Adapter USB-COM do-it-yourself: scheme, device and recommendations

In modern computers and laptops, manufacturers have stopped using the COM port, which was widely used earlier. If you have an old computer that you purchased about 7 years ago, you can find several such ports on it. The COM port was designed to connect different external equipment, such as a mouse, printer, modem, and other special tools. Now there are problems with connecting various outdated equipment to new laptops or computers. For example, the receiver is now quite difficult to connect, because for this you need to use the COM-port.

What to do in this case? There is a way out, of course. You can make USB-to-COM converter by yourself. How to do it? In this article you will find answers to your questions.

USB-COM adapter

Do-it-yourself USB-COM adapter

Now we will look at how to make a USB-COM adapter that will work with RX and TX signals.It will not work with other types of modem signals, but this is quite enough to connect standard equipment.
To proceed to the assembly, you will need to purchase such parts:

  • Two 10 nF capacitors and one per 1 microfarad.
  • Two resistors for 27 Ohms and one for 1.5 KΩ.
  • Connector mini-USB and DB-9 one by one.
  • Quartz 12.00 MHz.
  • PL2303HX.
  • Foiled PCB for 48 * 22 mm board.

After purchasing these parts, you can proceed directly to the assembly. First, we will need to consider the scheme of the USB-COM adapter (you can see the image below).

USB-COM adapter cable

Build and configure the adapter

In the installation of nothing complicated. First you need to take or make a fee. After that, drill four holes. Next, we need to solder all the parts, the location of which will correspond to the scheme. At this assembly is over.

To prevent the board from oxidizing, it must be blown out with polyurethane varnish. If this is not available, you can use any other quick-drying automotive varnish.

It's time to move on to customization. We connect our board to the computer. If everything is properly assembled, it will determine the device on which to install the drivers.

We install Prolific drivers for this port on the computer. Turning to the Task Manager, you can see that it has been defined and is now fully functional.


To test the performance of our adapter, it is necessary to close the contacts near the COM port with some thin object (see the image below for where it should be done).

DIY COM USB adapter

Next on the computer you need to run the program "Hyperterminal". This utility is standard on some operating systems. If you do not have it, you need to download from the Internet.

Go to the section "Connection Settings", where you need to select the COM-port. In the new window, you should configure the following parameters of our port:

  • signal transfer rate - 115200;
  • data bits - 8;
  • parity is not;
  • stop bits - 1;
  • flow control is not.

Then we confirm the changes by clicking the OK button. After that, start the connection. We turn to the English layout and print any words in the program. All the characters you clicked will be displayed on a white background.

In the event that it does not work, that is, the words are not displayed, it is necessary to change the installation. Configure it until the characters are displayed.

That's all.You can release the contacts that you clamped and start using a USB-COM adapter.

Adapter USB-COM Windows 7

USB RS232 DB9 COM adapter cable

If you can't make your adapter, you can purchase a USB RS232 DB9 COM. Its price varies within 1000 rubles, and it has quite good technical characteristics. Probably the main thing is a good connection speed. It is also supported by almost all operating systems. Unfortunately, it only works with USB 2.0.

Connect to computer

To connect the USB RS232 DB9 COM adapter to your computer, you need to find drivers on the Internet that fit your operating system. Download and unpack them. We connect the adapter to the computer, after which we indicate where the drivers are located. If everything was done correctly, the LED on the device will turn on, indicating that it is ready to work.

Possible problems

Unfortunately, even in the case when you use the purchased USB-COM adapter cable, problems may arise. They need to fight, and it is easy to do.

  1. The operating system could assign a port a number with which devices cannot work. In this case, you need to go to the OS settings.Go to the "System" section, then go to the "Device Manager". In the new window, look for the item "Ports", where the necessary converter should be located. Click on it with the right mouse button and go to the "Properties" item. Next, click on the "Advanced" button and in the new window set the port number. After that, restart your device and begin to use.Cable adapter USB RS232 DB9 COM
  2. Incompatibility of the connected equipment and the converter itself. If in the first case the problem is easily fixed, now nothing will be done. Unfortunately, such cases are possible, but they are extremely rare. To prevent this situation, you should consult with the seller before buying.
  3. As in the first case, there may be problems with the COM port number. It can not only go beyond the limits of visibility for other devices, but also be under a number that is already taken. In this case, just change the number. At the end, restart the computer and continue to work.

Alternative solution to the problem

If you need to install additional equipment that needs a COM port, you can also use a module with this connector.It simply installs directly in the computer and connects to the motherboard. After installing the COM port card or PIC-COM (second name), it is also assigned a number. As in the previous cases, this number can be changed if problems arise. If you are not sure that you can properly install this board in a computer, it is better to use the help of professionals, so as not to harm other equipment.

USB-COM adapter

Unfortunately, this solution can only be used for computers. In laptops, installing an additional fee is quite difficult, and it requires additional costs. First, the case itself will not allow the installation of an additional module, and secondly, it may not be supported by your model. Therefore, in the case of a laptop, you should use a USB-COM-adapter.


If you need to connect obsolete equipment that requires a COM port, you can make an adapter yourself. It is quite simple. If you do not want to do this, you can purchase a USB-COM-adapter. Its price is about 1000 rubles. Remember that you still have to install the drivers and check the performance of this equipment.On Windows 7, a USB-to-COM adapter is fairly easy to install, since you can find all the drivers on this Internet operating system.

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