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May 18, 2015

Introduction to Communication Interfaces and Standards – Part 2

by 4hathacker  |  in Robotronics at  6:39 PM

We have already discussed about the two wide classes of communication interfaces viz. serial and Parallel. Before going further in depth, let us discuss some important terms related to serial communication

1. Baud Rate – It is the measure of no. of signal elements or symbols Transmitted or received per second. Baud rate and Bit rate (data bit rate in bps i.e., bits per second) are different in asynchronous transmission due to the start and stop bits used during the transmission.
2.  Start Bit – The bit preceding every word that signals the receiver a data word is coming. It may be logic zero or logic one.
3. Parity Bit – When a bit is added at the end of data word. There are three cases for adding a parity: no parity bit, even parity bit and odd parity bit. These all even or odd or none only relates to the no. of ones transmitted.
4. Half Duplex – A two-way serial communication using only one line which means that line is required to transmit or to receive the data at a time.
5. Full Duplex – A two-way serial communication using two lines and here the data can be simultaneously transmitted and received.

One more important question:
What is Preamble in communication and what is the need of preamble in Communication?
Preamble is generally used in network communication where no. of transmitters or receivers is more than one. To actively work with a no. of transmitters and receivers an extra sequence of known bits is also sent or received along with the data bits. They are used as a reference signal identifying a frame to start. It is also known as sync word. It probably means that the data is about to be sent. It is also used to synchronize transmission timing between two or more systems.

Protocol:
Definition: A protocol is a standard adopted, which tells the way in which the bits of a frame must be sent from a device (or controller) to another device or system. It gives us knowledge about the frame of bits:
1. Sent in which mode of serial communication
2. Whether header bits are there or not
3. When a no. of devices are communicating with each other so only destined device activates and receives the bits without any interference
4. Effects the definition of address of transmitting device so receiving device can identify the transmitter
5. Frame length and frame size the receiving device aware of
6. Contents of frame specifying a command or control or data
7. Possess any error to detect and correct
8. Giving Minimum and maximum length permitted in a single frame of bit

Getting back on track to the types of Serial Communication Interface (SCI), we have
1. Synchronous Communication:
In Synchronous Communication, bits of a full frame are sent in a prefixed maximum time interval. The data byte or a frame of data is received or transmitted with constant interval of time maintaining a uniform phase relation. It is hereby noted that, clock information is also being transmitted either explicitly or implicitly.

Note: There is a special case in synchronous communication - when bits of a frame (full) are variable to be sent in a maximum time interval, it is said to be iso-synchronous.

Characteristics of Synchronous Communication:
·   Random message signal reception or transmission is not allowed. Data communication occurs at pre-fixed bps only.
·    At a certain rate, clock synchronization is present always.
Examples of Synchronous Communication:
LAN, Inter-processor communication, etc.

2.   Asynchronous Communication:
In asynchronous communication, the clocks of receiver as well as transmitter, both are independent and unsynchronized but of same frequency and variable phase differences between bytes or bits of two data frames, which may not be sent within any prefixed time interval.

Characteristics of Asynchronous Communication:
·   Frames are sent or received at variable time intervals, which in turn, facilitates in-between hand shaking between serial transmitter and serial receiver.
·  Clock is always implicit to asynchronous data receiver and is independent of the transmitter

Examples of Asynchronous Communication:
UART Serial, Telephone, RS232C between UART devices.
            
This is all about modes of serial communication, some characteristics and key points. If you would like to learn more about serial communication, there is a book in embedded systems you may refer to “Embedded Systems Architecture, Programming And Design Second Edition” by Rajkamal. It is a good book regarding embedded systems.

If you have any queries please feel free to ask.

In our next discussion, we will be learning about the two famous communication standards I2C and SPI, and some introduction to CAN Protocol.

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