Serial Communication is the simplest communication of all between a sender and receiver. It became popular owing to its low cost installation and less space requirements. This post will discuss Serial Communication, its protocols, comparison of RS Standards and advantages of Serial Communication over Parallel Communication.

What is Serial Communication

Serial communication can be defined as the method of sending one bit of data at one point of time in succession through a bus. Take an example of Bow an arrow. How does an arrow shoots from the bow? One at a time, isn’t it? Same is the case with Serial Communication.

This is directly opposite to the function of parallel communication, where several bits are sent together on a bus consisting of many wired lanes in parallel.

Serial communication takes place by using only a single wire or line. Hence, for a two-way digital communication, we will only need two wires between the transmitter and receiver. As the cost of establishing a parallel communication is very high and its configuration is lengthy and complex, the Serial communication is preferred for all major communication/computer networks.

Serial Communication – Data Flow through Single Path

Fig. 1 – Serial Communication – Data Flow through Single Path

At small distances, serial buses are becoming more common and popular because the disadvantages of parallel buses prevails their advantage of simplicity. Improved technology to ensure the integrity of signal as well as increased transmission and receiving speed of data per channel has made the serial ports a close competitor to parallel ports.

The cost of integrated circuits are a lot because of the large number of pins. It is therefore, many integrated circuits utilize the serial buses when the importance of speed does not exist. Examples of these low cost serial buses are SPI, I²C, and 1-Wire.

Few examples of systems consisting serial communication are Morse Code Telegraphy, RS-232, RS 422, RS-423, RS-485, Universal Serial Bus, FireWire, Serial Attached SCSI, Serial ATA, PCI Express, etc.

Devices Using Serial Communication

Fig. 2 – Devices Using Serial Communication

Serial Communication Protocols

Following are the Serial Communication Protocols:

  • CAN Protocol
  • I2C Protocol
  • SPI Protocol
  • USB Protocol
  • eSPI Protocol


Serial Communication Protocols

Fig. 3 – Serial Communication Protocols

CAN Protocol

1970’s was the era when automobile manufacturers started introducing new features such as anti lock braking, air conditioners, gear control, centrally operated door locks etc. These features warranted extra bulky wiring and complex designs which increased costs and risks.

To overcome these issues, Robert Bosch introduced CAN protocol in 1980s. This Serial Communication protocol was further standardized as ISO 11898 in 1993. It was the CAN protocol that completely changed the communication between advanced sensors.

The CAN protocol is utilized for electronics networking in automobiles, aeroplanes and medical systems.

I2C Protocol

I2C stands for Inter Integrated Circuits protocol. It is also known as IIC protocol. It allows connection of multiple slave devices with one or more than one master device. This serial communication is used for short distance communication between two Integrated Circuits on the same base (PCB). It uses two bidirectional transmission lines for the data transfer. It can support up to 3.4 Mbps which is considerably high data speed.

SPI Protocol

SPI Protocol stands for Serial Peripheral Interface Protocol. It was introduced by Motorola in 1980s and is widely used in embedded systems. It is used for short distance communication in embedded systems. Master Slave architecture is used while communication between devices in this protocol.

USB Protocol

USB Protocol needs no introduction. It is, by far, the most popular protocol in use. This protocol was introduced to standardize the peripheral connections to computer.

In USB protocol, the data communication happens in the form of packets of 8 bits or multiple of 8 bits.

eSPI Protocol

eSPI Protocol stands for Enhanced Serial Peripheral Bus Interface Protocol. Intel introduced eSPI Protocol as the replacement of Low Pin Count (LPC) bus. Its main purpose was to minimize the numbers of pins as compared to LPC bus.

Serial Communication Standards

RS standards are the most popular standards which are based on serial communication. Following are the main RS Standards that are widely used:

  • RS 232
  • RS 422
  • RS 485

RS Standards in Serial Communication

Fig. 4 – RS Standards in Serial Communication

RS 232

RS 232 stands for Recommended standard number 232. RS-232 ,as an interface, has been a standard from many years for interconnecting a Data Circuit Terminating Equipment (DCE) with a Data Terminal Equipment (DTE). Various forms of RS 232 interface are available in the market. However, being different in design and configuration, they are easy in conversion to other forms. Examples of this interface are RS 232 ‘C’, RS 232 ‘D’ , RS 232 V.10, RS 232 V.28.

RS 422

This standard was introduced to fix the shortcomings of RS-232C. These shortcomings included slow and short distance transmission. The clock timing and its purpose is defined in this standard. Examples of this standard are D Sub 9 pin and D Sub 25 pin connectors.

RS 485

This standard was introduced to fix the shortcomings of RS 422. The shortcoming of RS 422 was the inability to switch off the driver circuit. RS 485 utilizes three state logic which allows it to deactivate each transmitter separately.

Comparison between Common RS Standards in Serial Communication

Fig. 5 – Comparison between Common RS Standards in Serial Communication

Advantages of Serial Communication over Parallel Communication

There is a misconception that the serial ports/buses are slower than parallel ports/buses as the transmission of data is only a bit per unit of time. Even serial ports/buses may be clocked considerably at quicker rate than the parallel ports/buses and can accomplish a higher speed of data flow. The factors which makes Serial Communication better than Parallel Communication are:

No Clock Required: In case of unclocked and asynchronous type Serial Communication, the problem of clock skew between the lanes/ channels does not exist.

Requires Less Space: The Serial Communication configuration requires less space because the requirement of cable is less in serial connection. The availability of this additional space gives good isolation of the data lanes from the neighboring communication components.

No Cross-talks: There is a minimal presence of conductors in the nearby space. It is therefore, the chances of cross-talks are rare.

Low Cost: The cost of serial link is less in comparison to parallel links.

Also Read:
Communication Protocols in Embedded Systems – Types, Advantages & Disadvantages
Digital Communication – Introduction, Basic Components, How Signal Process Works and Advantages
Asynchronous Transmission – Communication Characteristics, Process of Data Flow, Advantages and Disadvantages