To build a HART Universal Command 3 (Read Dynamic Variables and Loop Current) Long Frame, you need the 5-byte unique address of your Endress+Hauser CM82.
Below is the precise C function to construct the command string, along with an example byte output.
## 1. C Function to Build Command 3 Long Frame
This function sets the long frame start delimiter to 0x82, inserts the 5-byte device address, specifies Command 3 (0x03), assigns a data length of zero (0x00), and computes the mandatory XOR checksum (LRC).

#include <stdint.h>#include <string.h>
// Calculates the mandatory Longitudinal Redundancy Check (LRC)uint8_t calculate_lrc(const uint8_t *frame, size_t start_idx, size_t end_idx) {
    uint8_t lrc = 0;
    for (size_t i = start_idx; i < end_idx; i++) {
        lrc ^= frame[i];
    }
    return lrc;
}
/**
 * Builds a HART Command 3 Long Frame.
 * @param buf        Destination buffer (must be at least 14 bytes).
 * @param long_addr  Pointer to the 5-byte CM82 unique address array.
 * @return           Total number of bytes written to the buffer.
 */size_t build_cmd3_long_frame(uint8_t *buf, const uint8_t *long_addr) {
    size_t idx = 0;
    
    // 1. Preamble (CM82 requires minimum 5 bytes of 0xFF)
    for (int i = 0; i < 5; i++) {
        buf[idx++] = 0xFF;
    }
    
    // Checksum calculation must start directly after the preamble bytes
    size_t checksum_start = idx; 
    
    // 2. Start Delimiter (0x82 = Master-to-Slave, Long Frame)
    buf[idx++] = 0x82;
    
    // 3. Address Field (5-byte Unique Device ID)
    memcpy(&buf[idx], long_addr, 5);
    idx += 5;
    
    // 4. Command Byte (0x03 = Read Dynamic Variables and Loop Current)
    buf[idx++] = 0x03;
    
    // 5. Data Byte Count (0x00 = No data bytes sent in request)
    buf[idx++] = 0x00;
    
    size_t checksum_end = idx;
    
    // 6. Checksum (LRC)
    buf[idx++] = calculate_lrc(buf, checksum_start, checksum_end);
    
    return idx;
}

## 2. Example Raw Byte String Output
Assuming your CM82 has an example 5-byte unique address of 11 9C 02 AB CD, the resulting 14-byte serial stream layout looks exactly like this:

| Byte Index | Hex Value | Description |
|---|---|---|
| 0 - 4 | FF FF FF FF FF | Preamble (Synchronization) |
| 5 | 82 | Start Delimiter (Long Frame Master-to-Slave) |
| 6 | 11 | Manufacturer ID (Endress+Hauser) |
| 7 | 9C | Expanded Device Type |
| 8 - 10 | 02 AB CD | Device Identification Number |
| 11 | 03 | HART Command 3 |
| 12 | 00 | Data Byte Count (0 bytes) |
| 13 | 3B | Checksum / LRC (82 ^ 11 ^ 9C ^ 02 ^ AB ^ CD ^ 03 ^ 00) |

Raw Byte Array String:
\xFF\xFF\xFF\xFF\xFF\x82\x11\x9C\x02\xAB\xCD\x03\x00\x3B
------------------------------
Would you like the C parsing code to extract the Loop Current and IEEE-754 floating-point process variables from the slave response frame, or do you need help setting up the UART serial port settings for the transmission?