Allen-Bradley 1786-XT ControlNet Coaxial Trunk Terminator

Allen-Bradley 1786-XT ControlNet Coaxial Trunk Terminator

The Allen-Bradley 1786-XT, also cataloged as the 1786-XT/A ControlNet coaxial terminating resistor, operates as a dedicated hardware component for 75 Ohm impedance matching and signal reflection suppression within ControlNet trunk line networks.

Suffix Breakdown & Model Matrix

• 1786-XT: Standard 75 Ohm ControlNet coaxial trunk terminator utilizing standard industrial environmental specifications (-40 deg C to +85 deg C operating temperature).
• 1786-XT/A: Series revision variant specifying strict ambient temperature handling protocols (0 deg C to +60 deg C operating temperature envelope).

Hardware Specifications

Deterministic Network Trunk Impedance Termination

In deterministic fieldbus architectures such as EtherNet/IP and ControlNet, physical media termination prevents high-frequency data transmission corruption. The 1786-XT maintains constant 75 Ohm line load across the 5 MHz transmission frequency spectrum, attenuating signal reflection at both physical drop cable endpoints. Backplane bus communication stability depends directly on clean waveform propagation; un-terminated trunk nodes introduce standing waves that trigger frame cyclic redundancy check (CRC) failures, node dropouts, and backplane packet drop delays.

Frequently Asked Questions

Q: Does the 1786-XT require external DC power or backplane current draw?

A: No. The 1786-XT is a purely passive resistive component that draws 0 mA from the backplane or external supply, operating entirely via passive termination across the coaxial signal path.

Q: Can standard 50 Ohm BNC terminators be substituted for the 1786-XT on a ControlNet network?

A: No. ControlNet media topology strictly mandates 75 Ohm characteristic impedance. Inserting a 50 Ohm terminator causes a severe impedance mismatch, resulting in signal reflections, increased bit error rates, and network failure.

Field Installation Guidelines

1. Install exactly two 75 Ohm terminators per ControlNet coaxial segment, placing one unit directly on the unused port of the BNC T-connector at each physical end node of the trunk line.
2. Ensure full 1/4-turn bayonet lock engagement on the BNC plug to maintain low-resistance grounding contact between the shield and nickel-plated connector body.
3. Keep coaxial cable run isolation compliant with standard industrial noise abatement practices, maintaining separation from high-voltage AC motor drives and field power conductors.