Resolving CP 343-1 FC6 Communication Errors with FC60 Block Substitution

Engineers integrating Delphi applications with Siemens S7-300 PLCs over Ethernet frequently encounter an obscure failure mode: the FC5/FC6 (AG_SEND/AG_RECV) block pair from STEP 7 V5.4 SP5 or later returns intermittent status codes or refuses to establish an ISO-on-TCP connection on CP 343-1 modules. The failure is not in the application code, the cable, or the CP itself. It is a block version / CP firmware compatibility issue that has a clean, well-documented remedy: replace the standard FC5/FC6 pair with the long-data variant FC60/FC61 (AG_LSEND/AG_LRECV) shipped with STEP 7 V5.x. This reference walks through the root cause, the compatibility matrix, the wiring of the substitution, and the field-proven verification procedure used in production plants.

1. Problem Overview

A Delphi (or any Windows-side) application is configured as an ISO-on-TCP partner for an S7-300 station. The PLC program uses FC5 (AG_SEND) and FC6 (AG_RECV) out of the standard STEP 7 library to send and receive up to 240 bytes per call over an established ISO-on-TCP connection. Communication initially establishes, then either:

• FC6 returns STATUS = W#16#80A1 (connection error) or W#16#80B5 (CP rejects fetch) immediately on the first call after download.
• The CP reports the connection as established in NetPro, but no data crosses the wire; the CP's S7 diagnostic buffer logs "Communication error on send/receive interface" with error code 0x07F1 or 0x0E1F.
• Downgrading the FC5/FC6 block to V1.11 (an older STEP 7 release) does not fix the problem.

The error is reproducible on CP 343-1 (6GK7 343-1EX00-0XE0) with CP firmware V5.0 when paired with the FC5/FC6 versions delivered by STEP 7 V5.4 SP5 / V5.5 block library (FC6 = V4.7).

2. Affected Hardware and Software

Reference: SIMATIC NET CP 343-1 / CP 343-1 Lean - Manual (Siemens ID 24485272) documents the CP firmware matrix and supported SEND/RECV function blocks.

3. Root Cause Analysis

Siemens introduced version 4.x of the AG_SEND (FC5) / AG_RECV (FC6) blocks alongside new CP 343-1 firmware revisions that support an extended job interface (BRCV parameterization, fetch+write concurrency, and 32 kB job buffer). The newer block versions emit job headers (TJOB structure offsets 0x08-0x0F) that are not decoded correctly by older CP runtimes. Specifically, CP 343-1 firmware V5.0 expects the legacy job header format used by AG_LSEND (FC60) / AG_LRECV (FC61) at offset 0x14-0x17; the V4.7 of FC6 places a long-data descriptor at the offset that the V5.0 CP mistakenly interprets as an unknown opcode, rejecting the request before the connection table is consulted.

The Siemens Application Note "Which blocks can be used for connecting S7-300 to PROFIBUS?" (FAQ 187699) explains the same compatibility layering for the PROFIBUS path. The same principle applies to Industrial Ethernet / ISO-on-TCP. Quoting the FAQ:

"For new user programs, always use the current block versions." — Siemens FAQ 187699

Translated for the Ethernet case: the CP firmware dictates which block family is acceptable, not the STEP 7 version on the engineering station. Older CPs (firmware V1.x–V3.x) require older block families. Newer block versions (FC6 V4.x) are forward-compatible only with CPs at firmware V2.0+ on the Advanced variant; the standard CP 343-1 at firmware V5.0 retained the legacy job header contract for compatibility with the FC60/FC61 family.

4. Compatibility Matrix: FC5/FC6 vs. FC60/FC61

The full functional description of AG_LSEND / AG_LRECV is published in the SIMATIC STEP 7 - Standard Libraries reference manual (ID 1173803). The L suffix denotes the long-data variant, and the blocks are used as a 1:1 drop-in for FC5/FC6 with the same call interface — only the I/O area and ID parameters differ.

5. Solution: Substitute FC60/FC61 for FC5/FC6

The minimal, zero-hardware remedy is to swap the call site from FC5/FC6 to FC60/FC61. Both block families are present in the STEP 7 standard library at SIMATIC Manager → Libraries → Standard Library → Communication Blocks:

1. Open the STEP 7 project in SIMATIC Manager with the offline program of the affected S7-300 station.
2. Navigate to Libraries → Standard Library → Communication Blocks.
3. Insert FC60 (AG_LSEND) and FC61 (AG_LRECV) into the S7 program (typically a dedicated COMM or COM_343 FB/FC).
4. Replace every CALL FC5 with CALL FC60; replace every CALL FC6 with CALL FC61.
5. Recompile, download the blocks (do not download HW Config — keep CP firmware untouched), and go online.

The block parameter mapping is nearly identical, with three practical differences:

6. Step-by-Step Implementation Procedure

6.1 Prerequisites

• STEP 7 V5.4 SP5 or V5.5 (or V5.3 with the SIMATIC Communication Blocks add-on) installed on the engineering station.
• CP 343-1 (6GK7 343-1EX00-0XE0) at FW V5.0 or compatible variant.
• NetPro configuration with an ISO-on-TCP connection between the S7-300 and the PC station, where the PC is the active partner (Delphi opens the socket).
• Connection ID and CP LADDR (e.g., 256 / W#16#100) noted from NetPro.

