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.
Connect the adapter to your computer and ensure you have an active internet connection to let Windows 7 or higher automatically load the driver.
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.
Check the device manufacturer's website to verify if your legacy device's specialized software is officially compatible with your current version of Windows.
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.
Open Device Manager and expand the Ports (COM & LPT) section.
Locate the "Prolific USB-to-Serial Comm Port" and note the COM port number assigned by Windows.
Open your serial communication application and configure it to use this newly assigned COM port number.
Right-click the adapter in Device Manager and select Properties if your legacy application requires a specific port range.
Select the Port Settings tab, then click the Advanced button.
Click the COM Port Number drop-down menu and select an available port within the required range.
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.
Check your legacy device's manual to find its required communication port settings.
Right-click the Prolific USB-to-Serial Comm Port in Device Manager and select Properties.
Select the Port Settings tab and adjust the adapter's settings to perfectly match the requirements of your legacy device.
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.
Check your legacy device's manual to determine if it requires a straight-through or null modem connection.
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.
