FOUNDATION Fieldbus H1 is a two-wire, digital, multi-drop field communication protocol designed for process automation. Developed by the Fieldbus Foundation (now FieldComm Group), it enables bi-directional digital communication with field devices while powering them over the same two wires.

How FOUNDATION Fieldbus Works

Unlike 4-20mA where each device needs its own pair of wires and carries only one variable, FOUNDATION Fieldbus H1 allows multiple devices to share a single segment. Key characteristics:

  • Communication speed: 31.25 kbps
  • Maximum devices per segment: 16 (practical limit: 6-12 with intrinsic safety)
  • Wire: standard instrumentation cable (twisted pair, shielded)
  • Maximum segment length: 1900 m (without repeaters)
  • Power: supplied on the bus wire itself via a fieldbus power conditioner

Function Blocks in Field Devices

The revolutionary aspect of FOUNDATION Fieldbus is that control function blocks execute inside the field device itself. A flowmeter can execute a PID block, outputting a control signal to a valve on the same segment. This distributed control reduces DCS controller load and improves loop response time.

Standard function blocks include: AI (Analog Input), AO (Analog Output), PID, Discrete Input (DI), Discrete Output (DO), and Signal Characterizer (SC).

Segment Design and Wiring

Critical design rules:

  • Termination: Exactly two terminators per segment – one at the fieldbus power conditioner, one at the far end of the longest spur
  • Spurs: Short branches off the trunk. Maximum spur length is 120 m (longer in non-hazardous areas)
  • Intrinsic Safety: Use entity parameters or FISCO/FNICO calculations to verify IS compliance when devices are in hazardous areas
  • Shielding: Ground the shield at one end only to prevent ground loops

Commissioning Steps

  1. Wire the segment with terminator at both ends
  2. Power up the fieldbus power conditioner and verify 9 VDC minimum at the furthest device
  3. Use a fieldbus diagnostic tool (Relcom FB-Test, Pepperl+Fuchs FieldConnex) to verify signal quality
  4. Commission devices in the host DCS using device descriptions (DD files)
  5. Configure function block parameters and link schedules
  6. Perform control loop checkout with process simulation