@param num (optional): maximum number of bytes to read. If non-zero, serialRead will read up to num characters from the buffer. If 0 or left out, serialRead will read up to and including the first newline character or the end of the buffer. Note that the returned string may not end in a newline if this character is not present in the buffer.
Reads characters from the serial port. The string is allowed to contain any character, including 0.
@status current Introduced in 2.3.8
none
str string. Empty if no new characters were available.