A control system including control devices with each control device including a control unit and a display device connected with the control unit via a dedicated communication line and being capable of display corresponding to a control state of the control unit. The display device includes a communication port connected with a control unit and a communication port connected with a data processing device, and a data processing section. Further, the data transmission method includes a dedicated protocol communication transmitting data with a processing device and a common protocol communication transmitting data with another processing device. In addition, the control-host computer used in the control system includes a control unit controlling a control target and a display device.