In this post it is showed how to test the RS-232 on Ardbox HF family versions.
20 IOs PLC: Ardbox Family Products
RS-232 Library: Industrial Shields RS-232 Library (Github)
Connecting an Ardbox with other equipment through RS-232.
On the Sketch is showed that you must send a 25 thought RS-232 to complete this test.
RS-232 Test sketch: