S7-1200 TSEND_C and TRCV_C: Ethernet Data Transfer Configuration

1. Overview

The SIMATIC S7-1200 supports peer-to-peer Ethernet data exchange between two CPU 1214C (or any S7-1200/S7-1500 combination) controllers using the TSEND_C and TRCV_C instructions from the "Communication" palette in TIA Portal. TSEND_C/TRCV_C are compact versions of the older TUSEND/TURCV pair and internally handle the connection establishment (TCON), termination (TDISCON), and option-byte handling. The transport protocol is ISO-on-TCP (RFC 1006) by default, which provides acknowledged, connection-oriented, point-to-point data transfer over the PROFINET interface of the CPU.

This reference applies specifically to a two-CPU 1214C topology (TIA Portal V11 or later), an unmanaged switch, and a default firmware of the S7-1200 family. The article resolves two recurring field issues:

1. The TSEND_C CONNECT input rejecting every data block with "incompatible data type".
2. Values written from an HMI tag are not propagated to the partner PLC, while values written from the program ARE propagated.

2. Prerequisites

3. Network Topology and IP Planning

Both controllers MUST reside in the same IP subnet. ISO-on-TCP uses port 0x1024 (decimal 1024) as TSAP for S7-1200; you may freely select local and remote TSAPs (16 bits each) provided that the active and passive sides mirror one another.

4. Configuring the Connection in TIA Portal

The most common beginner error is wiring the CONNECT input to a user-defined data block of the wrong structure. Siemens requires a TCON_IP_V4 System Data Type (SDT) data block. There are two ways to generate it:

1. Right-click the CPU in "Devices & Networks" and add an "ISO-on-TCP connection" between the two PLCs. TIA Portal auto-creates the connection data block.
2. Manually add a new global DB, declare a single CONNECT variable of type TCON_IP_V4 (V3.0), and fill the parameters.

Recommended path (Method 1):

1. Open "Devices & Networks" view of PLC_A.
2. Drag a connection line from the PROFINET port of PLC_A to PLC_B.
3. Choose "ISO-on-TCP connection" from the popup.
4. Click the connection line, go to Properties → General, and note the Connection ID (default = 1).
5. In PLC_A's project tree, expand "Program blocks → System blocks → Additional system blocks". A new DB named SENDCONNECTION_1 (or similar) appears. This is the DB you wire to the TSEND_C CONNECT input.

5. TSEND_C Block Parameters

6. TRCV_C Block Parameters

7. TCON_IP_V4 Connection Description

The "incompatible data type" error in the CONNECT input almost always means the user created a DB with the wrong UDT or used TCON_IP_V4 from a different version. The TIA Portal V11-V17 SDT contains:

TYPE TCON_IP_V4
VERSION : 3.0
STRUCT
   InterfaceId    : HW_ANY;       // PROFINET interface HW identifier, usually 64
   ConnectedMcId  : BYTE;         // Multicast ID, 0 = unicast
   LocalPort      : UINT;         // Local TSAP / port (e.g. 0x1024)
   RemoteAddress  : IP_V4_STRUCT; // 4 bytes + 2 bytes for port
   LocalAddress   : IP_V4_STRUCT; // optional, fill with 0.0.0.0
   ConnectionType : BYTE;         // 0x11 = TCP, 0x12 = ISO-on-TCP, 0x13 = UDP
   ActiveEstablished : BOOL;      // TRUE = active (client), FALSE = passive (server)
END_STRUCT
END_TYPE

Set ConnectionType = 0x12 for ISO-on-TCP. Set ActiveEstablished = TRUE on the TSEND_C side and FALSE on the TRCV_C side. The RemoteAddress must contain the partner's IP in network byte order (e.g. 192,168,0,2 + port 0x0000, port field stays 0 for ISO-on-TCP because TSAPs use LocalPort/RemotePort in the next fields).

8. Ladder / SCL Sample Code

PLC_A (sender, OB1 main scan):

// Edge-detect a "send now" trigger
A   M10.0            // Send request from HMI / program
FP  M10.1            // Edge memory bit
=   "SendTrigger"

// Call TSEND_C
CALL "TSEND_C_DB" , "TSEND_C_INST"
  REQ    := "SendTrigger"
  ID     := 1
  CONNECT:= "SENDCONNECTION".CONNECT
  DATA   := "DataDB".SendBuffer
  LEN    := 100
  CONT   := TRUE
  DONE   => "StatDB".SendDone
  BUSY   => "StatDB".SendBusy
  ERROR  => "StatDB".SendError
  STATUS => "StatDB".SendStatus

PLC_B (receiver, OB1 main scan):

// Always ready to receive
SET
=   "RecvEnable"

CALL "TRCV_C_DB" , "TRCV_C_INST"
  EN_R   := "RecvEnable"
  ID     := 1
  CONNECT:= "RCVCONNECTION".CONNECT
  DATA   := "DataDB".RecvBuffer
  LEN    := 0
  ADHOC  := TRUE
  DONE   => "StatDB".RecvDone
  BUSY   => "StatDB".RecvBusy
  ERROR  => "StatDB".RecvError
  STATUS => "StatDB".RecvStatus
  RCVD_LEN => "StatDB".RecvLen

9. Root Cause: HMI-Triggered Values Do Not Reach Partner PLC

This is the second and most common production symptom: a value written into SendBuffer by the PLC program is correctly transmitted to the partner, but a value written by the HMI is not. The reason is purely about REQ edge behavior.

