Synchronous and Asynchronous Serial Communication

Synchronous Serial Communication

Synchronous communication requires common timing signals and common clock for synchronization. This means that as well as the data, the clock signals must also be transmitted along. In synchronous method a block of data at a time is transmitted.

Asynchronous Serial Communication and data framing

The data coming in the receiving end of the data line in a serial data transfer is all 1s and 0s; it is difficult to make sense of data unless the sender and receiver agree on a set of rules, a protocol on how the data is packed, how many bits constitute a character, and when the data begins and ends. 

Start and Stop bits 

• Asynchronous serial data communication is widely used for character-oriented transmission, and block-oriented data transfers use the Synchronous method.
• In the Asynchronous method, each character is put between a start and stop bits. This is called framing. In data framing, for asynchronous communications, the data such as ASCII characters are packed in between a start bit and stop bit. 
• The start bit is always one bit, but the stop bit(s) can be one or two bits. The start bit is always a 0 (low) and the stop bit(s) is 1 (high).
• The example below gives the framing of the ASCII character “A”, where, binary 0100 0001, is framed in between the start bit and two stop bits. Notice that the LSB is sent out first.

Framing of ASCII “A” (41H) 

• Notice that the transmission begins with a start bit followed by D0, the LSB, and then the rest of the bits until the MSB (D7), and finally, the 2 stop bits indicating the end of character “A”.
• In asynchronous serial communication, peripheral chips and modems can be programmed for data that is 5, 6, 7 or 8 bits wide.
• In some systems in order to maintain the data integrity, the parity bit of the character byte is included in the data frame.