Introduction to Communication

 Introduction to Communication Interface

1. Parallel Data Transmission

Parallel ports were originally developed by IBM as a way to connect a printer to a PC. When IBM was in the process of designing the PC, the company wanted the computer to work with printers offered by Centronics, a top printer manufacturer at the time. IBM decided not to use the same port interface on the computer that Centronics used on the printer. Instead, IBM engineers coupled a 25-pin connector, DB-25, with a 36-pin Centronics connector to create a special cable to connect the printer to the computer. Other printer manufacturers ended up adopting the Centronics interface, making this strange hybrid cable an unlikely de facto standard.

When a PC sends data to a printer or other device using a parallel port, it sends 8 bits of data (1 byte) at a time. These 8 bits are transmitted parallel to each other. The standard parallel port is capable of sending 50 to 100 kilobytes of data per second.

Advantages of Parallel Data Transmission:

•  Fastest form of transmission -- able to send multiple bits simultaneously
•  doesn’t require high frequency of operation

Disadvantages of Parallel Data Transmission:

•  Requires separate lines for each bit of a word
•  Costly to run long distances due to multiple wires
•  Suffers from electromagnetic interference
•  Cable lengths more limited than a serial cable

Applications: Parallel ports can be used to connect a host of popular computer peripherals: such as prints, scanners, CD burners, external hard drives, Iomega zip, network adapters, and tape backup drives.

Types of parallel port

At the present time it is known four types of parallel port:

• Standard parallel port (SPP)
• Parallel port PS/2 (bidirectional)
• Enhanced Parallel Port (EPP)
• Extend Capability Port (ECP)

SPP/EPP/ECP The original specification for parallel ports was unidirectional, meaning that data only traveled in one direction for each pin. With the introduction of the PS/2 in 1987, IBM offered a new bidirectional parallel port design. This mode is commonly known as Standard Parallel Port (SPP) and has completely replaced the original design. Bidirectional communication allows each device to receive data as well as transmit it. Many devices use the eight pins (2 through 9) originally designated for data. Using the same eight pins limits communication to half-duplex, meaning that information can only travel in one direction at a time. But pins 18 through 25, originally just used as grounds, can be used as data pins also. This allows for full-duplex (both directions at the same time) communication.

2. Serial Data Transmission:

1. Synchronous Data Transmission: 

Data is transmitted one bit at a time, using a clock to maintain integrity between words.
Advantages:

• Only one (half duplex) or two (full duplex) wires are required to send/receive data.
• Low cost due to low number of wires.

Disadvantages:

• Lower speeds than parallel transmissions.
• Difficult to maintain data integrity due to problems with synchronizing clocks.

2. Asynchronous Data Transmission:

Data is transmitted on bit at a time using start bits and strop bits to maintain integrity between words.

Disadvantages:   Lower speeds than parallel transmissions.

                                    

Key words

Baud Rate:

The measure of the number of signal elements transmitted or received per second. Baud rates and data bit rates (bps-bit per second) are not equal in asynchronous transmission due to the start and stop bits.

Start Bit:

The bit preceding every word that signals the receiver a data word is coming. In some microcontroller (e.g., HC11) the start bit is logic low (0), while in others the start bit is logic high (1).

Parity Bit:

A bit sometimes added to the end of the data word. There are three possible settings for the parity: none, even, and odd. The setting represents the sum of the 1’s transmitted.

Stop Bit:

The bit or bits following every word that signals the end of a data word. In some microcontroller (e.g., HC11) the stop bit is logic high (1), while in others the start bit is logic low (0).

Half Duplex:

Two-way serial communication using only one line. With half duplex, the device can not transmit and received at the same time.

Full Duplex:

Two-way serial communication using two lines. With full duplex, data can be simultaneously transmitted and received.