I have a client(Windows 7 Pro) that make a connection to Windows terminal server through RDP, i have enabled COM port redirection for both server and client side, but the redirection doesn't happen ...
The client don't have a native mainboard COM port, but have an USB-TO-Serial adapter (Prolific USB-to-Serial), it is possible redirect this USB-TO-Serial COm Port ?
It's not currently possible without 3rd party software. Windows sees this device as a USB device, not a COM port (yes I understand the irony). RDP doesn't currently support remote USB devices (hopefully some day it will).
If you're running 2008r2 and win 7 ultimate, you do have the option to look at remotefx USB redirection... to redirect USB devices... or look at 3rd party software like this.
I don't think the accepted answer here is entirely correct.
Right next to me is a barcode scanner connected to a Prolific USB2Serial adapter connected to a Windows 7 client (with installed Prolific Drivers). The Client sees this Prolific Adapter as COM3.
Now if I connect to any terminal server while allowing ports to be mapped (Options > Local Resources > More. Check "Ports"), that virtual COM3 will be available on the Terminal Server with no problem.
So to answer the question:
If you can install drivers on the Terminal Server Client machine and you install the Prolific driver, then you can enable Port redirection on the Client and the Port will be available and working correctly on the Server.
If you can't install drivers on the client, you will need an external 3rd party software as it was linked by @TheCompWiz
I had a USB-based serial port for a credit card reader that I needed to redirect through RDP. I configured Remote Desktop to make ports available to the remote session. Initially, I had the serial port to COM100 (to easily avoid potential conflicts), and it didn't work. It did work, however, when I changed the port to COM2.
We specifically use these adapters professionally with POS receipt printers and they work fine. Just know that you can't use the windows drivers on the server and need something that maps the ports via the registry such as the OPOS drivers we use for the receipt printers.