ControlLogix Open Wire: Underrange Bit, Not a Wire-Off Flag

Fixes That Waste Your Shift

The HMI shows the level at 0 %, the operator swears the tank is half full, and the panel gives you nothing: module LED green, no controller fault, no message. Four fixes get tried first on this symptom. Three of them cost you a shift.

• Swapping the analog module. The module is not faulted. It is reporting exactly what the configured range tells it to report. A new module in the same slot with the same configuration gives you the same 0 %.
• Hunting for a dedicated "wire off" bit. Open-wire and no-load detection lives on the diagnostic digital I/O. On a ControlLogix analog voltage input there is no wire-off member in the tag structure. The break surfaces as ChxUnderrange. That is the flag you are looking for.
• Adding a low process alarm at 0 %. It cannot separate an empty tank from a dead transmitter, and it chatters every time the process legitimately sits at the bottom of the range.
• Leaving the channel on 0-10 V or ±10 V "so it covers everything." This is the fault, not a workaround. In those ranges 0 V is a perfectly legal reading, so a severed wire produces no flag at all.

Start with the range and the terminations. Everything else follows from those two.

How the Module Sees a Broken Wire

A 1-5 V signal is a live-zero signal: the bottom of the process range sits one volt above electrical zero. That 1 V gap is the entire detection mechanism. Cut the signal conductor, blow the fuse feeding the transmitter, or lose transmitter power, and the input collapses to 0 V — a full volt below the lowest legal process value.

The module converts, compares the result against the low limit of the configured range, and sets the per-channel status bits. On a 1756-IF16 in slot 7 of the local chassis, channel 4 gives you Local:7:I.Ch4Underrange and Local:7:I.Ch4Fault, with Local:7:I.Ch4Data pinned at the bottom of the scaled range. That underrange bit is your wire-off and your dead-loop-supply indication in one.

Voltage inputs are high impedance, and nothing inside the module forces an open input to 0 V. An unterminated channel floats. In single-ended mode the input multiplexer can carry charge from a neighbouring channel onto the floating input and hand you a stable, plausible, completely fictional mid-scale reading with no flags set. Terminate every voltage input at the RTB.

Check the Field Side First

1. Go online and expand the module's input tag in the tag browser. Read ChxUnderrange, ChxOverrange, ChxFault, and ChxData for the suspect channel. Member names vary slightly by catalog and firmware — read what is actually there rather than typing a name from memory.
2. Count the underranged channels. One channel points at one wire or one transmitter. A whole power group points at the fuse. Every channel with frozen data points at the module connection, not the field.
3. Meter DC volts across the channel terminals at the RTB, transmitter powered. 0 V at the RTB means the break is upstream of the module. Correct volts at the RTB with the tag still underranged means configuration or module.
4. Meter the field supply at the fuse holder, then again at the transmitter terminals. A fuse that measures good in the holder but drops under load is still your fault.
5. Only after the meter work, open Module Properties and read the configured range, scaling, and alarm limits.

Configure the Channel So the Break Is Unambiguous

1. Select the narrowest input range that covers 1-5 V. On the 1756-IF16 family that is the 0-5 V selection. Never leave a 1-5 V transmitter on 0-10 V or ±10 V.
2. Scale the channel to the live zero, not to the range: Low Signal 1.0 V → Low Engineering 0.0, High Signal 5.0 V → High Engineering 100.0. The module extrapolates linearly below Low Signal, so 0 V lands at -25.0 %. That negative value is impossible from a healthy loop.
3. Enable the low-low process alarm at about -5.0 % — below anything the process can produce, above the noise floor. That gives you a hardware-generated ChxLLAlarm bit dedicated to wire break. Enable alarm latching in the configuration if you want it to hold; clear it from the module output tag unlatch member, for example Local:7:O.Ch4LLAlarmUnlatch.
4. Terminate the input at the RTB. For a 4-20 mA transmitter, fit a precision burden across the input terminals — 250 Ω, 0.1 %, low tempco — which both produces the 1-5 V and forces a hard 0 V when the loop opens. For a voltage-output transmitter, fit a pull-down across the terminals sized so the loading error stays inside your accuracy budget; take the module input impedance from the module specifications.
5. Set the RPI to what the loop needs. Detection latency is bounded by RPI plus the scan of the task that reads the bit.
6. Handle bad quality in code instead of letting 0 % propagate into a PID:

WireOff_AI04 before it annunciates, so transmitter power-up and fuse-hold transients do not nuisance-trip it.

Verify It With the Wire Out

1. Inject 1.000 V and 5.000 V at the field end with a calibrator. Confirm Ch4Data reads 0.0 and 100.0.
2. Wind down to 0.900 V. The PV goes slightly negative and the low-low alarm stays clear. That proves your alarm limit is not inside the live process range.
3. Wind to 0.000 V. Ch4LLAlarm sets; Ch4Underrange sets once you cross the range floor. Time the transition against your RPI.
4. Lift one leg at the RTB. Same bits, and no neighbouring channel moves. If a neighbour twitches, your termination is missing or the wrong value.
5. Pull the field supply fuse. Every channel fed from it flags together — that is the group alarm you want for a supply failure, distinct from a single wire.
6. Confirm the PV holds its last good value and the controlling loop drops to manual or hold instead of winding the output open.
7. Reconnect and confirm the bits clear, including the unlatch path if you enabled latching.

Pitfalls That Bite on This Module Family

• Underrange does not latch. An intermittent conductor sets and clears the bit between RPIs, and the operator never sees it. Latch it in logic with a manual reset.
• ChxFault is a rolled-up bit: it sets for underrange and overrange, and also for calibration and connection problems. Annunciating on it alone tells the operator nothing about where to look.
• Single-ended versus differential mode changes the channel count and the terminal-to-channel mapping. Confirm which physical terminals feed Ch4 in the module wiring diagram before you chase what looks like an alarm on the wrong channel.
• 0.000 V sits right on the floor of a 0-5 V range, so underrange can hover. Treat the low-low alarm below live zero as the primary detection and underrange as confirmation.
• A sensor with no live zero — 0-5 V or 0-10 V from a true zero — cannot be diagnosed by the module at all, because a broken wire and a zero process produce the same volts. Convert the loop to 4-20 mA or 1-5 V, or add external loop monitoring.
• A broken return or shield leg on a single-ended group can drag several channels at once. Do not assume a multi-channel underrange is always the fuse.

Stop and escalate when the calibrator proves correct volts at the RTB terminals and the channel tag still disagrees, or when underrange and data stay frozen after a module reseat and a fresh download of the configuration. At that point it is a module or firmware issue, and Rockwell Automation Technical Support will want the catalog number, firmware revision, the exported module configuration, and the controller major fault log. Have the calibrator readings and the tag values side by side before you open the case.

FAQ