Reading Rice Lake Scale Data on a Siemens S7-1200 Through an RS-232 to Ethernet Converter

This reference walks through a field-proven configuration that pulls continuous weight data from a Rice Lake 400-series indicator (60,000 kg capacity) into a Siemens SIMATIC S7-1214 (CPU firmware 4.4 with TIA Portal V16) using a USR-TCP232-304 serial-to-Ethernet converter. The pattern is general: any S7-1200 with PROFINET/Ethernet interface (CPU firmware 4.2 or later recommended for Open User Communication stability) can be wired to any TCP-capable serial device server exposing a continuous-mode serial scale.

1. System Architecture and Data Flow

The communication chain has three nodes and one protocol translation step:

The PLC functions as the TCP client / active connection partner and opens a single socket to the converter, which is configured as a TCP server. The converter transparently forwards the bytes from the indicator's serial port into the TCP stream and vice versa. Open User Communication (OUC) is the Siemens-native way to handle this on the S7-1200, using the standard blocks TCON, TRCV, TSEND, and TDISCON from the "Communication" palette in TIA Portal.

2. Prerequisites

3. RS-232 Wiring Between the Indicator and the Converter

Most industrial indicators expose a female DB9 RS-232 port. Use a shielded cable, keep it under 15 m, and tie the shield to ground at one end only (the converter end is typical).

4. Configuring the USR-TCP232-304

Out of the box the converter runs a web configuration on http://192.168.0.200. The relevant parameters and the recommended values for this application are listed below.

5. Rice Lake 400-Series Continuous Output Format

The 400-series indicators in continuous mode emit one ASCII frame per weighment. A typical stable gross reading looks like:

ST,GS,+012345.67,kg<CR><LF>

Field-by-field:

The complete frame is 21 to 24 bytes depending on unit length. CR LF is the deterministic delimiter the parser should anchor on.

6. TIA Portal V16 Project Setup

1. Create a new project in TIA Portal V16 with the exact CPU order number, e.g. 6ES7214-1AG40-0XB0.
2. In Device View, open the CPU properties → Ethernet addresses and assign a static IP (e.g. 192.168.0.10) and subnet mask (255.255.255.0).
3. Confirm the CPU's PROFINET interface is connected to the same physical switch as the converter.
4. From the right-hand Instructions palette, expand Communication > Open User Communication and drag the TCON, TRCV, TDISCON, and TSEND blocks into the program (e.g., OB1 or a dedicated FB). Each will get its own instance DB.

7. TCON Connection Parameter Block

TCON takes a TCON_IP_v4 structure as the CONNECT input. Configure it as follows for the S7-1200 to act as the active partner that opens the socket to the converter:

// TCON_IP_v4
tconParams.InterfaceId       := 64;                  // Local PROFINET interface (HW ID)
tconParams.ID                := 1;                   // Connection ID (1..4095, unique)
tconParams.ConnectionType    := 16#0B;               // 0x0B = TCP/IP
tconParams.ActiveEstablished := TRUE;                // TRUE = PLC is the client
tconParams.RemoteAddress.ADDR[1] := 192;            // 192.168.0.200
tconParams.RemoteAddress.ADDR[2] := 168;
tconParams.RemoteAddress.ADDR[3] := 0;
tconParams.RemoteAddress.ADDR[4]   := 200;
tconParams.RemotePort        := 20108;               // USR converter port
tconParams.LocalPort         := 0;                   // Auto-assign (any)

Call TCON in OB1 (or in a startup FB) and gate the call with a one-shot REQ edge so the connection is only requested once.

// Connection start (rising edge from HMI or first scan)
"instTCON"(REQ := "startComm",
           ID  := 1,
           CONNECT := "cfgConn".tconParams,
           DONE => "statConn".done,
           BUSY => "statConn".busy,
           ERROR => "statConn".err,
           STATUS => "statConn".status);

8. TRCV with AdHoc Mode

This is the most important section for resolving the "connection is up but I get no data" failure. AdHoc mode is required whenever the application cannot predict the message length in advance — which is the case for a free-running continuous serial stream.

Configure the TRCV instance as follows:

// TRCV in AdHoc mode
"instTRCV"(EN_R := "statConn".done AND NOT "statConn".trcvBusy,
           ID   := 1,
           LEN  := 0,                                 // 0 = AdHoc
           DATA := "rcvBuf".bytes,                   // BYTE array
           NDR  => "statConn".ndr,
           ERROR => "statConn".rcvErr,
           STATUS => "statConn".rcvStatus,
           RCVD_LEN => "statConn".rcvLen);

Each successful receive sets NDR := TRUE for one scan and returns the bytes copied into DATA, with the byte count in RCVD_LEN. The application code must:

1. Append the received bytes to a circular / line buffer.
2. Scan the buffer for the CR LF (0x0D 0x0A) terminator.
3. When a complete frame is found, extract the weight value and clear the consumed bytes.
4. Reset NDR in the next cycle (the block does this automatically when re-triggered).

9. State Machine for Frame Assembly

Because the PLC can be called faster or slower than the indicator transmits, a small state machine is the cleanest implementation:

States:

• IDLE → first scan; jump to WAIT_CONN.
• WAIT_CONN → call TCON. Stay until TCON.DONE = TRUE.
• RECEIVING → call TRCV every cycle. On NDR, append to lineBuf.
• FRAME_OK → CR LF detected. Parse fields. Hand the value to the application. Return to RECEIVING.

