Sending serial bits in arduino

Question

I am a newbie in Arduino and serial communication. I am working on a project that use Arduino to send data bits to an actuator using RS232 communication protocol. In order to control the actuator, an instruction consists of 6 bytes must be sent with 9600 baud, no hand shaking, 8N1 format.

Here are the instructions: Byte 1 - Device, Byte 2 - Command, Byte 3 - Data(LSB), Byte 4 - Data, Byte 5 - Data, Byte 6 - Data(MSB)

For example to send 1, 20, 1, 1, 0, 0 (Move the actuator to absolute position of 257 microsteps) This is my draft code. How can I send start and stop bit? I am confused on sending start and stop bit. The Serial.write() is sending binary data as a byte. Does it mean that I write Serial.write(20), the compiler will send the data in 00010100 binary format? I am quite frustrated with serial communication.

Answer 1

The start and stop bit are already included as part of the UART protocol. When you send a byte it first sends the start bit, then the byte, then the stop bit. You don't need to do anything to make those bits send.

20 is actually sent as 0001010001

So your code will just be a series of Serial.write() calls for each byte you want to send.

Serial.write(1);
Serial.write(20);
Serial.write(1);
Serial.write(1);
Serial.write(0);
Serial.write(0);

Or as an array:

uint8_t data[6] = {1, 20, 1, 1, 0, 0};
Serial.write(data, 6);

Answer 2

Read the language reference: https://www.arduino.cc/en/Reference/Serial
..and for write(): https://www.arduino.cc/en/Serial/Write
....notice that it "writes" the bytes or buffers you define. You have already defined the baud rate and started the serial channel, then all you need to do is pass the characters or strings you want to send to the write function.

In your application it may be best to build a 6 character command string and send it all at once as a string. You must then remember that With "write" you are responsible for "writing" the cr and lf which is probably needed as a command terminator in your receiving device.

For your kind of application it may also be possible to use "println" which appends a cr/lf automatically for you. It depends on your protocol.