SERIAL COMMUNICATION STANDARDS
Most of devices are parallel in nature. These devices transfer data simultaneously on data lines. But parallel data transfer process is very complicated and expensive. Hence in some situations the serial I/O mode is used where one bit is transferred over a single line at a time. In this type of transmission parallel word is converted into a stream of serial bits which is known as parallel to serial conversion. The rate of transmission in serial mode is BAUD, i.e., bits per second. The serial data transmission involves starting, end of transmission, error verification bits along with the data. Any serial I/O involves the following concepts.
(a) Interfacing requirements (b) Alphanumeric codes (c) Transmission format (d) Error checks in data communication (e) Data communication over lines (f) Standards in serial I/O
The microprocessor has to identify the port address to perform read or write operation. Serial I/O uses only one data line, chip select, read, write control signals.
Fig. 5.1 Block diagram of serial I/O interfacing
Data transfer takes place using ASCII code (American standard code for Information Interchange) which is 7 bit code with 128 combinations. The data can be transmitted by taking various parameters into consideration such as synchronization or synchronization, direction of data flow speed, errors, medium of data transmission etc. In synchronous transmission both transmitter and receiver operate, in synchronous to each other.
Synchronization used for high speed operations. In asynchronous data transmission data is transmitted between Start and Stop bits with logic 1 as mark logic 0 as space. In asynchronous we get around 11 bits for data transmission one start, 8 bits of data, 2 stop bits. A synchronous data transmission is used for less than 20 Kbits /second transmission.