Originally authored by: Bernie Thompson, July 4, 2011

To change the COM port for a USB Serial adapter on Windows, you must adjust the advanced port settings within the Device Manager. This is often necessary because Windows may automatically assign a high COM port number, while many legacy applications expect a port between COM1 and COM4.

How to Change the COM Port

1. Open the Device Manager and expand the Ports (COM & LPT)
   
2. Right-click on your USB Serial adapter device and click on Properties
   
3. Select the Port Settings tab and click on the Advanced button
   
4. Click on the COM Port Number drop-down menu to change the port to the required port number, ideally between 1 and 4 (some legacy applications expect a COM port in this range)
   
5. Click OK on all open Property windows to apply and save the settings. The updated COM port number should now be reflected in the Device Manager
   

Applicable To

PL2303-DB9

Originally authored by: Bernie Thompson, July 5, 2011
Updated 2024-07-11 by Pat P.
Updated 2026-03-04 by Sam R.

The Plugable USB to Serial adapter (PL2303-DB9) can be installed on Windows 7 by downloading the recommended Prolific driver, running the installer, and then connecting the device. While Windows 7 may detect the adapter automatically, manual installation ensures the correct driver is used.

How to Install the Prolific Driver on Windows 7

1. Download the driver from the Plugable driver page
2. Unzip the downloaded folder to your desktop or another accessible location
3. Double-click the installer file to begin the installation process
4. Follow the prompts to complete the setup (no specific configuration questions are required):
   
5. Connect the USB to Serial adapter to the computer. A pop-up similar to this should appear:
   
6. Wait for the Windows notification confirming the device is ready to use:
   
7. Verify the installation by opening the Device Manager and looking for the adapter's entry under Ports (COM & LPT):
   

   
   

Installing the drivers after a fresh installation of Windows 7 in 2024

Windows 7 SP1 installation media does not include support for Windows 7's latest driver signing functionality and requires installing the following updates from Microsoft manually before installing the Prolific drivers, or running Windows Update:

• Update for Windows 7 (KB3112343)
• Update for Windows 7 for x64-based Systems (KB3135445)

After installing these updates the installation process should proceed normally.

Applicable To

PL2303-DB9

Originally authored by: David Roberts, December 9, 2013

The Plugable USB serial adapter allows you to connect legacy devices that use RS232 serial communications to computers that do not have a serial port. Although this usually works well, a number of problems can crop up because many of the devices people want to connect are older devices with various issues. A frequent problem is that the software that is necessary to connect to the device does not work under modern operating systems such as Windows 7 or above. Other problems can include using an improper cable, incorrect port settings, or a basic incompatibility between the Plugable USB serial adapter and the device it is being connected to.

Drivers don’t load correctly, or the cable cannot be seen in Device Manager

When the cable is plugged into Windows 7 or higher computers, the necessary driver should load automatically. If it does not load, make sure the computer is connected to the internet. If an internet connection is not available, download the latest drivers for the cable from the Plugable driver site here. For earlier Windows XP and Vista computers, the drivers must be downloaded from the Plugable website and installed. 

The drivers load successfully, but I can’t communicate with my device

Serial communications over RS232 is a legacy from an earlier age in computer when devices were not necessarily plug-and-play. Several different variables come into play, and all must be set up correctly to ensure communications.

Software

Many legacy devices came with specialized software that is used to communicate with them over a RS232 serial port. Many of these programs were written years ago, and are not as automated as modern applications. In many cases, the software is not compatible with modern Windows operating systems. If the software won’t run, or runs with errors, check the website of the device maker to see what Windows version it is compatible with. Many times you will see a warning notice saying that the software does not work with certain versions of Windows and no update is anticipated. For example, Nikon digital cameras from the late Nineties used a serial connection to transfer photos to a computer. However, this software does not support Windows Vista or later, and Nikon warns on its website that no update is anticipated. 

Some devices don’t come with special software. Instead you communicate with them using a standard serial communications program. A popular and easy to use program is Putty, which can be downloaded for free from the internet. It is also possible to send commands to the COM port directly from the Command Prompt, but this is for experts only. 

COM Port

In Windows, serial communications proceed through a COM port that is assigned by Windows when the USB serial adapter is plugged in. You can find out this COM port by opening Device Manager, going to Ports (COM & LTPT) and looking for Prolific USB-to_Serial Comm Port (COM#). The # symbol represents the port number of the COM port assigned by Windows. This port must be set correctly in whatever application is trying to communicate through that port. 

