Layer 1 of the OSI Reference Model is where we define the “physical” elements of a digital data network.
The H1 FF network exhibits the following properties:
- Two-wire (ungrounded) network cable
- 100 ohm (typical) characteristic impedance
- DC power is conveyed over the same two wires as digital data
- 31.25 kbps data rate
- Differential voltage signaling (0.75 volts peak-to-peak transmit minimum ; 0.15 volts peak-to peak receive threshold minimum)
- Manchester encoding
Since DC power is conveyed over the same two wires as the digital data, it means each device only needs to connect to two wires in order to function on an H1 network segment. The choice of a (relatively) slow 31.25 kbps data rate allows for imperfect cables and terminations which would otherwise plague a faster network. Manchester encoding embeds the network clock pulse along with the digital data, simplifying synchronization between devices.
As you can see, the layer 1 design parameters were chosen to make FF H1 networks easy to build in unforgiving industrial environments. The physical layer of FOUNDATION Fieldbus happens to be identical to that of Profibus-PA, further simplifying installation by allowing the use of common network validation tools and connection hardware.
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