Any tips on Modbus over RS485 on MDBT42Q?

Ahh - so in my case I was controlling an electricity meter and there were only 4 commands - so I just pulled those - including CRC - out of the datasheet.

But the calculation looks ok - there's even a flowchart on page 40 of https://modbus.org/docs/Modbus_over_serial_line_V1_02.pdf which seems to be implemented in https://stackoverflow.com/a/19994882/1215872

So... this would appear to work - at least it produces the values they have in their example:

function addCRC16(buf) {
  // https://stackoverflow.com/a/19994882/1215872
  var crc = 0xFFFF;
  for (var pos=0;pos<buf.length;pos++) {
    crc ^= buf[pos];    // XOR byte into least sig. byte of crc
    for (var 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
      }
    }

  buf.push(crc&0xFF, crc>>8);
  return buf;
}

var message = [0x02, 0x07];
addCRC16(message);
Serial1.write(message);