from the machine control guide...When Serial Time Code is enabled, the Clock Reference setting in the USD Setup section of the Session Setup window switches to Video Reference/Video In. Additionally, the Positional Reference popup is grayed out and displays Serial Timecode.Pro Tools automatically puts the USD in a special form of Generate mode and prohibits any changes to the USD Positional and Clock Reference settings. While Serial Time Code is being used, this Generate mode is engaged to calculate frame-edge accuracy in Pro Tools, but the generator itself is not actually running. It is only shown as active in Session Setup to avoid confusion over where the Positional Reference is coming from (in this case, the serial connection).