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What is a COM Port? - Everything about Serial Ports

Editorial Team Editorial Team
Updated: Oct 31, 2024

A COM port is an input/output interface used to connect serial devices to a computer. COM ports are also known as serial ports, directly referencing their functionality for connecting peripheral serial devices. Almost all modern computers are built without COM ports, but many serial port devices rely on accessing a serial interface. Medical equipment, laboratory instruments, and point-of-sale systems often operate via a serial connection.

This article provides readers with a comprehensive guide to COM ports.

  • We cover COM port basics such as how they work and their role in connecting peripheral devices to computers.
  • We explain: serial ports and the RS-232, RS-485, RS-422, and UART communication protocols used to control serial data transfer.

The guide also looks at various types of serial devices and how they are used in multiple usage scenarios like fire control systems, medical devices, networking, and industrial automation. We want to provide our readers with an understanding of the importance of COM ports in supporting modern and legacy technology.

Table of Contents:
  1. What is a COM Port:
    1. How do COM Ports Work?
    2. What is a Serial Interface?
    3. Types of Serial Ports.
    4. Serial Communication Protocols.
    5. What is a Serial Device?
  2. What are Serial Ports used for?
  3. The Role of COM Ports in Serial Communication.

What is a COM Port?

The definition of a COM port, short for a communication port, is an interface for serial data transmission that transfers data one bit at a time. COM ports are typically used to connect peripheral devices such as modems, printers, or industrial equipment to a computer. They are used in applications like medical devices and industrial automation that need a simple and reliable interface. COM ports can be hardware-based physical interfaces or virtualized through software to extend their functionality.
COM/Serial Port

How do COM Ports Work?

COM ports transmit data between a computer and a serial device via sets of pins. The pins include transmit (TX), receive (RX), ground (GND), and control lines. Serial data is converted into streams of bits and transmitted through the TX pin. The data is received through a device’s RX pin and interpreted into serial data by the device.

What is a Serial Interface?

Serial interfaces transmit data one bit at a time over a single channel, making it a reliable and cost-effective method of communication. The simple wiring and minimal interference impacting COM interfaces provide consistency and reliability. This feature makes them more appropriate for long-distance data transfer than more complex parallel communication methods.

Types of Serial Ports

Two different COM port types are typically available as male connectors with either 9 or 25 pins. Ports used to connect serial cables are either 9 or 25-pin female connectors.
Standard naming conventions define a computer’s serial ports as COM1 and COM2. LPT1 is used to specify a parallel port.
Each pin has a specific function related to sending and receiving data. All interfaces that comply with the RS-232 communication protocol are considered serial ports.
DB9
DB25

Serial Communication Protocols

Multiple protocols are employed to support serial communication. The most widely used protocols include:

  • RS-232: The RS-232 protocol is one of the original serial protocols and is widely used to implement serial communication. An RS-232 port supports communication over short distances of up to 15 meters and provides low data transfer rates. This makes the RS-232 protocol an excellent method of establishing simple and direct connections between a computer and peripheral serial devices like printers or modems.

  • RS-422: An RS-422 port provides support for faster data transfer rates over longer distances than an RS-232 interface. The RS-422 protocol is better at handling noisy environments and is often employed in industrial and scientific applications where data integrity is essential. The protocol supports communication between multiple devices with a single transmitter and several receivers.

  • RS-485: RS-485 is an advanced protocol designed to support complex industrial environments. An RS-485 port provides higher data throughput than RS-232 and supports distances of up to 1,200 meters. The protocol allows multiple devices to be connected to a single bus and is widely used for vital control systems and industrial automation implementations.

  • UART: Universal Asynchronous Receiver/Transmitter (UART) is a piece of hardware that translates data between serial and parallel ports. UART processing supports serial data transmission and sends information bit by bit. Data is then converted to a full byte by the other end of the connection. UART technology enables compatibility between parallel and serial ports for data transmission.

What is a Serial Device?

Serial devices are pieces of electronic equipment designed to support serial data transmission where bits are transferred one at a time sequentially over a single channel. The simplicity and reliability of serial data transmission is responsible for it being a very popular choice when data consistency and integrity are essential. The following are a few examples of the many types of serial devices in use today.
Serial Based Devices
  • Production printers: High-volume production printers use serial interfaces to transmit data between print servers or computers. Communication is used to manage large jobs to ensure accuracy when printing items like labels, books, and packaging materials.

