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Introduction

Table: Differences between synchronous and asynchronous data transfer
 
Asynchronous Data Transfer Synchronous Data Transfer
In asynchronous data transfer, a word or character is preceded by a start bit and is followed by a stop bit. The start bit is a logical 0. The stop bit(s) is (are) a logical 1. In synchronous data transfer, the transmission begins with a block header, which is a sequence of bits.
Data can be sent one character at a time. This can be used for transferring large amounts of data without frequent starts or stops.
When no data is sent over the line, it is maintained at an idle value, logic '1'. Since the data sent is synchronous, the end of data is indicated by the sync character(s). After that the line can be either low or high.
Parity bit can be included along with each word or character. Each character data can be 5, 6, 7 or 8 bits. Parity bit can be included along with each word or char-acter. Each character data can be 5, 6, 7 or 8 bits.
The start and stop bits are sent with each character. Generally, the stop bits may be either one or more bits. The stop bits must be sent at the end of the character. It is used to ensure that the start bit of the next character will cause a start bit transition on the line. In synchronous data transfer, the transmitter sends syn-chronous characters, which is a pattern of bits to indicate end of transmission.
Asynchronous mode data transfer is used for low speed data transfer. Data can move in simplex, half duplex and full duplex methods. Synchronous mode data transfer is used for high-speed data transfer. Data can move in simplex, half duplex and full duplex methods.
In this data transfer, the transmitter is not synchronised with the receiver by the same clock. The clock is an integral multiple of the baud rate (number of bits per second). Generally, this multiplication factor is 1, 16, or 64. In synchronous mode data transfer, the receiver and transmitter is perfectly synchronised on the same clock pulse.
Synchronisation between the receiver and transmitter is required only for the duration of a single character at a time. Synchronism between the transmitter and receiver is maintained over a block of characters.
Asynchronous data transfer can be implemented by hardware and software. Synchronous data transfer can be implemented by hardware.


This IC also receives serial data and converts them into parallel and sends the data in
parallel. It is available in a 28-pin dual in line package and has the following features:
  • Synchronous and asynchronous operation
  • Programmable data word length, parity and stop bits
  • Parity, overrun and framing error checking instructions and counting loop interactions
  • Programmed for three different baud rates
  • Supports up to 1.750 Mbps transmission rates
  • Divide-by 1, –16, –64 mode
  • False start bit deletion
  • The number of stop increase of asynchronous data transfer can be 1 bit, 1½ or 2 bits
  • Full duplex double buffered transmitter and receiver
  • Automatic break detection
  • Internal and external synchronous character detection
  • Peripheral modem control functions





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