Modbus CRC-16 used instead us regular CRC-16 on TX of messages coming for Jeenodes

[paularmand]

On receive the regular CRC-16 of the jeenodes is correctly calculated on the FLM side, but on transmit a different modbus CRC-16 is used to transmit the messages back to the Jeenodes. This requires some additional code on the jeenode to check message integrity.

// Compute the MODBUS RTU CRC
uint16_t modbus_crc (volatile uint8_t buf[], int len)
{
  uint16_t crc = 0xFFFF;

  for (int pos = -1; pos < len; pos++) {
    if (pos == -1) {
      crc ^= (uint16_t)0x2D;          // Added the sync byte
    }
    else {
      crc ^= (uint16_t)buf[pos];          // XOR byte into least sig. byte of crc
    }

    for (int i = 8; i != 0; i--) {    // Loop over each bit
      if ((crc & 0x0001) != 0) {      // If the LSB is set
        crc >>= 1;                    // Shift right and XOR 0xA001
        crc ^= 0xA001;
      }
      else                            // Else LSB is not set
        crc >>= 1;                    // Just shift right
    }
  }
  // Note, this number has low and high bytes swapped, so use it accordingly (or swap bytes)
  uint8_t temp_high_value = (uint8_t)(crc >> 8);      // Save high byte value
  crc <<= 8;                                          // Store low byte value in high byte
  crc = crc | temp_high_value;

  return crc;
}

[icarus75]

CRC-16 calculation in the FLM when tx'ing jeenode-type packets is correct. Should you still experience CRC-type issues, please re-open this FR.