Symptom
A float, limit switch, pressure switch, or auxiliary contact operates in the field but the input bit in the program stays off, the module indicator stays dark, or both.
The short answer
Inputs Not Reading
A discrete input reads when the field contact lets the wetting voltage through to the module and the module has a common to return it to. The input fails to read when the contact does not close, the wire is open, the wetting voltage or common is missing, the module and the device disagree about sinking and sourcing, or the module point is dead. Measure at the field device, at the panel terminal, and at the module, and the break is between the last place the voltage was present and the first place it was not.
Key points
- Confirm the field device is actually operating with a meter at its terminals; a stuck float is not a controller problem.
- The module indicator follows the voltage at the terminal. Dark with the contact closed means no voltage arrived.
- No wetting voltage on the field side, or a missing common, takes out a whole group at once.
- A sink and source mismatch reads never, or always, and looks like a dead module.
- The program can filter, invert, or force the input; check those before condemning hardware.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| The field device is not operating | Measure across the device contacts with it in both states. A float hung on a cable or a limit switch out of adjustment never closes. |
| Open wire between the device and the panel | Voltage at the device with the contact closed and none at the panel terminal. Ring the conductors out with the circuit isolated. |
| Missing wetting voltage | No voltage on the supply side of the field contact. A branch fuse for the field supply has opened, or the supply is off. Usually a group of inputs fails together. |
| Missing or open common | Voltage reaches the input terminal and the indicator stays dark. Measure between the terminal and the module common; if the common is missing the module cannot complete the circuit. |
| Sink and source mismatch | A sensor or a wiring change that connects the input to the wrong polarity. The input never reads, or reads constantly. Compare the wiring against the module datasheet diagram. |
| Input filter time too long | A short pulse from a fast contact or a flow switch is filtered out. Check the module input filter setting against the pulse width. |
| Forced or inverted in the program | A force on the input, or logic that reads the bit with the wrong instruction for the wiring convention. The bit may be on and the program still not act on it. |
| Wrong address or mapping | The wire is on point 7 and the program reads point 6. Compare the drawing, the terminal, and the tag. |
| Remote rack connection lost | Inputs on a disconnected rack are stale. The I/O status light and the connection status show it. |
| Module point failed | Voltage at the terminal, common good, indicator dark. Move the wire to a spare point; if it reads there the point is dead. |
Start at the device
The temptation is to start at the software. Start at the field device, because a float that has hung up on a cable, a limit switch that has been knocked out of adjustment, or a pressure switch with a plugged sensing line produces exactly the same symptom as a dead input module, and it is far more common. Operate the device by hand where that is safe, and measure across its contacts.
- 1
Measure at the device.
Across the contact terminals. Wetting voltage present with the contact open, near zero with it closed, means the device and its supply are fine and the problem is toward the panel. Nothing across the contacts in either state means no wetting voltage is reaching the device.
- 2
Measure at the panel terminal.
Between the input terminal and the module common. Voltage that appears when the device closes means the wire is good. Voltage missing means an open wire or a missing supply.
- 3
Check the common.
Between the module common terminal and the supply return. A missing common takes out every point on that group, and it is the cause that gets found last because nobody measures it.
- 4
Watch the module indicator.
Lit when voltage is at the terminal and the common is present. Lit indicator and an off bit is a program, mapping, force, or connection problem, not a wiring problem.
- 5
Look at the bit online.
If the bit follows the indicator, the hardware is doing its job and the logic is the question. If the bit does not follow the indicator, check forces, the I/O configuration, and the address.
Sinking and sourcing
A sinking input expects the field device to connect it to the positive supply; a sourcing input expects the device to connect it to the return. Two-wire contacts do not care, but three-wire sensors do, and so does any wiring that was drawn for one convention and installed for the other. The symptom of a mismatch is an input that never turns on, or one that is on all the time, with everything measuring plausibly. The module datasheet has the diagram, and the fix is to wire to it.
The program side
- Wiring convention
- A fail-safe contact wired normally closed reads as a bit that is on in the normal state. Logic written as if the bit were normally off inverts the meaning. Agree the convention on the drawing and read the bit accordingly.
- Filter time
- Discrete input modules filter short pulses to reject noise. A filter set for milliseconds swallows a pulse from a fast switch. Check the filter against the pulse width of the device.
- Forces
- A force on an input holds the bit regardless of the terminal. Check for installed forces early.
- Addressing
- The tag points at a slot and a point. A module moved to another slot, or a wire landed one terminal over, puts the signal somewhere the program is not looking.
Frequently asked questions
- The module light is on but the program bit is off. What is wrong?
- The module sees the voltage and the processor is not reading the module, or is reading it and something in the program is masking it: a force, a wrong address, a remote connection that is stale, or logic that reads the bit with the wrong instruction for the wiring convention. Check those in that order.
- Why did a whole group of inputs stop reading at once?
- The wetting voltage or the common for that group is gone. A branch fuse for the field supply has opened, or the common terminal has come loose. One point failing is a device or a wire; a group is a supply.
- What is a wetting voltage?
- The voltage that a closed field contact lets through to the input module. It comes from a supply in the panel, 24 VDC or 120 VAC depending on the module, and without it a closed contact delivers nothing to the input.
- Can I test an input by shorting the terminal to the supply?
- Jumpering the input terminal to the wetting supply, with the field wire lifted, is a legitimate way to prove the module point, provided the voltage and the polarity are right for the module and the circuit is one where making that input true is safe. Know what the input does in the program first.
Related topics
- PLC I/O SystemsDiscrete and analog input and output modules, sinking and sourcing, relay and transistor outputs, wiring conventions, and the module diagnostics that find field faults from the panel.
- Outputs Not EnergizingThe program says the output is on and the equipment is not running. Where the signal gets lost between the output image and the field device, and how to find the point in a few measurements.
- Logic Not Executing As ExpectedThe rung looks right and the plant does something else. The scan-order, duplicate-output, unscheduled-routine, and data-type traps behind it, and how to find which one you are in.
- Wet Well Level MeasurementHow lift station wet well level is measured, which technology suits which well, the failure modes that flood a station, and why the high level float must stay hardwired.
- Loss of Control VoltageControl voltage that drops out for a moment and comes back, resetting controllers and dropping contactors: utility sags when a large load starts, a transfer switch operation, a loose neutral, or an overloaded control transformer. Catching it with a recorder.
- Processor FaultedA PLC with its fault light on and no logic running. How to read what it is telling you, the causes in order of likelihood, and how to restart it without repeating the fault.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.