Serial communication

Arduino and simple Serial Communication

In order to establish communication with the Arduino, a serial protocol is used. This communication occurs when a script or program is uploaded to the Arduino, but is also possible as part of an interaction scheme, for instance when sensor values need to be visualized in a user interface. The serial communication can be explained in its most basic form as a transport line of individual characters between the Arduino and a computer. It is possible to deviate from this basic form; that will be explained later.

This section explains how to set up such simple communication between the Arduino board and the applications Max and Pd. Since these platforms are rather similar, patches for Max and Pd look somewhat similar. The Arduino can send data to Max/Pd, or it can receive data from it. In either case, data is represented by individual characters that are sent one at a time. Rather than the actual character, the transport line expects a numerical representation of that character. These representations are stored in a so called ASCII (pronounce: askey) table, which binds e.g. the letter ‘A’ to the value 65, or the character ‘7’ to the value 55. When sending data from Arduino, those ASCII representations remain obscure. All that is required is using the Serial.println command.

Serial.println("az");
int x = 17;
Serial.println(x);

The three lines above represents Arduino code that first sends the letters ‘a’ and ‘z’ over the serial port, and after that the value for x, which is 17, is sent. As said before, when sending, nothing shows that println uses ASCII representations. But at the receiving end, is will be ASCII that is received. The objects used are serial in Max and comport in Pd. The comport object is not vanilla and needs to be installed. Here is a how-to.

Max (left) and Pd (right) — connecting to a serial port

The connection to the serial port is created by providing a port reference. In Max this is a letter, and in Pd a number. With a specific message the information on available ports and their references can be printed. Arduino’s serial port is typically listed as ‘usbmodem’. The next step is to provide this identifier as argument, as well as the speed at which the communication is set up. The patches below show just this. The Max object that reads ‘poll’ is an attrui and is the easiest way to get data from serial.

Serial data sent to Max or Pd

The numbers that are printed out are in order: 97, 122, 13, 10, 49, 55, 13 and 10. After one second the exact same sequence appears. These values represent characters according to the before mentioned ASCII table. According to this table, the values 97 and 122 represent the characters ‘a’ and ‘z’, the values 49 and 55 the characters ‘1’ and ‘7’—indeed, numerical values are sent as individual characters. It is easy to see how this accords with the messages sent from the Arduino code. Twice, a combination of 13 and 10 appears. The ASCI table names these values ‘line feed’ (LF) and ‘carriage return’ (CR) respectively—terminology that suggests printer activity. These two values are inherent to the Arduino ‘println’ command and can be used in our patches as indicators that a complete message has been received.

Printing interpreted data from the serial connection

Max and Pd can transform the ascii values to the characters they represent using either the atoi or fudiparse object.

Important to take into account is that only a single application at a time can connect with the Arduino board over the serial connection. In order to allow one application to open the connection, another must close it.