Check out example codes for "arduino serial write". It will help you in understanding the concepts better.

Code Example 1

Writes binary data to the serial port. This data is sent as a byte or series of bytes; to send the characters representing the digits of a number use the print() function instead.

Serial.write(buf, len)

Serial: serial port object.
val: a value to send as a single byte.
str: a string to send as a series of bytes.
buf: an array to send as a series of bytes.
len: the number of bytes to be sent from the array.

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