如何通过Modbus TCP读取WAGO 753-483模拟输入模块的数据？

Let's break down the issues with your current setup and fix them step by step:

1. Fix the Read_AI Request Frame #

Your current Read_AI array has a critical mistake in the register count field (last two bytes set to 0,0), which tells the controller to read 0 registers—so you're getting no valid data back. Here's the corrected array:

byte Read_AI[12] = {0, 0, 0, 0, 0, 6, 1, 4, 0, 29, 0, 1};

Let's break down each part of the Modbus TCP frame to clarify why this works:

• Bytes 0-1: Transaction ID (can stay fixed or increment per request to match responses, either works for single-request workflows)
• Bytes 2-3: Protocol ID (fixed 0,0 for Modbus TCP)
• Bytes 4-5: Length of the following PDU (6 bytes total: Unit ID + Function Code + Start Address + Register Count)
• Byte 6: Unit ID (WAGO controller's station address, 1 is correct here as per your setup)
• Byte 7: Function Code (4 = Read Input Registers—this is the right choice for AI modules; FC2 is for discrete switches, FC3 is for writable holding registers, neither applies here)
• Bytes 8-9: Start register address (0,29 = decimal 29, which matches your documentation)
• Bytes 10-11: Register count (0,1 = read 1 register, since your voltage data lives in a single 16-bit input register)

2. Process the Response Data #

After sending the corrected Read_AI frame, you'll get a response from the WAGO controller. Here's how to parse it to get your voltage value:

Response Frame Structure #

A successful response will follow this format:

• Bytes 0-3: Match your request's Transaction/Protocol IDs
• Bytes 4-5: Length of the following data (should be 0,5 for this request)
• Byte 6: Unit ID (matches your request)
• Byte 7: Function Code (4 = no error; if it's 0x84, an exception occurred—check byte 8 for the error code)
• Byte 8: Number of data bytes (2 for 1 register)
• Bytes 9-10: The 16-bit register value (stored big-endian: high byte first)

Parse the Voltage Value #

Per your documentation, the data uses an 11-bit resolution (the two bytes correspond to the high/low parts of this 11-bit value). Assuming the 11 bits are the lower 11 bits of the 16-bit register (common for WAGO modules), here's how to calculate the actual voltage:

// Assume you've received the response into a byte array called 'response'
if (response[7] == 0x04) { // Check for successful read (FC4)
    if (response[8] == 2) { // Confirm we got 2 bytes of register data
        byte high_byte = response[9];
        byte low_byte = response[10];
        uint16_t raw_value = (high_byte << 8) | low_byte;
        
        // Extract the 11-bit effective value (mask with 0x7FF = 11 binary 1s)
        uint16_t effective_value = raw_value & 0x7FF;
        
        // Convert to voltage (0-10V range, 11-bit max value = 2047)
        double voltage = (effective_value / 2047.0) * 10.0;
        
        // Detect your target state (0V vs ~10V)
        if (voltage < 0.5) { // Adjust threshold to match your needs
            write("Voltage is near 0V");
        } else if (voltage > 9.5) {
            write("Voltage is near 10V");
        }
    }
} else {
    // Handle error cases
    write("Modbus read error - Exception code: " + String(response[8]));
}

3. Quick Validation Checks #

• Confirm your WAGO controller's Unit ID is indeed 1 (check the controller's web config if you're unsure)
• Double-check the register address: some docs use 1-based numbering, but Modbus uses 0-based—your request uses decimal 29 which matches the doc's "input register 29", so this is correct
• Ensure your TCP socket connection stays stable (you mentioned writes work, so this is likely not an issue)

内容的提问来源于stack exchange，提问作者Markus