CP 343-1 Blocks Folder: S7-300 Communication Configuration Guide

The CP 343-1 is a Siemens SIMATIC S7-300 communication processor that handles industrial Ethernet and PROFINET traffic, offloading the CPU from communication tasks. A frequent point of confusion in STEP 7 (SIMATIC Manager) is the Blocks container that appears under the CP object in the S7 program tree. This folder is not a programming workspace; it is a parameterization area managed by the CP firmware and STEP 7. Communication logic is implemented elsewhere, using the SIMATIC NET library and STEP 7 programming editors.

1. CP 343-1 Product Family and Catalog Numbers

Siemens has shipped several CP 343-1 variants over the lifecycle of the S7-300 platform. The catalog (order) numbers relevant to field engineers selecting or replacing a module are listed below.

2. The Blocks Folder: What It Is and What It Is Not

When an S7-300 station containing a CP 343-1 is opened in STEP 7 (SIMATIC Manager), the project tree shows the CP object as a sibling to the CPU. Under that CP object, a Blocks container appears. The Blocks folder on the CP contains system blocks and parameterization data blocks generated and managed by the CP firmware, not user-written code.

2.1 What the CP Blocks folder contains

• System data blocks (SDB) generated by HW Config and NetPro that describe the connection table, IP parameters, port assignments, and protocol configuration of the CP.
• System functions (SFB/SFC) for diagnostics automatically loaded by the CP firmware image.
• Parameterization blocks (DBs) that store CP-specific parameters such as keep-alive time, PROFINET device name, port MAC addresses, NTP settings, and web-server passwords.

2.2 What the CP Blocks folder is NOT used for

• It is not a place to write user logic (no STL/LAD/FBD blocks belong there).
• It is not a substitute for the CPU's Blocks folder. The user program, FBs, FCs, DBs, OBs, and the SIMATIC NET communication blocks all reside in the S7-300 CPU's S7 Program > Blocks container.
• It does not require manual block programming. The blocks are created, updated, and downloaded to the CP by the STEP 7 configuration tools (HW Config and NetPro).

3. Configuration Workflow Overview

Setting up a CP 343-1 follows a three-stage process. Skipping any stage leaves the CP either unparameterized or unable to use its connections.

1. Hardware configuration (HW Config): Insert the CP into the S7-300 rack, assign an IP address, subnet mask, and PROFINET device name, and configure the Ethernet port properties (MAC, link mode, port enable/disable).
2. Network configuration (NetPro): Define connections (S7, TCP, ISO-on-TCP, UDP) to partners. NetPro compiles the connection data into SDBs.
3. User program (STEP 7 programming editor): Call the SIMATIC NET library FBs/FCs (TCON, TSEND, TRCV, PUT, GET, BSEND, BRCV, etc.) from the CPU program to use the connections configured in NetPro.

4. Stage 1: Hardware Configuration in HW Config

Open the SIMATIC 300 Station in HW Config and drag the CP 343-1 from the hardware catalog onto an available slot in the S7-300 rack. Double-click the CP to open its properties dialog.

4.1 IP Address and PROFINET Device Name

The Ethernet interface properties dialog exposes the following fields:

4.2 Port Configuration

Each physical port (P1, P2 on two-port variants) can be configured for autonegotiation, fixed 10 Mbps / 100 Mbps / 1 Gbps, full- or half-duplex, and port enable/disable. PROFINET IO real-time traffic requires autonegotiation with 100 Mbps full-duplex as the deployed rate.

4.3 CP-Specific Parameters

The CP properties dialog also contains CP-specific tabs:

• Options > Time-of-Day Synchronization: Configure NTP servers, synchronization interval, and the CP as slave or master for plant-wide clock distribution.
• Options > Security (Advanced variant only): Firewall rules, VPN, port-filter rules.
• Options > Web Server: Enable/disable the integrated web server, set the read-write password.
• Options > PROFINET: Device replacement without programming device, topology editor configuration.

After saving the HW Config, the configuration data is compiled into SDBs that appear in the CP's Blocks folder. The SDBs are downloaded to the CP by selecting the CP in the project tree and choosing PLC > Download to Module > CP 343-1, or by selecting the entire station.

5. Stage 2: Connection Configuration in NetPro

NetPro is the central editor for S7 connection, TCP, ISO-on-TCP, and UDP connections. The flow is identical regardless of the protocol family: select the CP 343-1, right-click, choose Insert New Connection, and select the partner and connection type.

5.1 Connection Types Available on CP 343-1

5.2 Connection ID and Local ID

Every connection created in NetPro is assigned a unique Local ID (also called connection ID). This 16-bit word is the critical handle the user program uses to reference a specific connection when calling FBs like TCON, TSEND, TRCV, PUT, and GET. The Local ID is the first input parameter of nearly every SIMATIC NET FB and must be passed from the user program to the CP firmware at runtime.

