When you own a smart meter (in Dutch it’s called slimme meter), you probably know that these meters have a port for reading the meter’s data. This port is called the P1 port. This article will describe how you can connect to the P1 port and read the data coming from the meter using a Netduino Plus 2.
The first thing to do is connection the P1 port to the Netduino. The P1 port is a standard RJ11 socket. The P1 port is a half duplex serial port, with signals on TTL-level (0-5V) and with 4 pins connected. The RJ11 connector schema below will show the pins with the signal on each pin. As soon as the RTS-pin gets voltage, a data package will be send through the RxD pin. As long the RTS-pin has voltage, every 10 seconds data will be send.
Normally serial ports on TTL-level will have an inverted signal. For the P1 port that’s not the case. Therefore you cannot connect the P1 port directly to the Netduino, but you will have to invert the signal first.
The schema below shows how to connect the P1 port to the Netduino using a HD74LS04P Signal Inverter IC. The Signal Invert IC contains 6 inverters, but only 2 are used.
Once you have the hardware connected, you can start programming.
Opening the P1 port
You will need to use the following port settings to be able to read the P1 port’s data: baud rate: 9600, data bits: 7, parity: Even and stop bits: 1. Below the code for opening the port. In the above schema the P1 port is connected to the Netduino using connector D2, which means COM2.
string comPort = SerialPorts.COM2;
int baudRate = 9600;
int dataBits = 7;
var stopBits = StopBits.One;
var parity = Parity.Even;
// Create serial port
var serialPort = new SerialPort(comPort, baudRate, parity, dataBits, stopBits);
// Open serial port
Reading the data
Below you will find an example of the message as read from a P1 port. The first line will give information about the meter. As you can see, my meter is a ‘ISKRA ME382’. In a next article I will describe what the other message lines mean. A message will always start with a ‘/’ and end with a ‘!’.
The code below will show how you can read the message form the P1 port. The first method will use an infinite loop to continuously read the ports data. After a message was received, the code will pause a while before continuing reading the next message.
private static void ReadDataFromP1Port(SerialPort serialPort)
if (serialPort.BytesToRead > 0)
var msg = ReadMessageFromSerialPort(serialPort);
if (msg != string.Empty)
// Wait a while before continuing
The next method will show how the message coming from the P1 port is processed and how the readable message is composed. Because the Netduino Plus 2 handles the serial port a bit different than a Netduino 2, it is necessary to remove the parity bit (see ‘& 127‘) from the incoming byte. If you do not remove the parity bit, your data will be garbage.
private static string ReadMessageFromSerialPort(SerialPort serialPort)
string msg = string.Empty;
int singleByte = 0;
// Read the message
while (serialPort.CanRead && singleByte != _StopChar && msg.Length < _ProtectAgainstOverflow)
// Read byte and remove parity bit
// Without removing the parity bit, the message will contain garbage on a Netduino Plus 2
singleByte = serialPort.ReadByte() & 127;
// Add character to message
msg += (char)singleByte;
// Flush the serial port
// If loop was exited without the last character being the stop character, the message was invalid, so return an emtpy message
if (singleByte != _StopChar)
// Return the message
These are the used variables
private static int _StopChar = 33; // = !
private static int _TimeToWaitBeforePressingNextMessageInMilliSeconds = 8000;
private static int _ProtectAgainstOverflow = 600;