10. Parsing the Weight Value

Once a complete frame is in lineBuf (terminated by 0D 0A), the parser must isolate the third comma-delimited field. A clean SCL implementation uses a small state counter:

// Pseudo-SCL parser
FOR i := 0 TO lineLen - 1 DO
    IF lineBuf[i] = 16#2C THEN           // ','
        commaCnt := commaCnt + 1;
    ELSIF commaCnt = 2 THEN              // Inside the value field
        IF lineBuf[i] = 16#0D THEN       // End of frame
            EXIT;
        END_IF;
        valueStr := CONCAT(valueStr, CHAR_TO_STRING(CHAR#lineBuf[i]));
    END_IF;
END_FOR;

// Convert "+012345.67" to REAL
weightReal := STRING_TO_REAL(valueStr);
weightKg   := weightReal;               // kg as-is for this indicator

Edge cases that must be handled:

• US (unstable) — the indicator still emits a value; reject values flagged US in the application logic to avoid latching a moving value.
• OL (overload) — status OL should clamp the reading to the rated capacity (60,000 kg for this scale) and raise a maintenance alarm.
• Unit changes — if the operator switches units on the indicator, the parser still extracts the numeric portion, but the engineering conversion must be updated. Lock the unit on the indicator if possible.

11. Complete Program Skeleton (SCL)

FUNCTION_BLOCK "fbScaleComm"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
VAR
    tconInst   : TCON;        // instance DB auto-generated
    trcvInst   : TRCV;
    tdisInst   : TDISCON;
    tconParams : TCON_IP_v4;
    state      : INT;         // 0=IDLE, 1=WAIT_CONN, 2=RECEIVING, 3=FRAME_OK
    rcvBuf     : ARRAY[0..511] OF BYTE;
    lineBuf    : ARRAY[0..63] OF BYTE;
    lineLen    : INT;
    ndrOld     : BOOL;
    weightKg   : REAL;
    status     : WORD;
END_VAR
BEGIN
    CASE state OF
        0:  // IDLE
            state := 1;

        1:  // WAIT_CONN
            tconInst(REQ := TRUE,
                     ID  := 1,
                     CONNECT := tconParams,
                     DONE => , BUSY => , ERROR => , STATUS => status);
            IF status = 0 THEN
                state := 2;          // 7000h means established and idle
            ELSIF status = 16#7002 THEN
                ;                    // pending, keep waiting
            ELSE
                ;                    // log error
            END_IF;

        2:  // RECEIVING
            trcvInst(EN_R := TRUE, ID := 1, LEN := 0, DATA := rcvBuf,
                     NDR => ndr, ERROR => , STATUS => status, RCVD_LEN => rcvLen);
            IF ndr AND NOT ndrOld THEN
                // Append rcvBuf[0..rcvLen-1] to lineBuf
                ndrOld := TRUE;
                // Scan lineBuf for 0D 0A; if found, state := 3
            ELSIF NOT ndr THEN
                ndrOld := FALSE;
            END_IF;

        3:  // FRAME_OK
            // Parse lineBuf into weightKg
            // Reset lineLen := 0
            state := 2;
    END_CASE;
END_FUNCTION_BLOCK

12. Status and Error Code Reference

When the connection or receive fails, the STATUS output of the block identifies the cause. The most relevant codes are:

A full list is published in the SIMATIC S7-1200 Programmable Controller System Manual, section on Open User Communication error codes.

13. Troubleshooting Matrix

14. Verification and Commissioning

1. Converter reachability — From the engineering PC, ping 192.168.0.200. Expect < 1 ms on a managed switch.
2. End-to-end serial loopback — Open PuTTY raw TCP to 192.168.0.200:20108. Force a weighment on the indicator. A new line of ASCII should appear every cycle.
3. PLC online monitoring — In TIA Portal, go online and watch TCON.STATUS and TRCV.NDR. TCON.STATUS should settle to 0000h after one or two cycles; TRCV.NDR should pulse TRUE on each received chunk.
4. Watch table parse — Add the parser's lineBuf and weightKg to a watch table. Force a few weighments and confirm the parsed value tracks the indicator display.
5. Stability test — Leave the system running for at least one shift with motion on the scale. Watch the connection status and the weightKg value. Any drift toward US status or sudden zeros indicates a framing problem.
6. Alarm mapping — Wire ST vs US to a Boolean tag, and OL to a maintenance alarm. Confirm these surface correctly in the HMI.

15. Performance and Timing Notes

The S7-1200 user-program cycle is the bottleneck for how quickly the application can react to a new frame. Typical values:

If the application is reading fast-changing values, use a time-stamped capture (S7-1200 RD_SYS_T in the same cycle as the parser) so downstream consumers know exactly when the weight was sampled at the indicator.

16. Safety and EMC Considerations

• Route the RS-232 cable physically separate from VFD power cables; cross at 90° angles when necessary.
• Tie the cable shield to functional earth (FE) at the converter end only, using a low-impedance bond (short pigtail or 360° clamp).
• If the scale is in an Ex or harsh industrial area, fit a surge protector (e.g., Phoenix Contact DT-LAN-ARGO) on the Ethernet side of the converter.
• Use a managed switch with port-level MAC/IP filtering to prevent a rogue device on the plant network from injecting false weight frames.
• For load cells used in lifting or personnel applications, the weight path must be considered a safety function. This OUC integration is for informational/process data only; safety-rated weighing requires a SIL-rated indicator and dual-channel architecture.

17. Frequently Asked Questions