Document the Local IDs in a project spreadsheet. Losing the link between a NetPro connection and its Local ID is a common commissioning failure mode, especially when many connections are created and the IDs are not tracked.

5.3 Connection Properties and TSAP / Port Assignment

For S7 connections, NetPro exposes the TSAP (Transport Service Access Point) on both the local CP and the partner. The TSAP for CP 343-1 has the format 03.xx where xx is the slot of the CP, or a user-assigned value from the Properties > S7 Connection tab. For ISO-on-TCP, the TSAP is configurable to any 1-16 byte ASCII or hex string. For TCP and UDP, the partner port number is configurable (102 for ISO-on-TCP, 2000/2001 for S7, or user-defined).

6. Stage 3: Programming with the SIMATIC NET Library

Communication logic in the CPU is written using FBs and FCs from the SIMATIC NET library. These blocks are not in the CP's Blocks folder; they are copied into the CPU's program and called from the user logic. The library is shipped with STEP 7 and is located under Libraries > SIMATIC_NET_CP in the SIMATIC Manager.

6.1 S7 Communication (CPU-to-CPU)

For S7 connections to another S7 CPU, the following blocks are used:

The SIMATIC_NET_CP library also includes the legacy FC 5 (AG_SEND) and FC 6 (AG_RECV) for older projects; these are retained for compatibility but should not be used in new code with CP 343-1 EX30 and later variants.

6.2 Open Communication (TCP / ISO-on-TCP / UDP)

For open Ethernet protocols, the T-block family is the standard. These are universal, structured FBs that work with any S7-300/400 CP that supports open communication.

The TCON block must be called once at startup (or on-demand) to parameterize the connection. The parameter CONNECT of TCON is a UDT (UDT 65 for TCP, UDT 66 for ISO-on-TCP, UDT 67 for UDP) describing the local and remote endpoints. The TCON block writes the runtime connection parameters into the CP, which uses the configured connection from NetPro as the template.

6.3 Sample: TSEND Call in Structured Text

The following is a typical call to FB 67 (TSEND) to transmit 100 bytes of data over a TCP connection with Local ID 1.

// Instance DB for FB 67 (TSEND)
// Tag definitions
iTSEND_DB    : FB 67;   // Multi-instance or dedicated instance DB

iTSEND_REQ   : BOOL;    // Trigger to start send
iTSEND_BUSY  : BOOL;    // Operation in progress
iTSEND_DONE  : BOOL;    // Send complete
iTSEND_ERROR : BOOL;    // Error flag
iTSEND_STATUS: WORD;    // Status / error code
iTSEND_LEN   : INT  := 100;

// Call
iTSEND_DB.REQ     := iTSEND_REQ;
iTSEND_DB.ID      := 1;           // Local ID from NetPro
iTSEND_DB.LEN     := iTSEND_LEN;  // Send length in bytes
iTSEND_DB.DATA    := P#DB100.DBX0.0 BYTE 100;  // Pointer to source data
iTSEND_DB.DONE    := iTSEND_DONE;
iTSEND_DB.BUSY    := iTSEND_BUSY;
iTSEND_DB.ERROR   := iTSEND_ERROR;
iTSEND_DB.STATUS  := iTSEND_STATUS;
CALL FB 67, iTSEND_DB;

Triggers and edges must be managed by the user. The same pattern applies to TRCV, PUT, GET, and BSEND/BRCV. Always monitor STATUS on every call; CP-side errors surface here as detailed error codes (see Section 10).

7. Block Architecture in the CPU's Blocks Folder

After copying the SIMATIC NET FBs and FCs into the CPU, the S7 program tree should look like the following. The CP object's Blocks folder remains untouched for user code.

S7-300 Station
  └─ S7 Program(1)
       └─ Blocks  <-- CPU blocks folder
            ├─ OB 1        (main user program)
            ├─ FB 67       (TSEND, instance DB 100)
            ├─ FB 68       (TRCV, instance DB 101)
            ├─ FB 15       (PUT,  instance DB 102)
            ├─ FB 14       (GET,  instance DB 103)
            ├─ FC 100      (alarm/edge evaluation)
            ├─ DB 50       (send/receive data buffer)
            ├─ DB 60       (TCON parameter UDT 65 instance)
            ├─ DB 70       (TCON parameter UDT 66 instance)
            ├─ System Data  (SDBs compiled by HW Config/NetPro)
  └─ CP 343-1
       ├─ ...
       └─ Blocks  <-- CP blocks folder, populated by configuration tools
            ├─ System Data (connection SDBs, parameter SDBs)
            ├─ SFBs / SFCs (diagnostics only)

8. Protocol Selection Guide

Selecting the correct protocol is the most consequential design decision for a CP 343-1 link. The following guidance reduces the chance of an unsuitable protocol being selected and reworked later.

