Troubleshooting S7-1200 RCV_PTP RS232 Weighing Scale Data Loss

Problem Profile: RCV_PTP Returns No Data from RS232 Weighing Scale

Field experience with the SIMATIC S7-1200 PTP receive path is consistent: the RCV_PTP instruction is wired into OB1, the module reports no fault, the STATUS word returns 0x0000, and yet no bytes are copied into the receive buffer. The same characters visible in a Windows HyperTerminal session on the same COM port never appear in the instance DB. The symptom is independent of the weighing scale brand (Mettler-Toledo, CAS, Avery Berkel, Cardinal, GSE, Rice Lake, etc.) and points to one of five recurring classes of mismatch between the Windows terminal session and the PLC configuration: cable topology, interface configuration, receive trigger, buffer management, and termination character handling.

This reference walks through the diagnostic path for each class, with the parameter names, register/tag addresses, firmware versions, and SCL code fragments used in TIA Portal V15.1 through V18. It assumes the SIMATIC S7-1200 Programmable Controller System Manual and the CM 1241 Communication Module Manual are available as the primary reference, and builds the practical debugging procedure on top of them.

System Architecture for RS232 Weighing Scale Communication

The S7-1200 does not include an onboard RS232 port on the CPU body. RS232 communication requires a CM 1241 communication module installed on the left side of the CPU. The relevant part numbers are:

The CM 1241 RS232 supports baud rates of 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 76800, and 115200 bit/s. The most common weighing scale defaults are 9600 8N1 (CAS, Cardinal, GSE), 9600 7E1 (older Mettler-Toledo), and 4800 8N1 (Avery Berkel). Match these exactly in both TIA Portal and the Windows terminal session or the symptom will be identical: the terminal decodes the stream, the PLC discards every byte at the framing stage.

CM 1241 RS232 Hardware Setup

Insert the CM 1241 RS232 into the left bus of the CPU until the side connector locks. Power-cycle the CPU so the new module is detected, then confirm its presence in the device view of TIA Portal. Open the module's properties and verify the following before any PTP instructions are placed:

1. Hardware identifier — record the HW-ID displayed in the module properties. The CM 1241 RS232 typically assigns HW-ID 269 (decimal) when mounted in the first slot. This value is required as the PORT input of RCV_PTP.
2. Module diagnostics enabled — enable Diagnostics under the module's properties so that wire-break and frame errors surface as diagnostics events in the online diagnostics view.
3. Receive line initial state — for RS232 with RTS/CTS handshake, set RTS to ON in the idle state if the scale requires the handshake line asserted to begin transmission.

RS232 Cable Wiring and Null Modem Considerations

The CM 1241 RS232 exposes a DB9 male connector wired as DTE (Data Terminal Equipment). The weighing scale is also DTE. Two DTE devices cannot be connected with a straight-through cable; a null modem (crossover) cable is required. The minimum pinout for a 3-wire application (no hardware handshake) is:

The 2↔3 crossover is the single most common cabling error. HyperTerminal succeeds with a straight-through cable because the PC's COM port is wired as DTE, and the scale is also DTE — both TxD lines drive pin 3 and both RxD lines listen on pin 2. HyperTerminal still works because the application reads whichever physical line the operating system mapped to "receive data." The CM 1241 RS232, by contrast, is hard-wired to the EIA-574 pinout, and pin 3 on its connector is its transmit line, not its receive. If you wire pin 3 to pin 3, the PLC transmits into its own receiver and never sees a byte from the scale.

Field-proven verification:

1. Disconnect the cable at the scale end.
2. Use a multimeter in continuity mode between CM 1241 pin 2 (RxD) and the conductor reaching the scale's transmit pin (typically pin 3 on a DB9 scale or pin 2 on a 25-pin scale). A beep confirms the receive path.
3. Repeat for pin 3 (TxD) — continuity to the scale's RxD.
4. Confirm pin 5 continuity end-to-end. A missing ground return is the second-most-common cabling error and produces characters that appear with a corrupted high bit at the receiver.

TIA Portal Project Configuration

Open the device configuration for the CM 1241 RS232. Under Properties > Port Configuration, set the following parameters to match the weighing scale's datasheet exactly:

Compile the project and download. Online, open the module's diagnostics buffer to confirm zero active diagnostics alarms. A persistent Wire break alarm at this stage points to a physical layer issue, not a configuration issue — re-verify the cable and the scale's power supply.

RCV_PTP Instruction Programming

The RCV_PTP instruction is part of the Point-to-Point library that ships with TIA Portal. Place the block from Instructions > Communication > Point-to-Point. It generates an instance DB that owns the receive buffer pointer.

