On this board everything starts in a TIBPAL 16L8-25CN. When I looked up the data sheet It gives 9 pins are the input, but 18,19 and 12,13 are I/O. Then the signal goes to twin Darlington Pair chips that drive the IRFZ34 that direct link to the connectors of the led arrays. By sending a digital signal with the message in a stream on the I/O legs I could then interrupt the flow coming from the processor that comes with the sign and replace the signal from the Arduino. Then interrupt the clock and put in the clock from the Arduino.