What is a COM port? Explained

What are COM1 port

A COM port is a type of serial port used for communication between a computer and external devices or other computers. COM ports are commonly used in legacy devices, debugging embedded systems, virtualization, and networking in industrial or automation settings.

Since modern PCs have physical USB ports, you may be wondering what the purpose of the COM ports is. While COM ports may not be as commonly used as they once were, there are still several scenarios where knowledge of COM ports can be useful.

For instance, knowing about COM ports can be crucial if you need to connect to or use legacy devices.

In the rest of this article, you will learn more about a COM port and its usage in modern computing.

What is the COM header on the motherboard?

The technology of communication ports dates back to 1969. It is a simple serial protocol implemented using simple logic that suits the technology of the time.

The COM1, COM2, COM3, and COM4 ports on your motherboard can all represent the COM header, and each has unique PINs and functionalities. But the essence of this header is for communication between a computer and an external device or other computers.

The downside of using the COM header on a motherboard is that it operates at a relatively low speed, and the protocol cannot identify a device and its capabilities.

An improvement on this old technology is the USB protocol, which became common in the mid-1990s. Now featured on modern boards, USB headers operate at higher speeds and allow devices to identify themselves with a particular class.

The bottom line is that motherboards don’t require a COM port header since USB has successfully become the standard for connecting peripherals. This explains why many PCs and laptops will ship without serial ports but with USB ports.

Still, you can find some modern motherboards with a COM header.

What is a COM port header used for?

COM port headers are mainly used to connect external devices to the computer but are now generally replaced with USB. Still, some old devices don’t have modern (affordable) USB alternatives. So if you see that your old device has a COM port and it works, there’s really no point in replacing it.

Meanwhile, it is less about the PC needing the COM port. It is more about us needing it. (Just like in the case of CD players, despite the overwhelming presence of iPods and MP3s.)

For example, COM port headers can be used to build a simple and affordable PIC microcontroller program. In this case, due to its non-standard use, you aren’t necessarily using the port as an RS-232 port.

Basically, the use of COM port headers in modern computing revolves around niche cases because many programmers are now natively USB-based.

Still, a COM port can prove useful when you need to program a microcontroller for the first time while making a USB programmer.

What are COM3 and COM4?

The COM3 and COM4 ports are part of the COM port interface in DOS systems. These systems support up to four serial ports, which are COM2, COM3, and COM4.

But only two IRQ lines are reserved when some software that uses system interrupts tries to access the serial ports. In other words, these four COM ports will utilize two IRQ lines in pairs. That is, COM2 and COM3 use IRQ3, while COM1 and COM3 use IRQ4 and cannot be used simultaneously.

What plugs into the COM port on the motherboard?

The Com Port, or “Serial Port,” has nine pins. In older PCs, this 9-pin serial port is a header for connecting devices requiring fewer data transfers. In other words, devices that plug into the COM port on the motherboard include:

  • Mouse
  • The console of some electronic device
  • Camera
  • external modem
  • router
  • switch, and so on.

However, devices that need higher data transfer speeds, such as printers and scanners, would go into the parallel port, which was 40-pin, because it offered a higher speed than the serial port (COM port).

The COM port transfers data serially, meaning that a stream of data uses a single channel, making the flow slow, which explains why it has fewer pins than the parallel port. The parallel ports support simultaneous streaming of data, which makes transfers faster.

But the USB 4-pin connector was the game changer. It has fewer pins because it’s serial, but it’s still faster than both COM and parallel ports.

USB ports are younger than COM ports, which makes the latest technology overshadow the previous two connectors in terms of speed and reliability.

So just as devices such as a mouse, keyboards, cameras, consoles of electronic devices, headphones, pen drives, and whatnot would go into the COM port, it is now replaced with USB headers in modern PCs.

How do I assign a USB port as a COM port on the motherboard?

If you want to assign a USB port to your motherboard as a COM port, right-click the USB Serial Port line and select Properties from the popup menu.

Select the “Port Settings” tab on the menu and navigate to ” Advanced.”

You can choose the “COM port Number” dropdown box and select a COM port number from 2 to 4. The COM1 is usually the default in use.

What’s the difference between a COM port and a USB port on the motherboard?

There are a lot of obvious differences between serial ports and USB ports on motherboards that explain why the latter come with motherboards these days.

First, the only similarity between these two ports is that they send bits in order on a wire and are both external to the PC. Everything else is different.

COM portsUSB ports
A device connected to a busUSB is a consumer-ready external bus
Point-to-point, low-speed physical layer connectionFaster data flow
No interoperabilityHas more in common with ISA, PCI, PCIe, MCA, and others as a bus
Old technologyFeatured in modern motherboards
No knowledge of devices 

Final thoughts

While modern motherboards may have a wide range of features and connectivity options, there are still several situations where someone might need to know about COM ports.

For example, many older devices, such as barcode scanners, point-of-sale systems, and industrial control systems, still use serial communication through COM ports. If you need to connect to or use these devices, you must know about COM ports.

In addition, you may need to use a COM port to communicate with the device if you’re working with embedded systems, microcontrollers, or other hardware to debug any issues.

The most common situation is that virtualization software, such as VirtualBox, allows you to connect to virtual machines via a virtual COM port. So, if you’re using virtualization for testing or development purposes, you may need to configure COM ports.

I hope this article has given you the insights to understand COM ports, even on modern motherboards.

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