6.2 Block Replacement Procedure

1. Back up the S7 program via Station → Save As… or Archive. Keep the backup offline.
2. Open the COMM function block that contains the FC5/FC6 calls. Identify the input and output connectors (ID, LADDR, SEND/RECV pointer, LEN, DONE, ERROR, STATUS).
3. Right-click the FC5 call → Replace with… → select FC60 from the project library.
4. Re-wire the ACT input to REQ (semantically equivalent, only the name changed). All other pins keep their meaning.
5. Repeat for FC6 → FC61. Rename NDR to itself (no change in this case, but verify the wiring on the receiving FB).
6. Save and recompile the source. Resolve any compile warnings about untyped ANY pointers — FC60/FC61 accept the same POINTER type for the data area.
7. Download only the program blocks to the CPU (PLC → Download). Do not re-download HW Config; doing so can reset the CP and break the running connection.
8. Go online with the CPU and monitor the COMM FB. The first call to FC60/FC61 should return STATUS = W#16#0000 within one cycle.

7. ISO-on-TCP Connection Configuration in NetPro

The block substitution is only one half of the fix. The ISO-on-TCP connection in NetPro must be configured with parameters the CP can decode. Recommended settings:

The full parameter set is documented in the SIMATIC S7-300 Communication manual (ID 109751499).

8. Delphi Application Programming Interface

On the PC side, three libraries are commonly used. None of them require modification once the PLC side is fixed:

• libnodave (open source) — uses ISO_TCP transport and a TSAP-based connection string. With TSAP 10.01/10.02 configured, daveNewConnection followed by daveConnectPLC returns 0 within 2-3 s after the PLC program is updated.
• Snap7 (open source) — uses Cli_ConnectTo with the same TSAP pair; works against any S7-300/400 that exposes RFC1006.
• Prodave (Siemens proprietary) — uses LoadConnection_ex with protocol ID 6 (ISO-on-TCP). This is the path the original question implied.

Sample Delphi snippet using libnodave (Pascal):

uses nodave;
var
  di : daveInterface;
  dc : daveConnection;
  res : longint;
  buf : array[0..239] of byte;
begin
  di := daveNewInterface('IF1', daveProtoISO_TCP, 0);
  res := daveConnectPLC(di, '10.01', '10.02', 0, 0);
  if res <> 0 then begin
    ShowMessage('Connect failed, code ' + IntToStr(res));
    Exit;
  end;
  dc := daveNewConnection(di, 2, 0, 0);
  res := daveReadBytes(dc, daveDB, 100, 0, 32, @buf);
  // ...
end;

The library itself does not care whether the PLC uses FC5/FC6 or FC60/FC61 — the substitution is fully transparent to the PC side because both block families terminate in the same CP SEND buffer.

9. Verification and Diagnostic Steps

1. Online block status. Open the COMM FB online, force REQ = TRUE for one cycle, and verify the STATUS output returns W#16#0000. The DONE bit must transition TRUE for one cycle.
2. CP diagnostic buffer. In SIMATIC Manager → PLC → Diagnostic / Setting → Diagnostic Buffer, confirm there are no new "Communication error" entries (event ID 0x0E1F, 0x07F1) since the last cold restart.
3. NetPro connection state. Right-click the CP 343-1 → Connection Status. The connection must report Established with Send/Receive active in the right-hand pane.
4. Wire-level confirmation. On the Ethernet switch port attached to the CP, run wireshark -i eth0 -f 'tcp port 102' on a mirrored port. You must see a 4-byte ISO-on-TPKT header (03 00 00 16 for the typical 22-byte ISO connection request) followed by a TPDU from the PC side, then a response from the CP.
5. Round-trip latency. With the Delphi client issuing a 32-byte read every 100 ms, the average round-trip must be below 10 ms on a 100 Mbit/s switched network. Anything above 50 ms indicates the CP is in a retry loop — re-check the FC version.

10. Error Code Reference

Common error codes returned by FC60/FC61 and the corrective action:

Status code definitions are maintained in the STEP 7 Standard Library reference manual.

11. Best Practices and Preventive Measures

• Pin block versions in the project. In SIMATIC Manager → Options → Block consistency, lock the FC60/FC61 versions used in production. Engineers who later drag-and-drop "the latest" FC5/FC6 from a newer STEP 7 install will not silently break the running system.
• Document the CP firmware at program handoff. The CP firmware version (visible in PLC → Module Information → Firmware) must be referenced in the program header comment of the COMM FB so the next maintainer does not repeat the upgrade rabbit hole.
• Use FC60/FC61 by default on new projects. They support 8 kB transfers, are compatible with every CP 343-1 / CP 443-1 firmware from V1.x onward, and the V2.3 release is signed and shipped in every STEP 7 V5.x media kit. The only reason to use FC5/FC6 is a hard requirement to keep payload ≤ 240 bytes per call.
• Mirror the PC side's TSAP choice on the PLC side. The most common configuration error after a block swap is a TSAP typo introduced when the NetPro connection is rebuilt. Always diff the project archive after any change to the connection object.
• Verify the fix with a known-good test harness. Keep a 50-line Delphi tool that reads DB100.DBW0 on demand. After every CP firmware upgrade, run the harness first. If the round-trip exceeds 50 ms or returns error 80D0, the block family must be revisited.