Inputs:

• REQ — boolean trigger, latched. A rising edge starts the receive operation.
• PORT — HW identifier of the CM 1241 RS232 module. For slot 101 (first CM), this is decimal 269.
• BUFFER — pointer to a byte array in a global or instance DB. Example: P#DB100.DBX0.0 BYTE 1024.
• LEN — maximum number of bytes to copy into the buffer per call.

Outputs:

• DONE — TRUE for one cycle when a frame was received successfully.
• ERROR — TRUE for one cycle when an error occurred.
• STATUS — hex status word. 0x0000 means idle/no error. See the STATUS table in the diagnostics section.
• RCVD_LEN — actual bytes copied into the buffer on the last completed receive.

The canonical SCL block to receive continuously from a weighing scale in a cyclic OB looks like this:

// Weighing scale receive, SCL for use in OB1
IF "Scale_Run" AND "Scale_Trigger_Pulse" THEN
    "RCV_PTP_DB"(REQ := TRUE,
                 PORT := 269,                // HW-ID of CM 1241 RS232
                 BUFFER := P#DB100.DBX0.0 BYTE 1024,
                 LEN := 1024);
    IF "RCV_PTP_DB".DONE THEN
        "Scale_Frame_OK" := TRUE;
        "Scale_Bytes_Received" := "RCV_PTP_DB".RCVD_LEN;
        // Copy bytes to processing area or parse directly from DB100
        "Scale_Trigger_Pulse" := FALSE;     // arm next receive
    ELSIF "RCV_PTP_DB".ERROR THEN
        "Scale_Last_Status" := "RCV_PTP_DB".STATUS;
        "Scale_Trigger_Pulse" := FALSE;     // arm retry
    END_IF;
END_IF;

// Re-arm trigger on rising edge of a heartbeat or scan-cycle flag
IF "Heartbeat_1Hz" AND NOT "Heartbeat_Last" THEN
    "Scale_Trigger_Pulse" := TRUE;
END_IF;
"Heartbeat_Last" := "Heartbeat_1Hz";

Protocol Parameter Mapping Between HyperTerminal and TIA Portal

The Windows HyperTerminal session proves that the physical layer and the framing are correct: bytes are arriving at the PC's UART with the right polarity, at the right speed, with the right number of data bits and the right parity. The same five parameters must therefore be set identically in TIA Portal, even though the field names differ:

If any of these five values diverge by even one parameter, the UART in the CM 1241 will either mis-frame every byte (parity error, framing error) or fail to synchronize to the start bit (baud rate mismatch). HyperTerminal decodes the stream because it is set to the same values the scale is set to. Replicate exactly.

Trigger and Receive Buffer Management

The RCV_PTP instruction only completes a receive when one of three end conditions is met:

1. Termination character — the configured end-of-frame character (default 0x0A LF) is received. The complete frame, including the termination character, is copied into the buffer.
2. Buffer full — the configured buffer length is reached. The receive completes with the buffer partially filled. STATUS remains 0x0000.
3. Character timeout — no new character arrives within the configured timeout (in character times). The CM 1241 assumes the frame ended and copies whatever is in the receive FIFO.

Many weighing scales append CR LF (0x0D 0x0A) at the end of every frame. If the termination character is set to LF only, the trailing CR is copied into the buffer and the frame ends one byte after where it should. Set the termination character to whichever byte the scale appends last, or to a value that the scale's protocol never embeds inside the payload.

Buffer sizing: the receive buffer size in the port configuration sets the maximum frame length the CM 1241 will accept. The LEN input of RCV_PTP sets the maximum number of bytes copied into the instance DB per call. The two values are independent — set both to the same value to avoid truncation.

Diagnostic Procedures and STATUS Word Evaluation

When RCV_PTP completes with ERROR = TRUE, evaluate STATUS against the following table (hex values per the SIMATIC S7-1200 PTP instruction library reference):

Add an HMI tag or watch table entry that latches the STATUS value at every ERROR transition. The hex code alone resolves roughly 80% of "no data" symptoms without further instrumentation.

Common Pitfalls and Field-Proven Corrections

Verification Procedure

1. Place a watch table online with the CM 1241 module. Confirm zero diagnostics alarms.
2. Create a watch table for the receive DB and the instance DB of RCV_PTP. Observe DONE, RCVD_LEN, and the first 32 bytes of the buffer in real time.
3. Apply a stable load to the scale and verify a new frame appears in the buffer every cycle of the scale's transmission period.
4. Decode the buffer bytes as ASCII. Confirm the frame starts with the expected STX (0x02) or sign character and ends with the configured termination character.
5. Cross-check the gross/net/tare fields against the scale's display. A consistent offset of one byte typically points to a wrong termination character setting; a corrupted high bit points to a ground issue.
6. Disconnect the cable at the PLC and connect to a laptop running Tera Term or PuTTY with the same parameters. Verify identical bytes are received — if not, the scale itself is the variable.

Frequently Asked Questions