TSEND_C is edge-triggered. The send job executes exactly once per rising edge on REQ. The CPU only sends the current content of DATA at that moment. Two scenarios occur in the field:

1. HMI writes a new value, but REQ does not pulse — TSEND_C has no reason to fire, so the partner keeps the old data.
2. Program logic only sets REQ when a "process event" occurs — a value change alone does not generate an edge.

Typical cause: the program uses a button-driven or event-driven REQ (e.g. M10.0 set when a sensor trips) but the HMI just writes to DataDB.SendBuffer directly, without pulsing any send trigger.

10. Fix: HMI-Side Change Detection / Cyclic Send

Three proven solutions, ordered from most elegant to most brute-force:

10.1 Trigger REQ on every value change (recommended)

Add a "send enable" or "send on change" tag. In PLC_A, compare SendBuffer to a shadow copy. If a difference is detected, pulse REQ:

// In OB1 of PLC_A
"Shadow".LastSent := "Shadow".LastSent OR 0;
IF "DataDB".SendBuffer <> "Shadow".LastSent THEN
    "Shadow".ChangeDetected := TRUE;
END_IF;

// Edge detect the change flag
IF "Shadow".ChangeDetected THEN
    "SendTrigger"(FP := "Shadow".EdgeMem);   // SCL one-shot
    "Shadow".ChangeDetected := FALSE;
    "Shadow".LastSent := "DataDB".SendBuffer;
END_IF;

With this block in place, an HMI write to DataDB.SendBuffer will automatically trigger a transmission on the next PLC cycle.

10.2 Use a polling / heartbeat send

Send the entire buffer on a fixed time base (e.g. every 200 ms using a clock bit from OB35 or a self-generated flasher). This guarantees eventual consistency regardless of HMI writes:

// Using CPU clock memory, byte 1000 - bit 5 = 2 Hz
// 2 Hz is too fast for ISO-on-TCP, scale to 0.5 Hz via TON
"Clock_2Hz"( TP := 200ms, Q => "HalfHz" );
IF "HalfHz" THEN
    "SendTrigger"(FP := "EdgeSend");
END_IF;

10.3 Make the HMI pulse the send trigger

Bind the HMI button (or "value changed" event of the input field) to set a dedicated tag such as HMI_SendRequest for one cycle. The PLC detects the edge and fires TSEND_C. This is the least preferred approach because the HMI must know about the trigger variable, coupling display logic to communication logic.

11. Common TSEND_C / TRCV_C Status Codes

12. Diagnostic Procedure

1. Online → Diagnostics → Statistics in TIA Portal on the PROFINET interface of PLC_A: verify the partner IP appears as reachable.
2. Open a watch table on PLC_A. Monitor StatDB.SendBusy, SendError, SendStatus. After pulsing REQ once, BUSY should pulse to TRUE for one cycle and DONE to TRUE on success.
3. On PLC_B, monitor RecvDone and RecvLen. After a successful send, RecvDone = TRUE for one cycle and RecvLen = number of bytes transferred.
4. If BUSY stays TRUE for > 5 s, STATUS will report a connection error (0x80C4, 0x80E9). Capture the hex and cross-reference Section 11.
5. Open the S7-1200 "Diagnostics buffer" (online → Diagnostics buffer). The connection event includes the partner IP and the failing TSAP.

13. Verification Checklist

14. Field-Proven Caveats

• Do not place TSEND_C in OB1 with REQ tied to TRUE — the CPU will send at line speed and exhaust the Open User Communication buffer. Always use a finite-edge trigger or a timer.
• TRCV_C with LEN = 0 and ADHOC = TRUE is the most robust receive pattern: partial frames are copied immediately and RCVD_LEN reports the actual number of bytes received. Use this for variable-length messages.
• ISO-on-TCP TSAPs are 16-bit. The S7 convention is to put CPU slot number in the second byte (e.g. 01.01 = rack 1 slot 1). For a CPU 1214C use 01.01; this avoids collisions if you later add a CM module.
• Two TSEND_C blocks with the same Connection ID cause STATUS = 0x80A1. Always use unique connection IDs for each partner.
• CPU 1214C FW 4.0 and earlier will accept only the V1.0 SDT TCON_IP_V4 structure. If upgrading the project to a newer TIA Portal, right-click the block → "Update to latest version" to remap the structure.
• The same logic applies when the partner is a S7-1500. The block interfaces are identical; the only thing that changes is the maximum number of Open User Communication connections (S7-1500 supports far more).

15. Related Protocols

TSEND_C/TRCV_C are ISO-on-TCP only. If the requirement is to talk to a third-party device that does not support RFC 1006 (e.g. a SCADA system that expects raw Modbus/TCP or a generic TCP socket), use:

• MODBUSCLIENT / MODBUSSERVER instructions for Modbus TCP on the same S7-1200.
• TSEND / TRCV (the older, non-compact pair) for raw TCP without ISO-on-TCP header.
• Open User Communication as UDP with TUSEND/TURCV for non-acknowledged broadcasts.