The power board for MV IV is located behind the main pcb with the eight segment display and dip switches. This board provides power for communication between the main pcb and many of the other boards in the system, including communication board, inverter boards and the external pcb. Its relationship between the main pcb and so many other boards means that any internal board communication error for the system should begin with checking the output of this board.
There is a 20 pin molex connection in the upper left side of the main pcb. This connection will have eight pairs of wires going from the main pcb back to the power board. The black wire here is pin one on the corresponding chart:
As you can see from the chart, you are looking for 5, 12 or 15 vdc depending on which pins are being checked. You will need needle tips for your multi-meter to be able to check these values. This check is performed with the power to the system applied. Any deviation below the listed values can generate different communication errors with the associated boards.
If you suspect you have a power board problem and have multiple units on your site, it is always a good idea to take a power board from a working system and temporarily install it into the trouble unit. This will resolve your communication error and will reveal if the system has any other errors related to the event that caused the power board to fail. This can help save you making two trips to the site. Happy checking.