In a computer system, the CPU communicates with external devices using two main methods: parallel communication and serial communication. Parallel communication involves sending multiple bits of data simultaneously, while serial communication transmits data one bit at a time, sequentially.
**Classification of Serial Communication**
Based on how the data is synchronized during transmission, serial communication can be divided into two types: synchronous and asynchronous communication.
**1. Asynchronous Communication**
Asynchronous communication typically sends data in units of characters, where each character is framed into a specific format. The transmitting device sends one frame at a time, starting with a low signal and ending with a high signal. The receiving device captures these frames one by one through the transmission line. In this method, the transmitter and receiver use independent clocks, meaning they are not synchronized with each other.
The key to asynchronous communication is the character frame format, which helps the receiver know when a transmission starts and ends. This format is essential for proper data interpretation.
**Character Frame Structure**
A character frame, also known as a data frame, consists of four main components: a start bit, data bits, a parity bit (for error checking), and a stop bit. These elements ensure that both the sender and receiver can accurately interpret the data being transmitted.
**Baud Rate**
Another important factor in asynchronous communication is the baud rate. Baud rate refers to the number of signal changes per second, often used interchangeably with bits per second (bps). It indicates the speed of data transfer. A higher baud rate means faster data transmission. However, it's important to note that the actual character transmission rate depends on the character frame structure, as each frame includes additional control bits beyond just the data itself.
**2. Synchronous Communication**
Synchronous communication involves continuous, sequential data transmission, usually in larger blocks of information. Unlike asynchronous communication, which uses character-based framing, synchronous communication transfers entire data frames at once. These frames typically include a synchronization character, several data characters, and a check character (such as a CRC for error detection).
As shown in Figure 8.4, a single-sync character frame and a double-sync character frame both consist of three main parts: the sync character, data characters, and the check character. The sync characters can either follow a standard format or be custom-defined by the users. This method ensures precise timing and alignment between the sender and receiver, making it more efficient for large data transfers.
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