Because older Windows computers only offered COM ports 1 through 4, many legacy applications require the COM port to be set within this range. Some applications only allow ports 1 or 2 to be set. If the port is out of the range required by the application, you can change the port number by  right-clicking on the adapter in Device Manager, selecting Properties, selecting the Port Settings tab, then clicking Advanced. Clicking on the COM Port Number button reveals a list of ports, with unavailable ones listed as “in use.” Set the port you want here, then click OK to close the Properties dialog. Device Manager should update and show the number you selected. If it does not, make sure you have sufficient permissions to change the COM port settings. 

Port Settings

There are many other settings that must be the same for the port and the device being communicated with. Typically the manual for the device being connected will outline the correct settings. Sometimes the application that is used with the device will allow you to change the settings required by the device.
Generally, however, the settings for the port are changed to match the settings required by the device. These settings are changed by right-clicking on the adapter in Device Manager, selecting Properties and selecting the Port Settings tab. The default settings are appropriate for many devices. However, in many cases the Bits per second, or “baud rate” can be set higher for faster communications. Data bits, Parity, and Stop bits change the format of the data sent to the device. Typically, if these are set wrong, the device will not be able to understand anything sent to it. Flow control selects a hardware (requires a special cable) or software method for controlling communications between the devices that keeps their communications synchronized. Most devices don’t use flow control, and for them, this can be set to Off. 

Cable type

Historically, COM ports were used for serial communications with modems, that took the digital information and modulated it for transmission over telephone lines or radio. Information sent from one modem went to the receiver on the other modem, and was forwarded to the receiving pin on the COM port of the destination computer. Because this “crossover” function was performed by the modems, the cable from the computer to the modem was straight cable, connecting the send pin on the source computer to the send pin on the modem. 

However, when two computers were directly connected to each other, without the intervening modems that connected send to receive, a special cable, called a “null modem” or “crossover” was used that connected the send (Xmit or TX) pin on the source computer to the receive (RECV or RX) pin on the destination computer. 

Which cable to use depends on how the connected device sees itself in relation to the computer. Typically devices that actually do something will require a Null Modem cable, while devices that process data and send it somewhere else (like a modem) will use a straight cable. It’s always good to get the make and model of the device being connected and go online to find out which cable is needed. Many devices come with a cable, which should be used if it is available

If the software is compatible, the port settings are made correctly, and the right cable is used, there are still some cases where the device does not work well with a particular USB Serial adapter. Some manufacturers advise against USB serial adapters alltogether or their website or recommend using a particular chipset. The major alternative available is the FTDI chipset. It is more expensive and works in some situations where the Prolific chipset used in the Plugable serial adapter will not work. Check the manufacturer’s documentation and online forums for information. You can google “<product make and model> USB serial adapter” (without the quotes). Often you’ll find a forum post by someone who has tried using a USB serial adapter and has discovered what works.

Our updated USB to Serial Adapter with Prolific PL2303GT chipset requires installing the Prolific Serial Drivers. These drivers are provided for Windows 7 through 11 and comply with the latest Microsoft driver security requirements.

In general, we do not advocate the use of Windows 7 as a general purpose operating system as it is no longer receiving security updates, however in some cases it can be necessary to drive legacy hardware requiring a RS232 serial connection.

What you will see on a fresh install of Windows 7

A fresh installation of Windows 7 64-bit with Service Pack 1 (SP1) will not support the current Windows driver security requirements. This requires manually updating the operating system before it can install these drivers, or run Windows Update successfully.

After starting the driver installation there will be a Windows Security pop-up window stating "Windows can't verify the publisher of this driver software", but gives the option to "Install this driver software anyway". If you select this option there will be a second similar pop-up window and you can complete the installation.

However, when the USB to serial adapter is connected a new pop-up message from the "Program Compatibility Assistant" implies the USB-to-Serial Cable Driver is not digitally signed. The driver is signed, but Windows doesn't recognize the signature because it is newer than this version of Windows.

In the Device Manager the serial adapter will show up with a yellow triangle with exclamation mark and the device status will be "Code 52" indicating the driver is unsigned.

Required Windows updates

Windows 7 requires installing some additional updates before it will recognize the new driver signature, these updates are also required to enable Windows Update, which would normally provide these updates automatically.

• KB3033929
• KB3112343
• KB3135445

After downloading on another computer and transferring to the Windows 7 computer, run each of these updates following the on screen instructions, then restart the computer before connecting the USB to serial adapter.

