FOUNDATION Fieldbus (FF) is a Local Area Network (LAN) for process control sensors, actuators, and control devices. It is a digital, two-way, multi-drop network positioned to replace proprietary networks that connect Programmable Logic Controllers (PLCs) of many big companies. In traditional systems, a computer or expensive controller unit provides control for the network of devices. In recent years, some companies have developed instrument systems with more intelligence built into the devices. The main drawback of these systems is that they are proprietary and thus not interoperable. There are other open instrumentation standards, such as PROFIBUS and HART, but these protocols do not implement control -- a separate controller is still required. FF is an open standard that allows the field devices to run both the input/output and the control.
Traditional systems in this market are known as 4-20 mA systems, so named for the signal levels that control the devices. These devices take input measurements and send the information to a control unit for processing. The computer then performs the necessary calculations and tells devices what their outputs should be. To get a device's serial number or to calibrate or test a device in the field, a person would have to walk (or drive) to the physical device. With FF, these types of things can be done directly from the control room. What if the computer that is running the control goes down? In a traditional system, the devices would go into some pre-defined fail-safe mode, leaving the pumps, valves, and other equipment without any interactive control until communication with the computer resumes. With FF, control is brought down to the device level. After downloading the configuration, your control loop (a PID loop, for example) continues operating even if the monitoring computer is disconnected.
4-20 mA System
|Interoperability & Interchangeability||OPEN, interoperable, interchangeable.||PROPRIETARY, limited interoperability, no interchangeability.||OPEN, interoperable, interchangeable.|
|Variety & Availability of Equipment||Wide variety. Multiple suppliers.||Limited. The customer can use only the offerings of one supplier.||Actual FF devices are limited but growing. There are, however, many ways of incorporating standard 4-20 mA devices into an FF network.|
|Communication||One variable. One direction.||Multiple variables. Two-way.||Multiple variables. Two-way.|
|Wiring||Point-to-point.||Point-to-point or multi-drop.||Multi-drop.|
Table 1. Comparison of FOUNDATION Fieldbus and traditional systems.