Circuit design is a function of hardware engineering. Here, circuits can contain discrete components, Field Programmable Gate Arrays (FPGA) and micro-controllers. The circuits range from simple monitoring circuits and filters to more complex digital and analog circuitry.
There can be a number of ports interfacing the outside world, such as, serial (RS232, RS422, RS485, USB,SPI, etc.), Ethernet, Wireless, just to name a few. This enables communication to and from the circuitry. The type of components and complexity of design is dependent upon the application.