FOUNDATION Fieldbus is an all-digital, Serial communications, two-way communications system that serves as the base-level telecommunications network in a plant or factory automation environment. It is an open architecture, developed and administered by the Fieldbus Foundation.
It's targeted for applications using basic and advanced regulatory control, and for much of the discrete control associated with those functions. Foundation fieldbus technology is mostly used in process industries, but nowadays it is being implemented in powerplants also.
Two related implementations of FOUNDATION fieldbus have been introduced to meet different needs within the process automation environment. These two implementations use different physical media and communication speeds.
- Foundation Fieldbus H1 works at 31.25 kbit/s and generally connects to field devices. It provides communication and power over standard twisted-pair wiring. H1 is currently the most common implementation.
- HSE (High-speed Ethernet) works at 100 Mbit/s and generally connects input/output subsystems, host systems, linking devices, gateways, and field devices using standard Ethernet cabling. It doesn't currently provide power over the cable, although work is under way to address this.
FOUNDATION fieldbus was originally intended as a replacement for the 4-20 mA standard, but many delays in its development have prevented its large scale use, and has allowed other technologies such as Modbus, Profibus, and Industrial Ethernet to gain large installed bases. FOUNDATION fieldbus was developed over a period of many years by the International Society of Automation, or ISA, as SP50. In 1996 the first H1 (31.25 kbit/s) specifications were released. In 1999 the first HSE (High Speed Ethernet) specifications  were released. The International Electrotechnical Commission (IEC) standard on field bus, including Foundation Fieldbus, is IEC 61804.
A typical fieldbus segment consists of the following components.
- H1 - fieldbus interface card
- FPS - Fieldbus Power Supply and Signal Conditioner
- PS - Bulk power (Vdc) to Fieldbus Power Supply
- T - Terminator
- And fieldbus devices, (e.g. transmitters, transducers, etc.)
Typical Fieldbus Segment: