Gold fishing use an array of sensors under the bowls to detect the balls being thrown and display the colored light pattern. When the game boots up as the playfield sensors power on they report their ID number to the main board which it then displays the ID that it received from the playfield sensors on the balls left display. There are 25 sensors on the playfield and the main board will not continue its boot process until it has received an ID from all of the boards.
During the boot process when the game is first turned on all of the bowls will slowly turn green when they start their enumeration process and the heart beat indicator LED on the bottom side of the board will start to blink when this occurs, this activity is a function of the sensor board and only indicates that the sensor has started the process not that the main board has received an ID from the sensor. During the startup sequence the LEDS on the sensor can change different colors RED indicates the optics on the sensor are blocked, YELLOW indicates the sensor cannot determine the state of the optics. The voltage to the sensor boards can effect their operation the 12vdc to the sensor boards is critical and should be no lower than 12vdc at the furthest point in the harnessing (sensors 6, 17, and 25), low voltage can cause the sensor boards to reset during the boot process or incorrectly identify the state of the optics.
Please see the attached documentation for the game for detailed information on the sensor pin outs and the expected signals you can read during the boot process also test modes that are available to assist in troubleshooting the playfield.