A factory automation, FA, system comprising a plurality of controllers (100A to 100C) each of which is configured to control a driving device to be controlled, each of the plurality of controllers (100A to 100C) including: a buffer; a communication module configured to perform packet communication with one or more other controllers; a packet monitor module configured to buffer packet data generated in its own controller and packet data received by its own controller in the buffer; and a stop module configured to stop, in response to a predetermined stop condition being satisfied, a buffering function of the packet monitor module for a controller specified from the plurality of controllers (100A to 100C), characterised in that, the packet data has source information for identifying a source controller, and destination information for identifying a destination controller, and the FA system further comprises an external device configured to communicate with the plurality of controllers (100A to 100C), the external device including a communication module configured to receive packet data stored in the buffer of each of the plurality of controllers (100A to 100C) from each of the plurality of controllers (100A to 100C), and a display configured to display the source information and the destination information about each packet data received from each of the plurality of controllers (100A to 100C), wherein the display is configured to display packet data having matching source information and matching destination information in a corresponding manner. a buffer; a communication module configured to perform packet communication with one or more other controllers; a packet monitor module configured to buffer packet data generated in its own controller and packet data received by its own controller in the buffer; and a stop module configured to stop, in response to a predetermined stop condition being satisfied, a buffering function of the packet monitor module for a controller specified from the plurality of controllers (100A to 100C), characterised in that, the packet data has source information for identifying a source controller, and destination information for identifying a destination controller, and the FA system further comprises an external device configured to communicate with the plurality of controllers (100A to 100C), the external device including a communication module configured to receive packet data stored in the buffer of each of the plurality of controllers (100A to 100C) from each of the plurality of controllers (100A to 100C), and a display configured to display the source information and the destination information about each packet data received from each of the plurality of controllers (100A to 100C), wherein a communication module configured to receive packet data stored in the buffer of each of the plurality of controllers (100A to 100C) from each of the plurality of controllers (100A to 100C), and a display configured to display the source information and the destination information about each packet data received from each of the plurality of controllers (100A to 100C), wherein the display is configured to display packet data having matching source information and matching destination information in a corresponding manner.