Asynchronous Serial Communication

Asynchronous serial communication is a type of communication where data is transmitted one bit at a time, without any synchronization signal between the sender and receiver. In this type of communication, the physical layer provides two important functionalities, which are:

Start and Stop Signaling

The physical layer of the asynchronous serial communication provides start and stop signaling to the transmitted data. It means that at the beginning of each data byte, a start bit is transmitted, which indicates the start of the data byte. Similarly, at the end of each data byte, a stop bit is transmitted, which indicates the end of the data byte. The start and stop bits are used for synchronization between the sender and receiver.

Flow Control

The physical layer of the asynchronous serial communication also provides flow control to the transmitted data. Flow control is used to manage the rate of data transmission between the sender and receiver. In asynchronous serial communication, the flow control is achieved by using two techniques, which are:

- Hardware Flow Control: In hardware flow control, the sender and receiver use dedicated flow control lines to manage the flow of data. The sender uses a flow control line to indicate whether it is ready to transmit data or not, while the receiver uses a flow control line to indicate whether it can receive data or not.
- Software Flow Control: In software flow control, the sender and receiver use special characters to manage the flow of data. The sender sends a special character to indicate that it is ready to transmit data, while the receiver sends a special character to indicate that it can receive data.

Conclusion

In summary, the physical layer of the asynchronous serial communication provides both start and stop signaling as well as flow control to the transmitted data. These functionalities are essential for the proper transmission of data between the sender and receiver in asynchronous serial communication.

It makes use of two pins on the serial connector, RTS (Request to Send) and CTS (Clear to Send). These two lines allow the receiver and the transmitter to alert each other to their state. A transmitter raises its RTS line, which causes an interrupt on the receiver, If the receiver is in a position to receive the data it will check its CTS line, The raising and lowering of these lines allows device drivers which implement hardware flow control code to maintain a reliable data connection between transmitter and receiver.
These pins handles start and stop for data transfer. Also it will help to handle flow control with the above mechanism.