Unitrol 6080 excitation system - problem

Dear all,

We've used a Unitrol 6080 excitation system for our hydro power plant (01 CIO PC D235; 02 Unitrol 6080 run in redundant). The problem is in one of our Unitrol 6080 stop working. The Service control panel (SCP) of the error Unitrol display message "Establishing UNITROL communication". I've tried to replace our spare Unitrol unit but the display message was not change.
So I would like to ask what is the problem with our Unitrol system? How to establish a new Unitrol excitation system?
(By the way I do not know why there is not much document about Unitrol 6080 in ABB websites)

Thanks.