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.
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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