XNX XNX Transmitter Wiring Diagram
This is a free printable xnx xnx transmitter wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
The Honeywell XNX Universal Transmitter is a 4-20 mA gas detection device used in hazardous-area safety systems. This wiring diagram guide explains how to connect the XNX to its power supply, sensor head, and control panel, helping safety engineers and field technicians commission fixed gas detection systems correctly on the first attempt.
The Honeywell XNX Universal Transmitter operates as a loop-powered 4-20 mA device that bridges a remote sensor head and a central control panel or DCS. Understanding the exact wiring is critical because incorrect connections in a hazardous area can produce silent faults that only appear during a real gas event.
In the standard three-wire configuration, terminal 1 receives the positive 24 V DC supply from the control panel, terminal 2 carries the 4-20 mA signal return, and terminal 3 provides a dedicated ground reference. The 4 mA signal level represents 0 % LEL for flammable gas detection or 0 ppm for toxic gas detection; the 20 mA level represents full-scale concentration. Any signal below 3.6 mA indicates a hardware fault — either a cable break, sensor failure, or transmitter fault — and should trigger a distinct fault alarm at the control panel rather than a gas alarm.
Power supply selection is the most common source of commissioning problems. The XNX requires a minimum of 11 V DC at its own terminals after all cable and barrier voltage drops are subtracted. A 24 V supply powering a transmitter 200 meters away via 0.75 mm² cable loses approximately 4 V at 20 mA loop current, leaving 20 V — well within range. However, the same supply driving multiple transmitters on daisy-chained power can easily drop below the 11 V minimum.
Cable screening must be earthed at the control panel end only. Earthing the screen at both the panel and the field transmitter creates a ground loop that injects 50/60 Hz noise into the 4-20 mA signal, producing signal noise and false readings. The screen should float (unconnected) inside the XNX enclosure.
The XNX sensor head connects via a short pigtail to the transmitter body. The firmware automatically identifies the sensor type — catalytic bead, electrochemical, or infrared — via HART communication on power-up. No manual sensor selection is required. Configuration of alarm setpoints, gas type, and output range is performed using the supplied magnetic wand through the closed enclosure cover, or remotely via a HART-compatible handheld communicator or asset management software.
Cable glands must be certified for the enclosure\'s ATEX/IECEx rating. The XNX body carries Ex d IIC T6 certification for Zone 1 hazardous areas. Incorrect glands — even if physically fitting — void the explosion-proof certification and create an ignition risk. Unused cable entries must be sealed with certified blanking plugs tightened to the manufacturer\'s torque specification.
How to wire xnx xnx transmitter wiring diagram
- Isolate and verify zero voltage Lock out and tag out the loop power supply at the control panel. Use an ATEX-rated multimeter to confirm 0 V DC at all field cable conductors before touching any terminals inside the XNX enclosure.
- Mount the transmitter body Install the XNX on a vertical pipe stand or wall bracket at the correct height for the target gas — below breathing zone for heavy gases (LPG, H2S), above for lighter-than-air gases (methane, hydrogen).
- Fit certified cable glands Install Ex-rated cable glands in both cable entries. Seal unused entries with certified blanking plugs. Tighten all glands and plugs to the torque values specified in the XNX installation manual.
- Terminate field wiring Connect positive 24 V supply to terminal 1, signal/return to terminal 2, and ground to terminal 3. Bond cable screen to the earth bar inside the enclosure — leave the screen floating (unconnected) at the control panel cable gland.
- Connect sensor head Attach the sensor head pigtail to the sensor terminals on the XNX body. The XNX auto-identifies the sensor type via HART on power-up — no manual selection is needed.
Specifications
| Supply Voltage | 18–32 V DC |
|---|---|
| Output Signal | 4–20 mA (HART) |
| Fault Signal Level | <3.6 mA |
| Hazardous Area Rating | ATEX Ex d IIC T6 Zone 1 |
Safety warnings
- De-energize the loop power supply and verify 0 V DC at all terminals before making or breaking connections in the hazardous area.
