Configuring FDL Communication on Siemens S7-300 with CP 342-5

Overview

Fieldbus Data Link (FDL) is the Layer-2 service set defined by IEC 61158/EN 50170 Volume 2 (PROFIBUS). It exposes the PROFIBUS bus as a deterministic, peer-to-peer data channel between stations without requiring a DP master/slave cycle. On the S7-300 platform, FDL is delivered through the CP 342-5 communications processor (catalog number 6GK7 342-5DA02-0XE0) and is driven from user logic by the standard blocks FC5 AG_SEND and FC6 AG_RECV shipped with the SIMATIC NET library.

This guide walks through commissioning an FDL link between two CPU 315-2 DP stations (6ES7 315-2AF02-0AB0) using two CP 342-5 modules, including hardware addressing, STEP 7 / NetPro configuration, FC5/FC6 parameterization, status-code interpretation, and verification.

Protocol Background: Where FDL Sits in the PROFIBUS Stack

FDL implements PROFIBUS Layer 2 (Fieldbus Data Link). It provides connection-oriented and connectionless services:

• SDA (Send Data with Acknowledge) – connection-oriented, acknowledged.
• SRD (Send and Request Data with Reply) – connection-oriented, used by S7 FDL by default.
• SDN (Send Data with No acknowledge) – broadcast/multicast.
• CSRD (Cyclic Send and Request Data) – cyclic polling.

On the S7 side, the default FDL connection uses SRD semantics with a fixed maximum user payload of 240 bytes per telegram. Each CP 342-5 supports up to 16 simultaneous FDL connections, and the CP can simultaneously operate as a DP master, DP slave, or FDL station through the same physical interface (electrical RS-485 via the 9-pin D-sub, or fiber-optic on the FO variant).

Prerequisites

Before configuring FDL, verify the following hardware, firmware, and software baseline:

Hardware Configuration and Module Addressing

Insert the CP 342-5 into a free slot of the S7-300 rack (slot 4 through 11 are valid, slot 3 reserved for IM). In the STEP 7 hardware catalog, locate SIMATIC 300 > CP 300 > PROFIBUS > CP 342-5 and drag the module matching the catalog number to the rail.

1. Open the CP 342-5 properties dialog and assign a unique PROFIBUS address (e.g., station A = PROFIBUS address 2, station B = 3).
2. Set the transmission rate consistently across all stations on the segment (commonly 1.5 Mbps for FDL links).
3. Note the I/O start address of the CP. The diagnostic/status area appears in the process image. The example uses LADDR = W#16#FFB (= 4091 decimal), which is the system default for CP 342-5 placed in slot 4; adjust if your slot assignment differs.
4. For the FDL operating mode, leave the CP in "No DP" or "DP master" if DP must run concurrently; FDL services are independent of the DP role.

Save and compile the hardware configuration (Station > Save and Compile). Repeat for the second CPU.

STEP 7 / NetPro Connection Setup

FDL connections are configured graphically in NetPro, which is launched from the SIMATIC Manager (or from the project tree in TIA Portal under Devices & Networks > Networks view).

1. Open NetPro for station A. Drag a PROFIBUS subnet across the network view if one does not exist.
2. Select the CP 342-5 of station A, right-click and choose Insert New Connection.
3. In the connection dialog, set the partner to "unspecific" or to the partner CP 342-5 of station B. The connection type is FDL connection.
4. Specify the partner PROFIBUS address (in our example, partner address 3 when configuring from station A's perspective).
5. Assign the local connection ID. The ID is referenced by FC5 and FC6. In the example, ID = 2; valid IDs on a single CP 342-5 range from 1 to 16.
6. Set the maximum send/receive length. The default 240 bytes is recommended unless the application requires less.
7. Compile NetPro and download the connection configuration to both stations.

The compiled NetPro database is stored in the system data blocks (SDB) of the CPU. On startup, the CPU passes the connection parameters to the CP, which then autonomously establishes the SRD handshake on PROFIBUS.

Programmatic Interface: FC5 AG_SEND and FC6 AG_RECV

The SIMATIC NET library delivers two function blocks per connection:

• FC5 AG_SEND – transfers a user buffer to the CP for transmission.
• FC6 AG_RECV – reads received data from the CP into a user buffer.

