Modes of Data Transfer

Modes of Data Transfer

Data Transfer along the communication media can take place in two ways: -

. Serial
. Parallel

Serial Data Transmission

If data is sent only bit at a time, then only one data line is required, along with the ground or signal return, as shown in Fig. 1.11. This is referred to as serial communications. Since information in the computer is usually stored and moved in parallel on the system buses, the data to be sent from a port to a device must be converted from parallel to serial. In Fig. 1.11, module A converts the parallel input data to serial form and sends it to module B in accordance with the clock input to module A.

Fig. 1.11 Serial Communications

Then why use serial communications? For distance longer than a few feet (room-to-room, building-to-building, etc.), the expenses and difficulty of installing groups of parallel wires increases greatly. Also, noise pickup and signal deterioration increases rapidly on transmission lines as distance increases. In order to reduce noise pickup, sometimes a separate ground wire is twisted together with each signal wire. Thus, in these cases, the number of wires will double and increase the cost even more for parallel transmission. Serial transmission is used to overcome these problems.

Serial data transmission is the one normally chosen for long-distance data communications, and most microcomputers have at least one serial data port, typically using an RS232 (serial) connector for data communication purpose.

This is in contrast to the parallel scheme where are number of bits are sent at the same time. The advantage of a serial scheme is that it only requires one wire for each communication channel, and one other wire for the current return path. Therefore a perfectly serviceable two-way communication scheme can be set up between say, a terminal and a computer, using only three wires.

Parallel Data Transmission

Data can be sent either one bit at a time or several bits at a time. When more than one bit of data is sent at a time, the data transfer is said to be in parallel.

Typically, parallel transfers are in a multiple of eight bits and require a data bus of eight or more lines as illustrated in Fig. 1.12. Recall that eight bits are sent for each data transfer under the control of the Send and Reply control signals.

Parallel data transmission involves simultaneous transmission of bits along several wires-usually nice. Parallel data transmission requires that bits be assembled at starting gates like race horses and simultaneously released down parallel wires when all bits are ready. Because nice wires are used to transmit messages, an entire character can be transmitted at once, rather than one bit at a time, and communications speeds are enhanced accordingly. This is one reason why many microcomputers use parallel data transmission for printers and the other peripheral equipment, typically using Centronics (Parallel) connectors.

Fig. 1.12 Parallel Data Transmission

Transmission Modes

Transmission Modes

The transmission mode enables the receiving computer to know where one byte ends and the next byte begins on the transmission medium. The two transmission modes are: -

. Asynchronous Transmission
. Synchronous Transmission

Asynchronous Transmission

Asynchronous Transmission is also called as start and stop transmission. In this case a sender can send a character at any time convenient and the receiver will accept it. In this case data is transmitted character by character at irregular intervals. Each character is preceded with a start bit and succeeded with a stop bit.

This kind of transmission is well suited for keyboard type terminals and tape devices. The advantage of using this kind of transmission is that it does not require any local storage at the computer. Therefore it is economical to implement.

The main disadvantage of using asynchronous transmission is the idle time during character transmission.

Synchronous Transmission

In case of Synchronous Transmission, it involves a group of characters in the form of a block and each block is framed by a header and trailer information. The header contains synchronizing information used by the receiving device to set the clock time in synchronize with the clock time of sender. The trailer consists of information about sender and receiver, this kind of transmission is well suited for remote communication between a computer and devices such as buffered card reader and printers. Here the block length is about 80-130 characters long. It is also used with buffered terminals for communication in a network. The primary advantage of using synchronous transmission is efficiency because it offers higher data rate and lesser time difference between the blocks as compared to asynchronous transmission.