Verifying the serial adapter driver

After logging back into the desktop, reconnect the serial adapter, there should be no popup warnings on connection.

Open the Start Menu, search for and launch "Device Manager", expand the section "Ports (COM & LPT)", then double-click on the "Prolific PL2303GT USB Serial COM Port (COM#)" entry to open the properties. The device status should now be "This device is working properly.".

If you have any questions, please feel free to contact our support team via email at support@plugable.com we are happy to help answer any questions!

Up until recently macOS has included built-in driver support for our USB to RS232 serial adapter, however we have noted that new computers, and those with a fresh operating system installation are no longer automatically creating TTY and CU serial devices. To address this Prolific provides an Driver Extension installer through the Apple App store and we included step-by-step installation instructions below.

1. Open the Apple App store, and search for "prolific technology inc."

2. Select the "Get" button for the "PL2303 Serial" application

3. When the installation completes, open the "PL2303 Serial" app from the macOS Applications folder    
4. When the "PL2303Serial" would like to use a new driver extension" prompt comes up select "Open System Settings" to approve the new driver

5. Under the "Driver Extensions" toggle on the "PL2303Serial" extension

6. The system should now assign /dev/cu.P* and /dev/tty.P* devices to the serial adapter and can be confirmed with the terminal command ls /dev/{cu,tty}.P*

Three ways to send and receive data from Serial devices in Linux

Identify the serial device

There are a few ways to find the serial adapter connected to the computer.

For USB serial devices like our PL2303-DB9 USB Serial Adapter, connect the adapter to the computer, then open a terminal and run:

dmesg | tail

Look for output like:

[54221.132226] usb 1-2.2: pl2303 converter now attached to ttyUSB0

This indicates the serial adpater can be accessed at /dev/ttyUSB0

Alternatively, you can run the following command to print out the newest created serial adapter:

ls -tr1 /dev/tty* | tail -n 1

This will output something similar to:

/dev/ttyUSB0

Screen

Screen is a full-screen window manager capable of connecting to serial devices.

To connect to the serial device at 9600 baud (one of the more common baud rates) run the following command in the terminal:

screen /dev/ttyUSB0 9600

From here you can type text into the terminal to transmit to the connected device, or read output from the device.

Screen commands consist of "C-a" or ctrl+a (holding down the control key, and then pressing the 'a' key, then releasing both), to enter the command input mode, then commands may be entered with another key stroke.

To disconnect from the session ( while leaving it open ): Ctrl+a, Ctrl+d, you can then use the following to reattach to the session.

screen -r

To quit screen and close connection: ctrl+a, k. Then 'y' when prompted to kill the session.

Minicom

Minicom is a serial communication program that has a more user-friendly interface than screen, however with more options comes more choices.

To connect to the serial device at 9600 baud with minicom, from the terminal:

minicom -b 9600 -D /dev/ttyUSB0

Minicom will provide more output when connected, in my case:

Welcome to minicom 2.7.1

OPTIONS: I18n 
Compiled on Jul 22 2021, 00:00:00.
Port /dev/ttyUSB0, 10:42:24

Press CTRL-A Z for help on special keys

From here you can type into the terminal to send to connected serial device.

To quit, similar to Screen, the commands are accepted through key commands. Ctrl+A, z, x will disconnect and close the application.

Minicom may store presets and can be configured using the setup mode, but it is not necessary for basic useage.

minicom -s

Redirecting echo

We can directly write to the serial device from bash by redirecting the output of the 'echo' command to the serial device. First we need to set the serial port speed and then send the text "helloworld" to the serial device:

stty -F /dev/ttyUSB0 speed 9600
echo helloworld >/dev/ttyUSB0

This method will not print responses from the serial device, and is most useful for one-way communication with a device, for example sending a string of text to an LED matrix display.

Troubleshooting Connectivity

Permission denied

Most permission denied error messages can be resolved by adding your user account to the 'dialout' group or the group with control of the serial port. We can check the group ownership of the port with the following:

ls -l /dev/ttyUSB0

crw-rw----. 1 root dialout 188, 0 Jan 21 11:05 /dev/ttyUSB0

In this case I would need to add my user account to the 'dialout' group:

sudo usermod -a $(whoami) -G dialout

`$(whoami)` can be replaced with your user name. Once added to the group, close and reopen the terminal, log out and back in, or restart the computer for the changes to take effect.