Place FC5 and FC6 from the SIMATIC_NET_CP library into the S7 program. Below is a working OB1 example matching the parameters used in the field reference. Flags are deliberately chosen for clarity; replace with your project's tag names.

// OB1 - Cyclic OB (OB1)
// Trigger send every OB1 cycle (use a clock bit like M0.7 for 1 Hz)
// Pos-edge of M0.7 generates send pulse

      A   M    0.7          // 1-second clock bit (e.g. from CPU clock memory)
      FP  M    82.7         // edge memory bit
      =   M    82.0         // send trigger pulse

// --- FC5 AG_SEND ---------------------------------------------------
      CALL FC    5
        ACT    :=M82.0      // 1 = trigger send
        ID     :=2           // FDL connection ID from NetPro
        LADDR  :=W#16#FFB    // CP 342-5 logical base address (slot 4 default)
        SEND   :=P#DB80.DBX20.0 BYTE 10  // source DB / start / length
        LEN    :=10          // bytes to send (max 240)
        DONE   :=M82.1       // 1 = send completed without error
        ERROR  :=M82.2       // 1 = error occurred
        STATUS :=DB82.DBW2   // 16-bit status word

// --- FC6 AG_RECV ---------------------------------------------------
      CALL FC    6
        ID     :=2           // FDL connection ID from NetPro
        LADDR  :=W#16#FFB    // CP 342-5 logical base address
        RECV   :=P#DB82.DBX20.0 BYTE 90  // receive buffer
        NDR    :=M82.3       // 1 = new data received
        ERROR  :=M82.4       // 1 = error occurred
        STATUS :=DB82.DBW4   // 16-bit status word
        LEN    :=DB82.DBW6   // actual received length in bytes

FC5 / FC6 Parameter Reference

FDL Status and Error Codes

The STATUS word returns Siemens-standard diagnostics. The two-byte structure breaks into event class (high byte) and event number (low byte). Common values encountered when commissioning CP 342-5 FDL:

Additional codes are listed in the SIMATIC NET programming manual; refer to MN_FDL-API_76 (Siemens FDL Programming Interface manual) for the full diagnostic set.

Baud Rate and Segment Length Limits

PROFIBUS segment length is dictated by the configured transmission rate. Use the table below when designing FDL links that may share the segment with DP masters:

Verification and Commissioning Checks

After downloading the hardware configuration, NetPro connection, and user program, walk through this verification sequence:

1. Bring both CPUs to RUN. Confirm the CP 342-5 SF LED is off and the BF LED is off.
2. Open SIMATIC Manager > PLC > Monitor/Modify and force a 1 Hz pulse on M0.7.
3. Observe M82.1 (DONE) toggling on station A and M82.3 (NDR) toggling on station B. If both toggle, the FDL link is operational.
4. Cross-check DB82.DBW6 (received length) on station B equals 10 bytes.
5. Reverse direction by forcing M0.7 on station B; confirm station A receives.
6. Use CP Diagnostics in STEP 7 to view connection state, telegram counters, and error statistics (transmitted OK / transmitted errors / received OK / received errors).

Troubleshooting Matrix

Migration Notes and Alternatives

FDL is mature but increasingly replaced by PROFINET (open user communication via TCON / TSEND / TRECV) on new installations. When migrating from FDL to PROFINET:

• Replace CP 342-5 with CP 343-1 Lean / CP 343-1 Advanced, or use the onboard PN interface of newer CPUs.
• Substitute FC5 / FC6 with TCON, TSEND, TRECV, TDISCON from the standard IEC library.
• Recreate the point-to-point connection in the TIA Portal network view as a TCP or ISO-on-TCP connection.
• Verify a documented migration path using Siemens entry 17596048 (FDL S5 to S7 configuration) for cross-generational guidance.

For mixed-vendor PROFIBUS ecosystems where a Beckhoff EL6731 acts as an FDL gateway, the Beckhoff EL6731 FDL interface documentation describes ADS-based access to FDL telegrams. For TIA Portal-based S7-300 engineering, the TIA Portal FDL setup reference details the TCON_FDL system data type and direct parameter assignment.