This document discusses input/output interfaces and data communication basics. It describes two major types of I/O interfaces: serial and parallel. Serial interfaces exchange data one bit at a time and are slower but cheaper, while parallel interfaces handle multiple bits simultaneously and are faster but more expensive. The document also covers serial vs parallel communication, synchronous vs asynchronous serial data transfer, and their differences.

1. I N P U T / O U T P U T I N T E R F A C E S
•P A R A L L E L C O M M U N I C A T I O N
•S E R I A L C O M M U N I C A T I O N
Data Communication Basics

2. Introduction
 Peripherals connected to a computer need special
communication links for interfacing them with the CPU
 The purpose of those communication links are to resolve
the differences that exist between the processor and each
peripherals.
 The major differences are:
 Peripherals are electromechanical and electromagnetic
devices and their manner of operation is different from
operation of CPU and memory which are electronic
devices. So, conversion of signal value is required.

3. Introduction
The data transfer rate of peripherals is usually
slower than the transfer rate of CPU, and
consequently synchronization mechanism may be
needed.
The operating modes of peripherals are different
from each other and each must be controlled so as
not to disturb the operation of other peripherals
connected to CPU.
Data codes and formats of peripherals differ from
that of CPU and memory.

4. I/O Interfaces
 To resolve these differences, computer system
include special hardware components between the
CPU and peripherals, to supervise and synchronize
all inputs and outputs transfer.
 These components are called Interface Unit
because they interface between the processor bus
and peripheral devices.
 The two major types of I/O Interfaces are:
Serial Interfacing
Parallel Interfacing

5. Serial Interfacing
 Exchanges data with the peripherals in serial mode.
 Data are transmitted one bit at a time.
 Slow, inexpensive to implement.
 Serial interface converts parallel mode bus system to
serial mode.
 If the bus has ‘n’ data lines, the serial I/O interface
accepts ‘n’ bits of data simultaneously from the bus.
These ‘n’ bits are sent to the I/O devices, one bit at a
time, requiring ‘n’ time slots for transmission.
 Eg. Keyboard Interfacing

6. Parallel Interfacing
 Some I/O devices can handle data at speeds that cannot
be supported by serial interfaces. In such cases, parallel
interface is required.
 In parallel interface, ‘n’ bits of data are handled
simultaneously by the bus and on the links to the device.
 Faster interchange of data
 Expensive due to need of multiple wires
 Many I/O devices, particularly those requiring high data
transfer rate use this arrangement.
 Eg: Printer Interface

7. Method of Communication
 Generally, there are two methods of data
communication.
Parallel Communication
Serial Communication

8. Parallel Communication
 A word of ‘n’ bits is transmitted in parallel.
 Channel comprises of n-lines, last line is called common ground
 Time required to transfer one word is equal to time taken for a bit to transmit.
 In practical, for long distance transmission, since costs for large number of
lines increases.
Transmitter Receiver
D0
D7
1 0 1
1 1 0
0 0 1
0 1 0
1 0 1
1 1 0
0 1 0
0 0 1
GND

9. Serial Communication
 In serial data transfer, each bit of the word is sent in succession, one at a time
over a single pair wires.
 A parallel to serial converter is used to convert the incoming parallel data to
serial form and then data is sent out with the Least Significant Bit, D0 first and
Most Significant Bit , D7 at last.
 The time taken to transmit a word in serial data transmission will be ‘n’ times
more than the time taken in parallel data transmission.
8085
Serial
Peripheral
SOD
SID
IOW
IOR

10. Parallel Vs Serial Communication
Parallel Serial
Communication through AD0 –
AD7 pins in 8085.
Communication through SOD and
SID pins in 8085.
8085 transfers eight bits of data
simultaneously over eight data lines
One bit at a time is transferred over
a single line.
Parallel communication over a very
long distance can become very
expensive.
Parallel-to-Serial and Serial-to-
Parallel conversion needed at
transmission and reception side.
I/O uses the entire data bus. I/O uses one data line.

11. Types of Serial Data Transfer
 There are two types of serial data transfer
Asynchronous Serial Data Transfer
Synchronous Serial Data Transfer

12. Asynchronous Serial Data Transfer
 In this type of transmission, the receiving device
does not need to be synchronized with the
transmitting device.

13. Synchronous Serial Data Transfer
 Synchronous communication is used for transferring
large amount of data at a stretch without frequent
start and stop.

14. Synchronous Vs Asynchronous
Synchronous Asynchronous
The transmitter is synchronized with
the receiver in the same frequency.
The transmitter is not synchronized
with the receiver by the same master
clock.
A block of characters is transmitted
along with the synchronization
information.
Asynchronous format is character-
oriented. Each character carries the
information of the start and the stop bit
Generally used for high-speed
transmission (more than 20K
bits/seconds)
Generally used for low-speed
transmission (less than 20K
bits/seconds)