Building Your Own Terminators for the Network

Left unterminated, transmission lines (such as a Foundation Fieldbus segment) suffer from signal distortion due to signal reflections at each end of the line. To prevent this, an impedance matching unit is added to the end of each line.

Power hubs/termination blocks shipped by NI

National Instruments ships power hubs/termination blocks from Relcom, Inc. This kit comes with the combined power hub and termination block, a power supply for bus-powered devices, and cables. This provides the termination necessary for connecting a Fieldbus interface card and three devices. It is appropriate for benchtop applications of Foundation Fieldbus, such as those found in engineering labs for device development and instrumentation maintenance facilities. For actual plant-floor installations, making your own terminators is generally more appropriate.

This kit is included in the Foundation Fieldbus starter kits. It can be ordered separately for single board purchases.

777335-01 Fieldbus Benchtop Power Supply Kit, US 120 VAC
777335-31 Fieldbus Benchtop Power Supply Kit, EURO 220V Power

Note: Following the link below and changing the ship-to country to a country with 220V Power will cause the European option to appear.

See Also:
Catalog Entry for Power Hub/Terminator


Making your own

Because Foundation Fieldbus networks provide power along with the signals on the same set of wires, they require more than just a resistor for termination. A capacitor in series with a resistor blocks the DC voltage used to power the devices, but passes the AC signal. This allows the signal to be terminated without dissipating the bus power.

To make your own terminator, place a 100 ohm resistor in series with a 1microfarad capacitor.

For information on where to place terminators, see the related link Wiring and Signaling. Keep in mind that some devices, such as I.S. barriers and power supplies, might incorporate terminators into terminal blocks. Check the device's documentation to determine if this is the case.

See Also:
Wiring and Signaling

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