Order now 1786-XT to guarantee the electrical stability and signal integrity of your industrial ControlNet bus. The Allen-Bradley 1786-XT is a precision-manufactured 75-ohm resistor designed specifically to terminate the physical ends of a ControlNet trunk cable segment. In any high-speed industrial network, signals that reach the end of a cable will «reflect» back if not properly absorbed, causing destructive interference and data corruption. The 1786-XT acts as an electrical sink, matching the characteristic impedance of the RG-6 quad-shielded cable to ensure that signals are cleanly terminated. Utilizing genuine OEM terminators is critical for preventing the intermittent «Node Offline» or «Media Fault» alarms that often arise from using improper resistances. By securing the 1786-XT at both ends of every network segment, you ensure the 5 Mbps deterministic performance required for mission-critical PLC and I/O communications.
What Is the 1786-XT?
The 1786-XT is a BNC-style male terminator containing a precision 75-ohm resistor. It is an essential component of the ControlNet physical layer, required for every network segment regardless of the number of taps or nodes. Its ruggedized construction is designed for factory floor use, offering resistance to vibration and environmental contaminants. In a redundant media system, a total of four 1786-XT terminators are required per segment—two for Trunk A and two for Trunk B. Without these terminators, the ControlNet signal will suffer from standing waves, leading to high error rates and potential total system failure.
Key Technical Specifications
- Part Number: 1786-XT
- Manufacturer: Allen-Bradley / Rockwell Automation
- Resistance: 75 Ohms (+/- 1%)
- Connector Type: BNC Male
- Compatibility: All ControlNet RG-6 Trunk Taps (1786-TPS, TPR, TPYS)
- Power Rating: Optimized for industrial signal levels
- Operating Temperature: -20 to +70 Celsius
- Package Quantity: Sold as individual units (Standard configurations require two per segment)
System Integrity and Signal Reflections
ControlNet is a high-speed, Manchester-encoded network that is extremely sensitive to physical layer discontinuities. The 1786-XT prevents the signal from bouncing off the end of the wire and returning as noise. This is technically known as avoiding a High Standing Wave Ratio (SWR). Even a single missing 1786-XT can cause an entire network to stop functioning once a second node begins to transmit, as the reflected pulses overlap with new data packets. This terminator is the final step in certifying a ControlNet installation for factory production.
Maintenance Tips
A frequent expert maintenance tip is to check the 1786-XT during annual shutdowns using a digital multimeter. The resistance should read exactly 75 ohms across the center pin and the outer shell; if the reading drifts above 80 ohms or below 70 ohms, the internal resistor has likely suffered from thermal stress or moisture ingress and should be replaced. Additionally, ensure the terminator is locked with a full quarter-turn. Loose terminators are a common cause of intermittent network jitter that is notoriously difficult to diagnose with software alone. Always verify that no dust or metallic shavings have entered the BNC jack before connecting the 1786-XT.
Frequently Asked Questions
Q: Can I use a 50-ohm terminator from an Ethernet network in place of the 1786-XT?
A: Absolutely not. Using a 50-ohm terminator will cause a significant impedance mismatch, leading to signal reflections that will likely bring down the entire ControlNet segment.
Q: Does every tap require a 1786-XT?
A: No. Only the taps located at the physical ends of the trunk cable run require a terminator on their unused trunk port. Intermediate taps should have their trunk ports connected to the next tap in the line.
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