- Never open the XNX enclosure in a live hazardous atmosphere — the Ex d rating requires the enclosure to be de-energized or the area declared gas-free first.
- Use only certified cable glands matching the enclosure's Ex rating; incorrect glands void the ATEX/IECEx certification and create explosion risk.
Tools needed
- ATEX-rated calibrated multimeter for hazardous area use
- HART handheld communicator (e.g., Emerson 475)
- Insulated flat-head and Phillips screwdrivers
- Ferrule crimper and cable stripper
Common mistakes
- Earthing the cable screen at both ends, creating a ground loop that injects noise into the 4-20 mA signal.
- Insufficient loop supply voltage — not accounting for cable resistance drop, leaving less than 11 V at the transmitter terminals.
- Connecting the sensor head wiring to the wrong XNX body terminals, causing an immediate fault signal or sensor damage.
Troubleshooting
- Signal reads below 3.6 mA immediately after power-up
- Cause: Open circuit in loop cable, reversed polarity at terminals, or insufficient supply voltage at transmitter Fix: Measure polarity and voltage at terminals 1 and 2. Voltage must be ≥18 V. Check cable continuity with multimeter. Verify loop fuse is intact at the control panel.
- Signal drifts between 4 and 6 mA with no gas present
- Cause: Ground loop caused by cable screen earthed at both ends, or electromagnetic interference from nearby VFDs Fix: Remove screen earth at the field end so it floats inside the XNX enclosure. Install HART filter if VFD interference is confirmed. Avoid routing signal cable parallel to power cables.
- Display reads correct value but control panel reads incorrectly
- Cause: Incorrect burden resistor value at panel analog input, or wrong scaling in PLC analog channel Fix: Verify burden resistor matches the panel's analog input impedance — typically 250 Ω for 1-5 V conversion. Confirm PLC channel is scaled 4 mA = 0 % and 20 mA = 100 %.
Frequently asked questions
What supply voltage does the XNX transmitter require?
The XNX requires 18-32 V DC at its power terminals. Always calculate cable voltage drop — with 0.75 mm² cable at 200 m and 20 mA, you lose roughly 4 V. Verify at least 11 V remains at the transmitter terminals under worst-case current. A regulated 24 V DC supply with dedicated loop fusing is standard practice for XNX installations in process plants.
What does a signal below 3.6 mA indicate?
A signal below 3.6 mA is the XNX fault state — it signals cable break, sensor failure, or transmitter hardware fault. The control panel must be configured to annunciate a fault alarm distinct from gas alarms. Immediately check supply voltage at the transmitter terminals, inspect sensor head connections, and measure loop continuity with a multimeter before declaring the instrument failed.
Can the XNX be installed in Zone 0?
No. The XNX transmitter body is certified only for Zone 1 (ATEX Ex d IIC). The specific certified sensor heads may be rated for Zone 0 by their own certification, but the transmitter enclosure itself must remain in Zone 1 or safer. Review the specific sensor head certificate to confirm its zone rating before installation near continuously explosive atmospheres.
How do I configure the XNX without a HART communicator?
The XNX includes three Hall-effect magnetic switches that respond to a supplied magnetic wand operated through the sealed enclosure cover. Full configuration — gas type, alarm setpoints, output scaling, and calibration — is possible without opening the enclosure. This is the preferred field configuration method in hazardous areas where opening certified enclosures is restricted by site permit-to-work procedures.
What cable type should I use for XNX wiring?
Use armored, screened, twisted-pair instrument cable rated for the ambient temperature range. Typical selection is 0.75 mm² or 1.0 mm² conductors for loop lengths up to 1 km at 24 V supply. The cable must be compatible with the certified Ex cable glands fitted to the XNX enclosure. SWA (steel wire armored) cable provides mechanical protection required in many process plant cable routing codes.