A control system comprising an input/output (I/O) module and a controller, wherein the controller can be connected to a field device via the input/output (I/O) module, the I/O module comprising: a PWM (pulse width modulation) demodulator configured to demodulate a PWM signal that is input from the controller; a current source configured to output a current based on an output signal of the PWM demodulator to the field device; a resistor connected in series with the current source; and a signal conversion unit configured to feed back an output current of the current source to the PWM demodulator via the controller, wherein the signal conversion unit comprises an A/D converter configured to convert an analog signal to a digital signal, and a current of an analog input that is detected via the resistor is input to the controller via the A/D converter, wherein the control system is characterised in that the controller is adapted to output a respective PWM signal corresponding to a respective one of modes including an analog output mode in which a current of a setting value is caused to be output from the current source via resistor to the field device and an analog signal is thus output to the field device, a digital output mode in which a current corresponding to on and off of a digital output value is caused to be output from the current source via resistor to the field device and a digital signal is thus output to the field device, an analog input mode in which a power supply voltage to the field device is caused to be supplied from the current source via resistor and an analog signal is input from the field device, and a digital input mode in which a power supply voltage to the field device is caused to be supplied from the current source via resistor and a digital signal is input from the field device, and wherein in the analog input mode, the converted digital signal of the A/D converter is input to the controller as an analog input measuring value, and wherein in the digital input mode the converted digital signal of the A/D converter is input to the controller and used as a high/low, H/L, determination value of the digital input value. a PWM (pulse width modulation) demodulator configured to demodulate a PWM signal that is input from the controller; a current source configured to output a current based on an output signal of the PWM demodulator to the field device; a resistor connected in series with the current source; and a signal conversion unit configured to feed back an output current of the current source to the PWM demodulator via the controller, wherein the signal conversion unit comprises an A/D converter configured to convert an analog signal to a digital signal, and a current of an analog input that is detected via the resistor is input to the controller via the A/D converter, wherein the control system is characterised in that the controller is adapted to output a respective PWM signal corresponding to a respective one of modes including an analog output mode in which a current of a setting value is caused to be output from the current source via resistor to the field device and an analog signal is thus output to the field device, a digital output mode in which a current corresponding to on and off of a digital output value is caused to be output from the current source via resistor to the field device and a digital signal is thus output to the field device, an analog input mode in which a power supply voltage to the field device is caused to be supplied from the current source via resistor and an analog signal is input from the field device, and a digital input mode in which a power supply voltage to the field device is caused to be supplied from the current source via resistor and a digital signal is input from the field device, and wherein in the analog input mode, the converted digital signal of the A/D converter is input to the controller as an analog input measuring value, and wherein in the digital input mode the converted digital signal of the A/D converter is input to the controller and used as a high/low, H/L, determination value of the digital input value.