CP 343-1 Lean S7 Communication: Server-Only Connection Setup

CP 343-1 Lean Hardware and Communication Architecture

The CP 343-1 Lean communications processor is the cost-optimized Industrial Ethernet / PROFINET interface for the SIMATIC S7-300 family. The module occupies a single slot in the S7-300 rack and is configured through STEP 7 V5.x HW Config or the TIA Portal Devices & Networks editor. It connects the S7-300 CPU to a 10/100 Mbit/s Ethernet network and supports ISO-on-TCP (RFC 1006) and TCP transport for S7 communication.

The Lean variant is positioned below the full CP 343-1 (6GK7 343-1EX11/EX21/EX30) and the CP 343-1 Advanced (6GK7 343-1GX21). To reach a lower list price, the Lean deliberately omits the following functions found in the full CP:

• S7 communication is restricted to server only — the CP cannot open an outgoing S7 client connection.
• No S7 routing between networks.
• No PG/OP routing.
• No IP access control list / stateful firewall.
• No FTP server, no web diagnostics, no file system.
• No PROFINET IO controller role (device role only on PROFINET-capable firmware).

These constraints are documented in the device manual CP 343-1 Lean manual (PDF). Any application that requires the CP to initiate an S7 connection — for example a PUT/GET call originating in the Lean-attached CPU — cannot use the Lean as the active partner.

Server-Only S7 Communication Constraint Explained

The phrase "S7 communication — server only" appearing in catalogs and STEP 7 means that the CP 343-1 Lean will only accept incoming S7 connection requests on a configured TSAP. It cannot open a client connection toward another PLC, PC station, or HMI panel. The partner always owns the active connection establishment.

The technical consequences for typical plant architectures are:

• Two CP 343-1 Lean modules cannot communicate with each other because neither side can become the active partner.
• A SIMATIC NET PC station can read/write tags on a CP 343-1 Lean because the PC station (acting as OPC DA server) is the active client.
• A full CP 343-1 can read/write tags on a CP 343-1 Lean when the full CP 343-1 owns the connection.
• An OP/TP HMI panel built on a full CP can read tags from a Lean; an HMI built on a Lean variant cannot initiate reads against another Lean.
• An S7-1200 or S7-1500 station can act as the active partner to a CP 343-1 Lean via standard PUT/GET or BSEND/BRCV calls.

This server-only behavior is firmware-bound and is not configurable. Firmware V3.x of the CP 343-1 Lean (e.g., 6GK7 343-1CX11-0XE0) added PROFINET IO device support but did not promote S7 communication to client role.

Connection Type Comparison: One-Way vs Two-Way vs Unspecified

In NetPro (STEP 7 V5.x) and in the TIA Portal Devices & Networks editor, each S7 connection carries an active-partner flag. Three values are possible:

When a connection is created with the PC station as the active partner and the CP 343-1 Lean as the passive server, NetPro shows the connection entry only on the PC-station side of the project. The PLC project hosting the Lean does not require a matching connection entry — the Lean simply listens for incoming connection requests on the configured TSAP. This is the "one-way" or "configured at one end" relationship that the original poster was asking about.

SIMATIC NET PC Station Configuration with iFix

The reference architecture described in the source question is:

• Three S7-300 CPUs each equipped with a full CP 343-1 — existing configuration with two-way S7 connections to a SIMATIC NET PC station.
• New S7-300 CPU planned with a CP 343-1 Lean — to be added to the same SCADA pool.
• SIMATIC NET PC station running as OPC DA server (Softnet-S7 / IE S7 OPC) — active partner of all S7 connections.
• iFix SCADA with the iFix OPC client driver — subscribes to tags exposed by the OPC server.

The key to integrating the Lean is that the SIMATIC NET PC station remains the active partner of every S7 connection in the pool. The Lean modules are passive servers; the PC station opens the TCP connections. No changes are required on the existing three full CP 343-1 connections.

NetPro Configuration Procedure (STEP 7 V5.x)

1. Open the STEP 7 project that contains the PC station with the OPC server.
2. Open NetPro and select the OPC server (S7-OPCSrv) on the PC station.
3. Right-click the OPC server and choose Insert New Connection.
4. If the Lean CPU is in the same STEP 7 project, select the Lean CPU as the partner; otherwise select "Unspecified CPU accessible via S7 connection".
5. In the connection wizard, select the CP 343-1 Lean interface as the partner's communication endpoint.
6. Mark the connection as one-way, with the PC station as the active partner. If the wizard offers only Unspecified, finish the wizard and edit the connection properties — set the Lean side to passive / server.
7. Save and compile NetPro (Station → Save and Compile). Watch the compile output for warnings about partner role conflicts.
8. Export the XDB file (Station → Export).
9. On the PC station, open the Station Configuration Editor and import the XDB (Import Station).
10. Restart the SIMATIC NET OPC server service and verify that the new connection appears as Established.

