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Communication Protocols · Free Tool

Foundation Fieldbus H1 Segment Calculator

Check an FF H1 or PROFIBUS PA segment: total current, voltage drop along the trunk and spur, and the voltage left at the farthest device.

Device currentVoltage drop9 V minimumCable length
V
mA
mA
m
m
Ω/km
V
Total segment current250 mA
Voltage at farthest device17.35 V (OK)
Trunk voltage drop6.6 V
Spur voltage drop0.0528 V
Margin above minimum8.347 V
Total cable if every spur is the longest1,320 m
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Segment Check

I = n × Idevice + Iother
Vend = Vs − I × R × Ltrunk − Idevice × R × Lspur

R is loop resistance per km (both conductors); type A fieldbus cable is about 44 Ω/km loop (22 Ω/km per conductor). Devices need at least 9 V.

Worked example

24 V, 12 devices at 20 mA plus 10 mA, 600 m trunk, 60 m spur, 44 Ω/km: I = 250 mA, drop 6.6 V trunk and 0.05 V spur, about 17.3 V at the last device.

Key insight: leave a voltage margin of a few volts above 9 V for future devices and ageing; segment design tools from your host system vendor give the final check.
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Frequently Asked Questions

What is the maximum H1 segment length?

1900 m total for type A cable, including all spurs.

How many devices per segment?

Up to 32 in principle; projects often limit to about 12 to 16 for risk and speed.

Does this apply to PROFIBUS PA?

Yes, PA uses the same IEC 61158-2 physical layer.

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