Does usb to serial multi port communicate only for one port at a time?
There might be issues with the EXACT device you have, but in my experience (which is with FTDI based devices), there's no problem running bytes out multiple ports at the same time. Now, be careful with your programming because if you want to do this you have to make sure that you are either using multiple threads, or starting your reads and writes in such a way that they don't block the calling thread.

What's the exact chipset? If you don't know (and well you might not), then try looking up the VID &amp; PID for the USB device (in Windows, you can find that via the device manager control panel. In Linux, try 'lsusb' as root.)

The VID is specific to a manufacturer, while the PID is specific to a device. Using the VID/PID, you can track down what device is inside the thing, and thus look up the data sheet and check for any oddball gotchas.