The full procedure with screenshots is documented in Siemens Knowledge Base entry 15355168.

PUT/GET Function Placement Rules

The PUT/GET blocks (SFB 14 GET, SFB 15 PUT) execute inside the S7-300 CPU, not inside the CP. Their ability to reach a partner CPU depends on the underlying CP's communication role. On a CP 343-1 Lean:

• SFB 14 GET / SFB 15 PUT cannot be used to initiate a connection — the Lean is not an S7 client.
• The CPU attached to the Lean can still be polled by PUT/GET running in another CPU whose CP is a full CP 343-1 (or an S7-1200 / S7-1500 / PC station acting as client).

The data exchange pattern is asymmetric: data is written into or read from the Lean-attached CPU by the active partner, never by the Lean CPU itself. If a user program inside the Lean CPU needs to push data to another station, a separate connection with a full CP 343-1 on the partner side must be used, with the partner owning the active role.

For projects with two CP 343-1 Lean modules in different racks, Siemens explicitly states that data exchange via PUT/GET is not possible because neither side can be the active partner. The same restriction applies to BSEND/BRCV (SFB 12/13) and USEND/URCV (SFB 8/9) when initiated from a Lean CPU. Status codes 80B1 / 80B2 are returned. This is documented in Siemens Knowledge Base entry 18610307.

Sample PUT/GET Code in SCL (executed on the FULL CP 343-1 side)

// Write 20 bytes of data from DB30 to the partner Lean CPU's DB20
CALL "PUT", DB15
  REQ    := M10.0;                  // Positive-edge trigger
  ID     := W#16#1;                 // Local connection ID from NetPro
  DONE   := M10.1;                  // Set when complete
  ERROR  := M10.2;                  // Set on error
  STATUS := MW12;                   // Detailed error/status word
  ADDR_1 := P#DB20.DBX 0.0 BYTE 20; // Partner destination area
  SD_1   := P#DB30.DBX 0.0 BYTE 20; // Local source area

// Read 20 bytes of data from the partner Lean CPU's DB20
CALL "GET", DB16
  REQ    := M11.0;
  ID     := W#16#1;
  DONE   := M11.1;
  ERROR  := M11.2;
  STATUS := MW14;
  ADDR_1 := P#DB20.DBX 0.0 BYTE 20; // Partner source area
  RD_1   := P#DB40.DBX 0.0 BYTE 20; // Local destination area

Equivalent Ladder Logic Excerpt

Network 1: Trigger PUT on rising edge of M10.0
  A   M10.0
  FP  M10.3
  =   DB15.DBX    0.0        // REQ

Network 2: Latch DONE for one scan
  A   DB15.DBX    4.0        // DONE
  S   M10.1
  A   OB1_SCAN_1
  R   M10.1

Network 3: Branch on ERROR for fault handler
  A   DB15.DBX    4.1        // ERROR
  JCN  noer
  L   DB15.DBW   12          // STATUS word
  T   MW12
noer: NOP 0

OPC Server Integration with iFix

Once the SIMATIC NET PC station has imported the updated XDB file and the OPC server is running, iFix connects through its OPC client driver (or the OPC Power Tool). Tag addressing follows Siemens S7 absolute syntax:

• DB1.DBD0 — Data Block 1, double-word at offset 0
• DB10.DBB4 — Data Block 10, byte at offset 4
• MB10 — Merker byte 10
• EW0 — Input word 0 (process input image)
• AW4 — Output word 4 (process output image)
• PEW100 — Peripheral input word 100
• PAW200 — Peripheral output word 200

Optimized block access (S7-1200 / S7-1500 symbolic mode) is not available on S7-300; only absolute addressing is used. Each OPC read/write cycle maps directly to an S7 read/write request from the OPC server through the active S7 connection to the CP 343-1 Lean. The Lean is fully transparent to the SCADA application.

When creating an iFix database entry that targets the Lean, use the OPC item path returned by the OPC server (typically S7:[S7 connection name]DB1,BYTE0). Configure scan time to match the actual process dynamics; a 1-second scan rate is typical for slow processes.

Network Configuration and IP Planning

Before adding a Lean to an existing S7-300 fleet, plan the IP address space and TSAP allocation to avoid conflicts:

The Lean supports only one Ethernet interface. The TSAP (Transport Service Access Point) is encoded as rack.slot in the format RR.SS. For example, a Lean in rack 0, slot 4 uses TSAP 00.04 on the Lean side; the PC station typically uses TSAP 01.01 (its OPC server slot).

