Instruments communicate with some sort of signal, often adhering to a standard. This signal may be defined by standards associations, or it may be a proprietary standard. Some standards include:

These devices are generally connected to some sort of control system which responds to the instrument's measurement. The response programmed into the control system manipulates control devices attached to the process. This cycle of changing of manipulated variables, measurement of controlled variables, and proper response is the basic concept behind process control. The control systems used are often considered part of the field of instrumentation, and can include simple Programmable Logic Controllers(PLCs) or Remote Terminal Units (RTUs), to more advanced Distributed Control Systems (DCSs). Inputs can vary from a few measured variables, to thousands of measured variables.