side issues in this case were caused by HMI configuration (driver, IP/port, slot number, fetch/write permissions) rather than by the CP. A non-Siemens HMI panel or SCADA must be configured for the same transport that the CP advertises in NetPro:

Pre-flight Checklist for a New CP 343-1 Lean Commissioning

1. Mount the CP on the S7-300 rail and connect to the backplane.
2. Assign a unique IP address, subnet mask, and (if required) router address via HW Config > CP Properties > Ethernet Interface.
3. Open NetPro and create the required connections (S7, ISO-on-TCP, TCP, UDP). Set the local CP to the correct role (active / passive / both) for the partner device.
4. Download HW Config to the CPU/CP first.
5. Download the user program (OB, FC, FB, DB) to the CPU.
6. Download the connection configuration from NetPro (this is the step that resolves the Send/Receive STOP symptom).
7. Cycle power on the CP to clear the runtime connection table.
8. Verify with the CP's online diagnostics that all communication channels report RUN.

Diagnostic and Connectivity Matrix

Edge Cases and Field-Proven Caveats

• Firmware / project upload mismatch. If the project was last saved against a newer STEP 7 version than the offline station, STEP 7 may refuse the connection download. Re-compile or use the same STEP 7 version that originally generated the project.
• Multiple CPs in one station. The LADDR in FC5/FC6 is the logical base address of the CP, not the slot number. A wrong LADDR produces STATUS=8184H.
• Connection ID collisions. Each Send/Receive connection has a unique ID in NetPro. Reusing IDs across two connections makes the lower-numbered one invisible to FC5/FC6.
• Power-on sequence. A cold restart of the CPU does not always reload the CP's connection table on legacy CP 343-1 Lean firmware. A power cycle of the CP is the deterministic way to force the table to be re-read.
• Firewall on the HMI server. Windows Firewall blocks port 102 by default on a fresh server image. The ICMP ping succeeds while the TCP/ISO-on-TCP port is blocked - a classic source of confusion.
• Switch port security. A managed switch with sticky MAC learning can silently drop frames from a CP that was previously on another port. The CP itself is healthy, but the network refuses its MAC.

How to Reset a CP 343-1 Lean to Factory Defaults

When the configuration state is unclear, resetting the CP to factory defaults is a safe way to start from a known state.

1. Open HW Config, right-click the CP 343-1 Lean and select Object Properties > Ethernet Interface > IP Address.
2. Click Reset CP to Factory Settings (German: "CP auf Werkseinstellungen zurücksetzen"). The CP will reboot with the configuration cleared.
3. Re-assign the IP address and re-download HW Config.
4. Re-download NetPro connections.
5. Cycle the CP and verify diagnostics.

Related Manuals and Official References

• CP 343-1 Lean - Device Description (Siemens Support, PDF)
• Siemens Industry Online Support - search entry "CP 343-1 Lean"
• STEP 7 V5.x - Programming and Operating Manual (S7-300 / C7 / M7)

FAQ