San Clemente, CA
The ribbon cable carries the address and data information for communication from the MPU to other boards on the system. If any board is tying any of these lines to Vcc or GND, then communication is not possible. The system may not boot or other strange issues can result.
This can happen either because of a misconnection of the cable on one of the headers, because a board is not receiving 5V and ground correctly, or because of a fault on the board or ribbon cable connector.
Check to make sure each of the boards if correctly powered and grounded and receiving 5V between the gray and black wires on the power connector.
