In accordance with the invention, there is described a sign display panel made up of a controller connected in a serial communication network with a set of display units. Each of the display units receives messages from the controller and detects errors in the messages. The controller sends a series of communication integrity messages to the set of display units. When a display unit receives the communication integrity message, it sends it to the next display unit or back to the controller. If a display unit does not receive a communication integrity message within a set time interval, the display unit sends an error message that is received by the controller.