Main port (the corresponding icon is in Windows CE or Windows Mobile Pocket PC) is a port which is allowed to manage serial port control lines. There may be only two ports of this kind in a ports bundle: one is assigned for an In-side and another one – for an Out-side. The presence of control port in a bundle is optional. If no main port is set, then neither port has the right to manage control lines.
To set main port, please, select serial port you want to set as main in a bundle and click the corresponding icon in Windows CE or Windows Mobile Pocket PC.
To set main port in Windows Mobile Smartphone, please, select serial port you want to set as main in a bundle, click left soft key, click "4" button and then "3" button: