How to get stable COM-ports for USB serial dongles in Windows XP?

In my experience, plugging the device into the exact same USB port each time results in it getting the same COM port number.

It's also possible to change the port number assigned to a device (Device Manager, edit Properties of the COM port) if you need to plug it into a different USB port. In the past I have labeled a device with a COM port number, and then plugged it into each USB port on my laptop and reassigned the COM port to be that number, regardless of where it's plugged in.

The FTDI driver will assign serial ports to the same COM ports by the serial number of the device connected. So, the ports should remain the same regardless of USB port. However, if you plug in a new USB device, it will not reuse the previously assigned COM ports.

FTDI has an app note explaining the process in more detail.

I've had general stability problems with Prolific drivers, so I do not use Prolific USB-to-serial devices and can't comment on their mapping strategy.

Use ComPortMan to bind COM port numbers to devices. Taken from the Help page, here's an example ComPortMan.INI:

[ComPorts10]
DeviceID=USB\VID_067B&PID_2517
PortName=COM5

[ComPorts20]
DeviceID=USB\VID_1234&PID_5678
PortName=COM6

Note: I've found that sometimes a * needs to be appended to the Device ID for this to work.