Scrambled Text

This is most often caused by incorrect baud rate settings on either the computer or device. Most serial devices like network routers and switches will have either a sticker with the serial port settings, or these can be found in the user manual for the device. For device with additional settings minicom may prove to be the easiest to configure.

The RS-232 connector on PL2303-DB9 uses a standard DTE pinout. The pin configuration is as follows:

The pin numbering can be found inside the DB9 connector of the adapter, and is as follows:

The original PL2303-DB9 was revised with an updated chipset in April of 2021. The original version of PL2303-DB9 featured a Prolific PL2303HXD chipset which was discontinued by Prolific in 2020. This version of the adapter featured a blue plastic casing as shown below:

The adapter has since been updated to use the more recent PL2303GT chipset from Prolific. This chipset offers many of the same features as PL2303HXD and is a drop-in replacement in most applications. The new version of the adapter features a grey plastic casing as shown below:

If you have any questions about the different versions please contact us at support@plugable.com and we'll be happy to assist.

A loopback test is a simple way to verify that a USB to RS232 adapter is functioning correctly. This test involves bridging the transmission (TX) and reception (RX) pins and sending data to the adapter and checking if it is received correctly.

Step 1: Identify the Adapter’s COM Port

Windows

1. Connect the USB to RS232 adapter to the computer.
2. Open Device Manager (Win + X > Device Manager).
3. Expand Ports (COM & LPT) and note the COM port assigned to the adapter (e.g., COM3).

macOS

1. Connect the adapter to the Mac.
2. Open Terminal and run:
   

   ls /dev/tty.*

3. Identify the device name (e.g., /dev/tty.usbserial-XXXXX). 

Linux

1. Connect the adapter to the system.
2. Open a terminal and run: 

   ls /dev/ttyUSB*

3. Note the assigned device (e.g., /dev/ttyUSB0).

Step 2: Bridge the TX and RX Pins

1. Identify the TX (Transmit) and RX (Receive) pins on the adapter. PL2303-DB9 uses a standard RS232 DTE pinout which assigns TX to pin 3, and RX to pin 2
   
   
2. Position a jumper wire, paperclip, screwdriver, or any other conductive object so it's touching pins 2 and 3 to electrically bridge them.

Step 3: Open a Terminal and Test Communication

Windows

1. Open a serial terminal application (e.g., PuTTY, Tera Term, or RealTerm).

2. Configure the connection:

   • Select the adapter’s COM port.

   • Set baud rate (e.g., 9600), data bits (8), parity (None), stop bits (1), and flow control (None).

3. Open the connection and type in the terminal. If the characters you type appear on screen, the adapter is working.

macOS & Linux

1. Open Terminal and run:

   screen /dev/ttyUSB0 9600

   Note: the name of the port you connect to should match the port name found in step 1.

2. Type in the terminal. If the characters you type appear on screen, the adapter is functioning.

3. To exit screen, press Ctrl + A, then K, and confirm with Y.

Troubleshooting

If no characters appear when performing the loopback test, ensure:

• The correct COM port is selected when connecting to the port.

• TX and RX pins are properly bridged.

• The adapter appropriate drivers are installed.

Windows historically assigns communications ports (COM ports) a number based on their order of detection. This worked fine when system boards had built-in Serial ports where the COM port number could be assigned in hardware or firmware settings, or Serial expansion cards which reliably detected in the same order on system boot. However USB connected serial adapters can be connected to, or disconnected from, the running computer in any order, and may detect in a different order on system reboot depending on other devices connected to the computer.

COM port retention stores the COM port number and settings for each USB to Serial adapter based on a unique identifier within the adapter chipset. This allows Windows to restore the COM port number and settings after a system reboot, or USB serial adapter unplug and connection.

This ensures that a serial adapter will be assigned the same COM port number reliably, even if there are multiple USB to serial adapters connected to the system. This provides the most benefit when using multiple serial devices on the same host computer by providing a reliable assignment of COM port numbers to specific devices.

Settings stored include

• The COM port number
• Bits per second (sometimes referred to as "baud rate")
• Data bits
• Parity
• Stop bits
• Flow control
• And additional advanced settings

Serial adapter settings can be found in the Windows Device Manager > Ports (COM & LPT) section on serial device Properties > Port Settings tab.

COM port retention is included with our latest PL2303-DB9 USB To RS232 Serial Adapter with brown over-molding, our older blue adapter does not include this feature.

If you have any questions please feel free to contact us at support@plugable.com and we'll be happy to assist!