 Another representation of the PID controller is the series, or "interacting" form
where the parameters are related to the parameters of the standard form through
with

This form essentially consists of a PD and PI controller in series, and it made early (analog) controllers easier to build.When the controllers later became digital, many kept using the interacting form.The analysis for designing a digital implementation of a PID controller in a microcontroller (MCU) or FPGA device requires the standard form of the PID controller to be "discretized".