On Series 1 and Series 1 Pro machines shipped after late January 2015, there is a change to the way the RUMBAs indicate serial port status.
Serial Numbers Affected:
Series 1 2000+
Series 1 Pro 10000+
Before late-January, the lights would be ON if there was no connection, where after late-January the lights would be OFF if there was no connection.
The earlier RUMBAs can be identified in the image below, followed by the newer RUMBAs:
You can see that the older RUMBA can be identified because it has two similar fuses.
The newer RUMBAs, however, have one yellow fuse, and one replaceable fuse.