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
Updated May 7, 2026

Applicable To

PL2303-DB9

Serial communications over RS232 is a legacy standard, and several variables—including driver loading, software compatibility, COM port assignment, port settings, and cable type—must be correctly configured. Verifying these configurations usually resolves communication problems between the adapter and your legacy device.

Troubleshooting Steps

Driver Installation Issues

Systems running modern versions of Windows usually handle driver installation automatically, but manual installation is sometimes required for older operating systems or offline computers.

1. Connect the adapter to your computer and ensure you have an active internet connection to let Windows 7 or higher automatically load the driver.

2. Download and install the latest drivers manually from the Plugable website if an internet connection is unavailable or if you are using Windows XP or Vista.

Software Compatibility Issues

Many legacy devices use specialized software written for older operating systems, which may fail to function correctly on modern versions of Windows.

1. Check the device manufacturer's website to verify if your legacy device's specialized software is officially compatible with your current version of Windows.

2. Download and install a standard serial communications program like PuTTY to test the connection manually if your device's software is incompatible or missing.

Incorrect COM Port Assignments

Older Windows applications often restrict communication to specific COM port numbers (typically COM 1 through 4), requiring manual reassignment if Windows assigns the adapter a port outside this range.

1. Open Device Manager and expand the Ports (COM & LPT) section.

2. Locate the "Prolific USB-to-Serial Comm Port" and note the COM port number assigned by Windows.

3. Open your serial communication application and configure it to use this newly assigned COM port number.

4. Right-click the adapter in Device Manager and select Properties if your legacy application requires a specific port range.

5. Select the Port Settings tab, then click the Advanced button.

6. Click the COM Port Number drop-down menu and select an available port within the required range.

7. Click OK to close the Properties dialog and save the settings.

Mismatched Port Settings

Serial devices require exact matches for settings like baud rate, data bits, parity, stop bits, and flow control to successfully transmit and receive data.

1. Check your legacy device's manual to find its required communication port settings.

2. Right-click the Prolific USB-to-Serial Comm Port in Device Manager and select Properties.

3. Select the Port Settings tab and adjust the adapter's settings to perfectly match the requirements of your legacy device.

4. Ensure these same settings are identically configured within your third-party communication software.

Incorrect Cable Type

RS232 devices require either a "straight-through" or "null modem" (crossover) wiring configuration, and using the wrong type will prevent communication even if all software settings are correct.

1. Check your legacy device's manual to determine if it requires a straight-through or null modem connection.

2. Attach a null modem adapter to the end of the Plugable PL2303-DB9 if your device requires a crossover connection, as the Plugable adapter uses standard straight-through wiring natively.

Expected Result

Device Manager will update and display the newly selected COM port number and settings, and the adapter will use the correct wiring configuration, allowing your legacy application or serial communications program to successfully communicate with the connected device.

If the Issue Persists

Contact Plugable's support team at support@plugable.com for further troubleshooting assistance.

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*

You can send and receive data from serial devices in Linux using command-line utilities like Screen or Minicom, or by redirecting the echo command. Before establishing a connection, you must identify the correct serial port path, which is typically found at /dev/ttyUSB0 for USB serial adapters.

How to Identify the Serial Port

1. Connect the serial adapter to the computer.

2. Open a terminal and run dmesg | tail to look for the attached converter path, or run ls -tr1 /dev/tty* | tail -n 1 to print the newest created serial adapter.

How to Connect Using Screen

1. Run screen /dev/ttyUSB0 9600 in the terminal to transmit and read text.

2. Disconnect while leaving the session open by pressing Ctrl+a then Ctrl+d, or quit completely by pressing Ctrl+a then k, and 'y' to kill the session.

How to Connect Using Minicom

1. Run minicom -b 9600 -D /dev/ttyUSB0 to open an interface to send text.

2. Quit the application by pressing Ctrl+a, then z, then x.

3. Configure presets using the setup mode by running minicom -s.

How to Write Directly to the Serial Device

1. Set the port speed with stty -F /dev/ttyUSB0 speed 9600.

2. Send text for one-way communication by running echo helloworld >/dev/ttyUSB0.

Workarounds for Serial Device Errors

• Resolve permission denied errors by adding your user account to the 'dialout' group or the group with control of the serial port. Check group ownership with ls -l /dev/ttyUSB0 and add your account by running sudo usermod -a $(whoami) -G dialout, then log out and back in or restart the computer for the changes to take effect.

• Fix scrambled text by correcting baud rate mismatches on either the computer or the device. Check the device's sticker or user manual for the correct port settings and configure them accordingly.

Applicable To

PL2303-DB9

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!