send method
Send request to the remote server
Implementation
@override
Future<ProtocolDataUnit> send(int slaveId, ProtocolDataUnit request) async {
_transactionId = (_transactionId + 1) & 0xFFFF;
final (head, adu) = _encodeTCPFrame(_transactionId, slaveId, request);
final aduResponse = await sendRawFrame(adu);
final (rspHead, pdu) = _decodeTCPFrame(aduResponse);
final response = ProtocolDataUnit(pdu[0], pdu.sublist(1));
_verifyTCPFrame(head, rspHead, request, response);
return response;
}