How about just answering the poor lad's question
Here's the answer. (We had a similar situation in the test 76C++ last year, so I had looked it up)
When Test switch (Initiated Test) is touched, if the DECU ARINC is not being received the digital display will read "ArNO", if the DDR data is not being received the digital display will read "drNO", and if the Raw N1 signal is not being received the display will read "rANO".
As you observed, it takes a bit for the DDR & DECU to start sending their outputs. Until then, you'll get the appropriate error message.
Good Luck.
Hoss