Сommunication protocols. Main types of serial data transfer

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Table of contents:

  1. What are the types of serial communication?
  2. Asynchronous communication protocols
  3. Synchronous serial protocols


What are the types of serial communication?


Serial communication is usually accomplished using one of the two main serial protocols. These protocols support asynchronous data transfer and synchronous data transfer. Read on to learn more about these two distinct communication protocols used with serial applications and devices.

Asynchronous data transfer


When employing an asynchronous serial protocol the sender needs to supply a synchronization signal prior to transferring any data. This needs to be done before each transmitted message. The characteristics of an asynchronous protocol include:

  • No clock signal is required between the message’s sender and recipient.
  • Longer distance data transmission is supported.
  • Reliability is increased with asynchronous data transmission.

Synchronous data transfer


When the synchronous serial protocol is used, both the sender and receiver use the same clock signal during data transfer. Some other characteristics of synchronous transmission are:

  • Synchronous protocols support higher data transfer rates.
  • The clock signal must be communicated between the sender and receiver.
  • A master/slave configuration is required to implement synchronous data transfer.

Asynchronous communication protocols


Here are some of the more common asynchronous communication interfaces that you will encounter. They are examples of different types of serial protocols that all use asynchronous data transfer to enable communication between devices or applications.

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RS232 protocol


RS232 is the first serial protocol and was developed to connect modems to teletype machines. The RS stands for Recommended Standard and was initially put forth by the EIA (Electronic Industries Alliance). The organization in charge of specifying standards is now known as the TIA ( Telecommunication Industry Association).

With the RS2332 protocol, you can connect one transmitter to a single receiver. The standard is used in modems, the familiar computer mouse, and computed numerical computing (CNC) devices.

The RS232 protocol supports cable lengths of up to 50 feet and provides the means to implement full-duplex transmission at baud rates of up to 1Mbps.

Serial ports can employ the RS232 protocol. You may be interested to know how the bytes of data stored in a device’s memory are converted to binary bits suitable for serial transmission. This feat is accomplished through the use of an internal chip inside the serial port. The chip is a Universal Asynchronous Receiver Transmitter (UART) chip that can convert parallel data in the form of bytes into bitwise data fit for serial transmission.

Serial Data

RS232 Wiring Connection


The standard RS232 serial port contains nine pins and can have either male or female connectors. An updated version of the protocol is named RS232C and supports all features of RS232. RS232C has 25 pins instead of 9, but only three pins are used to connect terminal devices.

RS232 Wiring Connection

RS422 Interface


The RS422 protocol offers extended capability when implementing serial communication. Up to ten transmitters and ten receivers can be connected through a single bus with this must-drop serial interface. Data is transmitted using a differential configuration that employs two twisted-pair cables. RS422 supports baud rates of up to 10Mbps with a maximum cable length of 4000 feet.

Wiring Connection

RS485 Interface


The RS485 protocol is the preferred serial protocol used in industrial implementations. Using this protocol you can design a differential configuration that comprises up to 32 line drivers and 32 receivers. Only one transmitter can be active at any time and is known as the line driver.

RS485 Wiring Connection

Note: Connections must be manually terminated when using the RS232 or RS485 protocols.

Long-distance communication benefits from using asynchronous wired protocols. The main drawback is the installation costs which can get very expensive when more transmitters and receivers need to be connected.

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Synchronous Serial Protocols


Onboard computer peripherals usually make use of synchronous serial protocols. These protocols allow one bus to interface with multiple devices simultaneously. Here are some of the synchronous protocols you will see in the field.

I2C Protocol (Inter-integrated circuit) - is used for onboard communication due to its data transfer speed of up to 400kbps. It uses a two-wire bidirectional approach to exchange data between devices located on the same bus.

SPI Protocol (Serial peripheral interface) protocol is used to implement high-speed data transmission and can send and receive a continuous flow of data without interruptions.

SPI uses four wires which each serve a distinct function. There is a master in/slave out (MISO) wire, a master out/slave in (MOSI) wire and one each that carries the clock and slave select signals.

CAN Protocol is a protocol used predominantly in autos and other motor vehicles. The protocol is designed for messaging and reduces the amount of copper required for multiplex electric wiring.

The USB interface is one of the best alternatives to serial or parallel ports. It provides a standard communication port that performs at higher throughput speeds than the older interfaces.

Microwire is a three-wire serial communication synchronous protocol. Its microcontroller has an on-board serial I/O port that can interface with the chips of peripheral devices.

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