Connection Compatibility Matrix

PUT/GET and Active/Passive Distribution Patterns

For architectures that involve multiple data exchanges, the active partner role must be assigned explicitly. Common patterns include:

  Active side (client)                          Passive side (server)
  --------------------------                    -------------------------
  SIMATIC NET PC station (OPC)        -->       CP 343-1 Lean    (read/write tags)
  Full CP 343-1 in another CPU        -->       CP 343-1 Lean    (PUT/GET blocks)
  S7-1200 / S7-1500                   -->       CP 343-1 Lean    (PUT/GET blocks)
  HMI panel (full CP)                 -->       CP 343-1 Lean    (read area pointers)
  S7-400 with CP 443-1                -->       CP 343-1 Lean    (PUT/GET blocks)

The CP 343-1 Lean can be the target of multiple S7 connections from different active partners. Each connection is independent, uses its own TSAP, and consumes one resource slot in the Lean. Plan accordingly when mixing HMI, SCADA, and inter-PLC traffic on a single Lean.

Verification and Commissioning Procedure

1. Online CP diagnostics — In STEP 7, connect online to the Lean CPU and open the CP's diagnostic buffer (CP 343-1 Lean → Module Information → Diagnostic Buffer). Verify that no "connection aborted", "TSAP in use", or "resource exhausted" entries exist.
2. SIMATIC NET diagnostic — Open the SIMATIC NET diagnostic tool and confirm all configured S7 connections show state Established with the partner IP / TSAP matching the Lean configuration.
3. OPC test client — Use an OPC test client (Matrikon OPC Explorer, Siemens OPC Scout) to read/write a single tag through the new connection. Expected result: GOOD quality, value updated within the OPC refresh rate (typically 1 s).
4. iFix tag verification — In iFix Database Manager, force a tag to a known value from the OPC server side and confirm the SCADA reflects the change within the configured scan time.
5. Load test — Add 50 to 100 tags and run the system for 30 minutes. Watch for OPC quality degradation that would indicate connection-count saturation or TCP keepalive issues.
6. Failure injection — Disconnect the Lean Ethernet cable for 30 seconds and verify that the OPC server marks tags BAD, then re-establishes the connection within 30 seconds of cable reinsertion.
7. Audit log — Record the final configuration: Lean IP, TSAP, Rack/Slot, NetPro connection ID, OPC item path, and iFix tag name. File in the project documentation folder.

Performance and Resource Limits

The Lean does not cache data — each OPC read/write results in a request to the CPU. High tag counts at aggressive scan rates can saturate the CPU OB1 scan. Monitor OB1 cycle time during commissioning and throttle OPC scan rates if the cycle exceeds 80% of the OB1 watchdog.

Troubleshooting Matrix

CP 343-1 Lean Order Numbers and Firmware Reference

The CP 343-1 Lean has been revised several times. Common order numbers seen in the field:

• 6GK7 343-1CX00-0XE0 — first generation, ISO-only transport.
• 6GK7 343-1CX10-0XE0 — added PROFINET IO device support.
• 6GK7 343-1CX11-0XE0 — current production, firmware V3.x, integrated 2-port switch on selected variants.

Firmware V3.x retains the server-only S7 constraint. PROFINET IO device functionality is added but does not change the S7-communication role. Always cross-check against the device manual CP 343-1 Lean manual (PDF) for the specific order number and its firmware revision.

Best Practices for Mixed Fleets (Full CP 343-1 + CP 343-1 Lean)

• Always make the PC station or another full CP 343-1 the active partner for any S7 connection terminating on a Lean.
• Document the connection direction in the project header so future engineers do not try to add a Lean-to-Lean link.
• Reserve TSAP numbers in a project standard (for example 10.00 for Lean in slot 4, 10.01 for Lean in slot 5) to avoid TSAP conflicts when multiple Leans are added.
• Use the SIMATIC NET diagnostic tool weekly during commissioning to catch silent connection drops.
• When migrating from a full CP 343-1 to a Lean, refactor any PUT/GET calls that originate from the affected CPU — they must be moved to a partner CPU with a full CP.
• Reserve a small contingency in the OPC license (typically one extra connection slot) for adding future Leans without license rework.
• Tag the CP 343-1 Lean physically with the project TSAP and active-partner role to avoid field wiring confusion during maintenance.

Migration Considerations When Adding a Lean to an Existing Project

When adding a Lean to an existing S7-300 fleet that uses full CP 343-1 modules, follow this migration order to avoid runtime disruption:

1. Add the Lean to the STEP 7 project on paper, compile, and validate offline.
2. Update NetPro on the PC station with the new one-way S7 connection.
3. Compile and export the new XDB file.
4. Schedule a maintenance window to import the XDB and restart the OPC server.
5. Install the Lean hardware in the target slot of the new S7-300 rack.
6. Download the hardware configuration to the new CPU.
7. Verify the OPC connection comes up automatically and iFix tags populate.
8. Document the change in the project history.

Existing CP 343-1 connections are not affected by the XDB import; only the new connection is added to the OPC server. The service restart is required once.

Frequently Asked Questions