This documents the serial protocol when taking to an Arduino Duemilanove or Uno, or similar device via its serial line (connected to the host via USB)

The serial line is fixed at 115200 baud, 8-bits, 1 stop bit and no parity.

In the following commands, the pin number is represented by a binary number, so if typing the command in via (e.g.) minicom, then to represent pin number 3 you need to type a Control-C character. All the command codes are lower-case letterers.

• Set pin mode: There are 3 commands to set the mode of a digital pin: ‘o‘ (0x6F) sets it to output, ‘i‘ (0x69) sets to input and ‘p‘ (0x71) sets it to PWM output. These are then followed by the pin number. So to set pin 3 to output, send the following 2 bytes: 0x6F 0x03. There is no acknowledgement to this command.

• DigitalWrite: Send the character ‘0‘ (0x30) or ‘1‘ (0x31) to indicate a logic low or high followed by the pin number. So to set pin 5 to high, send the following 2 bytes: 0x31 0x05. (Digit ‘1‘ followed by numeric 5) To set pin 13 low, send 0x30 0x0D.

• PwmWrite: Send the character ‘v‘ (0x76) followed by the pin number then a third byte to represent the value from zero to 255. E.g. To set pin 9 to a duty cycle of 50%, send: 0x76 0x09 0x80.

• DigitalRead: Send the character ‘r‘ (0x72) followed by the pin number to read the state of that pin. A single byte will be returned, ‘0‘ or ‘1‘ representing the logic level of the pin. E.g. To read pin 12; Send the two bytes: 0x72 0x0C then read a single byte expecting ‘0‘ (0x30) or ‘1‘ (0x31) in return to respect the logic levels low or high.

• AnalogRead: Send the character ‘a‘ (0x61) followed by the pin number to read the analogue value on that pin. Two bytes will be returned, the most significant byte first. E.g. To read analog pin 4, send 0x61 0x04 then read two bytes back. If the device were to send back 0x01 then 0x5C, the combined number is 0x015C which is 384 decimal.

• Ping: To test for basic operation, then you can send a ping command – this is ‘@‘ or 0x40. The remote device should return a single ‘@‘ in return. You may need to flush any serial input buffers and send one or more ping commands to a device to establish initial connectivity.

When testing from a terminal (e.g. minicom) then there is two additional commands you can give – ‘A‘ (capital A). This causes the unit to cycle, reading and printing the analog ports until you press the space-bar. Similarly, ‘X‘ will read and print the digital inputs in a loop until you press the space-bar. This should allow you to test all the functions on the target device before connecting up a program on the host.