research would show that there are numerous ways for two arduinos to talk to one another. But i was looking for a 'basic robust protocol' (..it doesn't sound basic ....I know!), to allow for two arduinos to talk to one another. The idea is , Arduino One(slave), is linked to a sensor and it reads data from the sensor and sends the data to Arduino Two on 'request' of the data by Arduino Two(master). Arduino Two would have to 'request' for the data from Arduino One, before it can read the data, if its available. A popular method on most sites (e.g https://learn.sparkfun.com/tutorials/connecting-arduino-to-processing/shaking-hands-part-1) is the use of the method below. I added a comment in search of clarity.

              void establishContact()
                     {  //This while loop does baffle me as it suggests that
                        //while there is nothing in serial, send A???
                        while (Serial.available() <= 0)
                         Serial.println("A");   // send a capital A

Now this method was used in handshaking between Arduino and processing. This method was called on the Arduino sketch. On the processing sketch, a method there just checks to see if 'A' was received. If it is, it reads in the data from serial, clears the serial buffer and writes 'A' back to Serial. I tried using this method on both of my arduinos as i felt it was 'basic' enough, but it didn't work at all. Does anyone has a better idea on how my arduinos could handshake?.

1 Answer 1


That while loop makes perfect sense. It' basically nudging the other end repeatedly until it replies with something - anything.

It's like sitting next to someone and poking them in the ribs every 300ms until they respond to you. Once they have responded you can then react to that response - be that to run very fast or tell them your important news.

The idea of handshaking like that is all to do with synchronisation. It ensures that both ends are ready for communication at the same time. Once synchronised you know, to a high probability, that what you are sending will be interpreted properly, and any replies you get back will be correct.

For basic communication between two Arduinos it's really not necessary.

The simplest call-and-response protocol involves Arduino A sending a single byte as a request and Arduino B replying with a single byte. It is up to you what those bytes represent, but ASCII letters for requests are ideal since they are understandable by a human, and numbers between 0 and 255 or -128 and +127 fit in to one byte (unsigned or signed respectively). Should you want bigger numbers being sent back then the reply will have to be delimited in some way. The simplest way of doing that is to format the response as ASCII data and send it with a CR and/or LF.

Something like:

if (Serial.available()) {
    char command = Serial.read();
    switch (command) {
        case 'T':
        case 'P':

The master end then sends either 'T' or 'P' to request temperature or pressure. It then reads characters up to a line feed and stores them in a string which it can then convert into the desired numeric data type depending on what temperature and pressure are (integer or floating point).

  • Always to the rescue @Majenko....Thanks alot
    – dada
    Apr 10, 2016 at 9:59

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