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.