AG_LRECV FC60 Maximum Buffer Size for S7-300/S7-400 Industrial

AG_LRECV (FC60) on SIMATIC S7-300/S7-400: Maximum Receive Buffer Size, Protocols, and Status Codes

The AG_LRECV function block, implemented as FC60 in the SIMATIC S7-300 / S7-400 library for SIMATIC NET Industrial Ethernet CPs, is the standard open-communication receive interface for jobs routed through CP 343-1 / CP 443-1 modules. Engineers who integrate legacy hosts (VAX, RS/6000, AS/400, custom Unix servers) frequently exceed the receive buffer because the limit is protocol-dependent, not block-dependent. This reference consolidates the protocol-specific maximum payload, buffer transfer semantics, the FC50/FC60 pairing, status-code matrix, and configuration paths in STEP 7 V5.x and TIA Portal V21.

1. AG_LRECV Function Block Overview

AG_LRECV is part of the SIMATIC NET "Open Communication Services" block family for the SIMATIC S7-300 and S7-400 PLC families. It is delivered in the standard library and operates in conjunction with an Industrial Ethernet CP (e.g. CP 343-1, CP 343-1 Lean, CP 343-1 Advanced, CP 443-1, CP 443-1 Advanced). The CPU calls FC60 in the user program (OB1 cycle or a dedicated watchdog OB), and the CP returns the received data through the configured job interface.

AG_LRECV supports three transport protocols on Industrial Ethernet:

• TCP (native RFC 793, raw socket)
• ISO Transport (ISO 8073 / TP4 over Ethernet, OSI layer 4)
• ISO-on-TCP (RFC 1006, the SIMATIC default for S7 communication)
• UDP (RFC 768, connectionless datagrams)

Refer to the official TIA Portal V21 documentation for the AG_LRECV parameter description.

2. Protocol-Specific Maximum Buffer Limits

The maximum number of bytes that can be passed through the RECV parameter of AG_LRECV depends entirely on the transport protocol negotiated at the configured connection. The limits are not enforced by the FC60 block itself but by the CP firmware when the receive job is dispatched.

For Profibus variants of AG_LRECV (FC60 over a Profibus CP), the limits are tighter and depend on the Profibus CP used; the Industrial Ethernet numbers above do not apply to Profibus. Always verify against the Profibus CP-specific indications when deploying over PB.

3. FC50 (AG_SEND) vs FC60 (AG_LRECV) Pairing

Open Communication Services on S7-300/S7-400 are symmetric: FC50 (AG_SEND) transmits a buffer to the partner, and FC60 (AG_LRECV) accepts the partner's transmission. Both blocks are tied to the same configured connection ID. The relevant Siemens KB article for programming the pair is "How do you program the communication blocks FC50 and FC60?".

Both blocks must use the same ID and LADDR (logical base address of the CP). Mismatching these two parameters is the most common source of 0x8180 ("no data yet available") status returns on otherwise-healthy connections.

4. Industrial Ethernet CP Compatibility

AG_LRECV (FC60) is supported on the following CP families as of firmware listed:

5. TCP Buffer Transfer Behavior

A critical field-engineering detail is that for TCP (and ISO-on-TCP) connections, the receive buffer is only delivered to the CPU when the buffer is completely filled. This is a direct consequence of how the CP implements TCP framing: the CP cannot know whether the partner has finished transmitting until either the partner closes the segment or the configured receive buffer reaches its length limit.

Practical consequences:

1. If the partner sends fewer bytes than the configured LEN, AG_LRECV will remain in the "waiting" state (status 0x8180 "no data yet available") until either the buffer fills or a partner-initiated close is detected.
2. If variable-length messages are expected, size the buffer for the largest expected message — or switch to UDP and rebuild framing at the application layer.
3. If the partner always closes the segment after writing (graceful shutdown of each TCP send), the partial buffer is delivered immediately.
4. Streaming payloads (e.g. 16 KB split across many smaller bursts) will not deliver until either the configured LEN is reached or a segment FIN is observed.

UDP does not have this behavior — every datagram is delivered as soon as it arrives, regardless of length. UDP's only constraint is the 2048-byte datagram ceiling.

6. AG_LRECV Interface Definition

The formal interface of FC60 in STEP 7 V5.x and TIA Portal V21 (industrial Ethernet variant):

The RECV ANY pointer must reference an area large enough to hold the configured LEN; if the ANY pointer specifies a smaller area, the CP will report a length-mismatch error. For example, with a TCP connection and a 8192-byte buffer, declare RECV as P#DB100.DBX0.0 BYTE 8192.

7. Status and Error Codes

The STATUS output of FC60 follows the standard SIMATIC NET error class scheme. Common values:

Siemens KB article "Possible status value of the block FC 60 (AG_LRECV) in the S7-300 with connection not set up" documents the special case where 0x8180 is returned when the CP has no active connection for the configured ID. This is distinguishable from "no data on wire" only by examining CP online diagnostics (NCM diagnostics > "Connections").