9. Sample Programs: SIMATIC NET Quick Start Collection

Siemens published a SIMATIC NET Quick Start Collection that contains complete, ready-to-compile STEP 7 projects for the most common CP 343-1 use cases. The collection covers:

• S7 connection PUT/GET between two S7-300 stations
• ISO-on-TCP link to a PC SAPI-S7 application
• TCP send/receive with a Windows Winsock example
• UDP broadcast send/receive
• PROFINET IO controller configuration with a SIMATIC ET200S as device
• SNMP polling from a PC to the CP 343-1

The collection was historically distributed on the Applications & Tools portal of Siemens Industry Online Support and bundled as the Auflösung der SIMATIC NET Quick Start Collection in German. The English-language version is available under the SIMATIC NET application examples portal. Each example ships as a STEP 7 project archive with detailed PDF documentation covering HW Config settings, NetPro connection parameters, and a complete program listing with comments.

When commissioning a new CP 343-1 link, locate the matching sample in the collection before writing code from scratch. The samples use the official SIMATIC NET library version that matches the firmware of the CP, eliminating version-mismatch errors that are common in custom implementations.

10. Diagnostics: Status Codes and Error Handling

The STATUS word returned by SIMATIC NET FBs encodes the CP-side error state. The codes are standardized; the most frequently encountered values are listed below.

For a complete list of status codes and their extended meaning, refer to the CP 343-1 manual and the SIMATIC NET function block documentation. The online help in STEP 7 also displays the description of any STATUS value returned in a VAT (variable table) when the FBs are monitored online.

11. Commissioning and Verification Procedure

1. Compile HW Config and confirm no errors in the message window.
2. Compile NetPro and confirm that the connection list reports all expected connections and that the Local IDs are unique.
3. Download HW Config to the S7-300 station. This loads the SDBs to both the CPU and the CP.
4. Verify CP online: Open the CP in STEP 7 with PLC > CP 343-1 > Online and check the diagnostics buffer. The CP should report its IP address, the active connections, and any link errors. The LINK LED on the CP confirms the physical Ethernet connection; the RX/TX LEDs confirm traffic.
5. Test the connection from the CPU side using a VAT. Call PUT/GET or TCON/TSEND with a simple test request and monitor DONE, BUSY, ERROR, and STATUS.
6. Test from the partner side: From a PC, use a SAPI-S7 tool, a TCP terminal, or a Modbus test client (if applicable) to verify the partner sees the connection and exchange test data.
7. Enable diagnostic blocks in the user program. SFB 52 (RDREC) and SFB 53 (WRREC) read and write PROFINET records, including diagnostic data. SFC 51 (RDSYSST) reads system state lists including connection status.
8. Save the final project to the Memory Card of the CPU so the configuration persists across power loss.

12. Common Configuration Errors and Resolutions

13. Firmware Considerations

The CP 343-1 has undergone multiple firmware revisions since its introduction. Newer firmware versions add PROFINET conformance classes, security features, and additional simultaneous connection counts. Always confirm that the firmware version loaded on the CP matches the version expected by the STEP 7 project; STEP 7 will refuse to download a project with newer features to an older firmware CP.

Firmware can be updated via the SIMATIC S7 firmware update tool, distributed as a free download from Siemens Industry Online Support. The process involves selecting the CP in HW Config online, choosing PLC > Update Firmware, and selecting the appropriate UFW file. The CP is automatically restarted after a successful update, which interrupts live communication. Plan firmware updates during a maintenance window.

14. Migration Notes for Modern Systems

For new projects, Siemens recommends the S7-1500 platform with the integrated PROFINET interface of the CPU rather than a separate CP. The CP 343-1 remains supported for installed base and for legacy system extensions. If porting a CP 343-1 application to S7-1500:

• The T-block family (TCON, TSEND, TRCV) is retained in the S7-1500 with the same call interface, simplifying code reuse.
• S7 connections are configured in the device's Properties > Communication view in TIA Portal rather than NetPro.
• Local IDs are still used but the value range is 16-bit and assigned automatically by the compiler in TIA Portal; manual tracking is no longer required.

For STEP 7 (SIMATIC Manager) projects that must coexist with TIA Portal projects on a shared network, the CP 343-1 firmware supports both PROFINET IO controller and PROFINET IO device modes. Verify that the S7-1500 project explicitly adds the CP 343-1 to its PROFINET IO device list with the correct GSD file (GSDML-Vx.x-Siemens-CP343-1-yyyymmdd.xml).

15. Summary

The CP 343-1 Blocks folder is a system-managed area containing configuration SDBs, not a programming workspace. Communication setup follows the well-defined three-stage workflow: HW Config for IP and port parameters, NetPro for connections with their Local IDs, and the SIMATIC NET library FBs/FCs in the CPU program to send and receive data. The SIMATIC NET Quick Start Collection provides ready-to-run sample projects for every supported protocol. For a reliable installation, document Local IDs, monitor STATUS on every call, and use the CP diagnostic buffer to resolve any startup-time issues.