  • 3D printers: Serial communication is used to transmit the meticulous instructions necessary to build models by 3D printers. Serial interfaces are used to receive directions that control the settings and movement of a 3D printer.

  • Industrial automation equipment: Specialized industrial automation equipment such as robots communicate with control systems via serial interfaces. Accurate communication is required to attain the precision necessary for performing automated tasks such as welding or assembling components.

  • Computer numerical control (CNC) machines: Precise communication provided by serial interfaces is required by CNC devices like mills, lathes, and routers. Accurate instructions are necessary to support the CNC’s operation when it is engaged in machining or manufacturing.

  • Automated guided vehicles (AGVs): AGVs are often used to transport materials in factories and warehouses. These autonomous vehicles employ serial protocols to communicate with management systems. Communication with these systems is used to optimize navigation through the facility and coordinate movements with other automated systems and devices.
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What are Serial Ports used for?

Following are some of the uses of COM ports in business and industry.

  • Industrial automation: COM ports are necessary to support industrial automation by facilitating communication between PLCs, sensors, controllers, and other automated devices. Serial ports are used to provide human-machine interfaces (HMIs) so operators can interact with industrial devices and manage machine operations.

  • Fire control panel remote access: Modern fire safety systems rely on hardware and software to identify potential issues and generate alerts to essential personnel. Remote monitoring systems employ serial devices connected to centralized control panels.

  • Medical Devices: A Laboratory Information Management System (LIMS) is a software solution used to automate workflows and tracking for improved efficiency. The system communicates with lab instruments through serial ports.

  • Consumer Electronics: Many consumer devices utilize COM ports to furnish simple and accurate communication. Examples include barcode scanners, measuring equipment, and GPS devices.

  • Farm management: Many types of equipment used on modern farms communicate via serial ports. A wide range of serial devices are used to help control pollution, monitor livestock health, automate irrigation activities, and perform many other farm-related operations.
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The Role of COM Ports in Serial Communication

An understanding of COM ports helps you manage and optimize serial devices and systems. This knowledge can be used to efficiently configure industrial equipment, troubleshoot communication issues between devices and computers, or streamline network communication. Knowing how COM ports work is also useful when interacting with consumer electronics or serial medical devices.

COM ports are an essential component of serial communication. By transmitting a single bit of data at a time, serial communication provides enhanced reliability compared to parallel communication methods. The steady flow of data furnished by COM ports is vitally important for supporting industrial, medical, and other systems requiring high data reliability.

Frequently Asked Questions about COM port

A virtual serial port is a software construct that emulates the functionality of a serial interface to enable communication with serial devices using a network or other type of physical port. Virtual serial ports are essential when using serial applications with computers that are not equipped with physical serial interfaces.

Virtual serial ports

The protection level of your network connections will depend on the solution you choose. Serial to Ethernet Connector by Electronic Team is one of the most reliable programs when it comes to the security of data transfer. This application deploys advanced traffic encryption that protects your data from unauthorized access or interception. That means all the connections you establish with this software are totally secure.
With Serial to Ethernet Connector, you can share an unlimited number of real and virtual COM ports at a time.
In network terminology, serial tunneling means creating a connection between COM ports of two serial device servers over Ethernet. The communicated serial data is packetized in both directions into TCP/IP packets and redirected across Ethernet.
In RS-232 port connections, the data transfer rate is measured by bits per second (bps). The standard data exchange rates for asynchronous serial communication include:

110, 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 56000, 57600, 115200, 128000 and 256000 bps. Note that these values refer to the effective transmission rate, i.e., the rate is determined without taking into account the service bits - start, stop, and parity.

For a long time, the serial port speed above 115,200 bps hasn’t been supported by any operating systems or hardware. But the advanced manufacturing technology of modern motherboards chipsets makes it possible to increase the data exchange rate to more than 1 Mbit/s.

Note that the data transfer rate depends largely on the length of the connecting cable and how it is run relative to the power supply wires.
According to the RS-232 standard, the maximum cable length is 50 feet (or 15.24 m).
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