8. STEP 7 Classic Configuration (V5.x)

For S7-300 / S7-400 deployments on STEP 7 V5.5 / V5.6:

1. Insert the SIMATIC NET CP into the hardware configuration (e.g. CP 343-1 / 6GK7 343-1EX30-0XE0). Note the CP's logical base address (default W#16#0100 on slot 4 for S7-300).
2. Open NetPro and add a new "Open Communication" connection on the CP. Choose the transport protocol (TCP / ISO Transport / ISO-on-TCP / UDP).
3. For TCP/ISO-on-TCP, configure the partner IP address and port. For UDP, configure the partner IP/port pair but note the connection is still "configured" in NetPro (it just uses datagrams).
4. Save and download the connection configuration to the CP.
5. Insert FC60 (AG_LRECV) from the library: Standard Library > Communication Blocks > Open Communication (or the renamed "SIMATIC NET CP" library depending on installation).
6. Wire the ID and LADDR parameters from NetPro into the FC60 instance.
7. Call FC60 in OB1 (or a faster OB for low-latency polling).

9. TIA Portal Configuration (V21)

On TIA Portal V21, the equivalent procedure:

1. Add the CP in the Device Configuration; configure its Ethernet port and assign an IP address.
2. Open Devices & Networks and switch to the "Connections" view.
3. Right-click the CP → "New Connection" → choose "Open User Communication" → TCP / ISO-on-TCP / UDP / ISO Transport as required.
4. Set the connection name, partner IP, and partner port. Save and download.
5. Drag the AG_LRECV (FC60) instance from the Open Communication program blocks into the user code; assign ID and LADDR from the connection properties.

Refer to the official TIA Portal V21 AG_LRECV documentation for the V21-specific behavior and parameter mapping.

10. STL and SCL Programming Examples

STL (Statement List) for OB1 cycle call

// AG_LRECV (FC60) call example — TCP, 8192-byte max
// Inputs prepared from NetPro connection
      CALL FC60
       ID     := 1                 // Connection ID from NetPro
       LADDR  := W#16#0100         // CP logical base address
       RECV   := P#DB100.DBX0.0 BYTE 8192  // Receive buffer
       NDR    := M100.0            // New-data-received pulse
       ERROR  := M100.1            // Error flag
       STATUS := MW102             // Status word
       LEN    := MW104             // Actual received length

// Edge evaluation
      A   M 100.0
      JCN no_new
      // Process LEN bytes from DB100 starting at offset 0
no_new: NOP 0

SCL (Structured Control Language)

// AG_LRECV call in SCL
IF "first_scan" THEN
  // Initialize trigger
  recv_trigger := TRUE;
END_IF;

IF recv_trigger THEN
  AG_LRECV(
    ID     := 1,
    LADDR  := W#16#0100,
    RECV   := P#DB100.DBX0.0 BYTE 8192,
    NDR    => ndr_flag,
    ERROR  => err_flag,
    STATUS => status_word,
    LEN    => recv_len
  );
END_IF;

IF ndr_flag THEN
  // Process recv_len bytes in DB100
  "ProcessBuffer"(len := recv_len);
END_IF;

IF err_flag THEN
  "LogStatus"(code := status_word);
END_IF;

11. Troubleshooting Matrix

12. Best Practices for Third-Party Hosts (VAX, RS/6000, Legacy Unix)

Integrating Siemens AG_LRECV with non-Siemens partners such as a VAX/VMS host requires strict attention to protocol semantics:

1. Prefer ISO-on-TCP (RFC 1006) over native TCP for legacy hosts. ISO-on-TCP carries explicit length prefixes that eliminate the "TCP buffer-not-full" stall documented in Section 5. Configure the partner to write TSAP + data + length per the SIMATIC framing rules.
2. If native TCP must be used, size the buffer to the largest possible message and configure the partner to close the write side after each message (TCP half-close / shutdown(SHUT_WR)). Otherwise data will sit in the CP until either the partner closes the entire socket or the buffer overflows.
3. For UDP integrations, keep each datagram ≤ 2048 bytes. Fragment larger messages at the application layer using an explicit sequence-number header.
4. Validate RECV ANY size at compile time — declare the receive buffer DB with sufficient length before downloading; a runtime ANY-mismatch surfaces only when the first long message arrives.
5. Poll continuously. AG_LRECV is not edge-triggered; the same call must execute every OB1 cycle (or every cycle of the chosen watchdog OB) for the CP to deliver data promptly.
6. Use connection diagnostics in NCM Diagnostics (STEP 7 V5) or Online & Diagnostics (TIA Portal) to distinguish "no data on wire" from "connection not established."
7. Watch the CP firmware version. Pre-V2.0 CP 343-1 modules cap at 4000 bytes; CP 343-1 Lean caps at 240 bytes. If the VAX side sends 2602 bytes the Lean CP is incapable regardless of protocol choice.

Frequently Asked Questions