A control system includes a control section provided with a CPU (processor) for controlling a machine to be controlled, an operating section provided with an input/output card interface for enabling connection of an input/output card, and a communications circuit for performing communications between the control section and the operating section, the control section comprising a first serial communications interface connected to the CPU via a first communications control circuit, the operating section comprising a second serial communications interface connected to the input/output card interface via a second communications control circuit, and the communications circuit providing a connection between the first serial communications interface and the second serial communications interface.