Before we start to communicate with a serial device, we must know the communication parameters it uses, that is, which are the specific configuration settings of the serial data that we wish to transfer. So, we must know the speed, data bits, parity, and stop bits settings.
For the speed, only the fixed values are typically allowed; in fact, we must choose from 75, 110, 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200 bit/s.
In reality, other speed settings can be used. You should carefully read the datasheet of the serial device to check the allowed speeds. Regarding data bits, the usual setting is 8 (that is, 8 bits are used to transfer the information); we can even choose 6 (rarely used), 7 (for ASCII), 8, or 9 (rarely used). In the next examples, I'm going to use the value 8 for this setting.
Parity and stop bits are deeply related to the serial communication protocol, which I will not explain here, so you must forgive me if I don't spend many words...