The short answer
Digital Signals
A digital signal is a two-state signal, on or off, carried by a contact, a transistor, or a defined current level, and the reason it fails is usually a mismatch between how the sending device produces it and how the input expects it. A dry contact closes a circuit the input powers; a wetted contact provides its own voltage, which must match the input; a transistor output sinks current to common or sources it from the supply, and the input must be wired for the one it is; a two-wire proximity sensor draws its own power through the signal and leaks a small current when off that a sensitive input may read as on; a NAMUR sensor signals with two current levels so that a broken wire or a short is detected as a fault. Alarms and safety signals are wired so that the healthy state is a closed contact or a live current and a broken wire reads as alarm. With those rules on the drawing and a contact rated for the small currents an input draws, a discrete signal is the most reliable signal in the plant.
Key points
- Know what the sender produces: dry contact, wetted contact, sinking or sourcing transistor, two-wire sensor, or NAMUR current levels.
- Match the input: powered or not, sinking or sourcing, voltage, and threshold.
- Wire alarms and safety signals fail-safe: closed or live when healthy, so a broken wire alarms.
- Use contacts rated for low current on 24 volt inputs; a power contact at milliamps corrodes and reads open.
- Debounce mechanical contacts and filter noisy inputs in the controller, with the delay documented.
Signal types
| Type | What the sender does | What the input needs | Notes |
|---|---|---|---|
| Dry contact | Closes or opens a circuit with no voltage of its own | A powered input that supplies the wetting voltage | Float switches, limit switches, relay contacts |
| Wetted contact | Provides its own voltage when closed | An input matching that voltage and referenced to the same common | Run feedback from a starter at 120 volts; isolate or use an input relay |
| Sourcing transistor, PNP | Connects the signal to the positive supply when on | A sinking input that expects positive | The common convention in North American panels |
| Sinking transistor, NPN | Connects the signal to common when on | A sourcing input that supplies the positive | Common on Asian-market sensors; check before wiring |
| Two-wire sensor | Powered through the signal loop; switches the loop current | An input that tolerates the off-state leakage and provides the minimum on current | Proximity switches; leakage of a milliampere or two |
| NAMUR | About 1 milliampere off, about 3 or more milliamperes on, per the standard | A NAMUR input or an isolator amplifier that reads the levels and detects breaks | Intrinsically safe applications; wire-break and short detection |
| Solid-state relay output | A semiconductor switch, often AC rated | A load above the minimum holding current | Leakage when off; not for low-current inputs without a burden |
Sinking and sourcing
The words describe the direction of current at the input terminal. A sinking input draws current in from the field device to its common, so the field device must source current from the positive supply: a sourcing device drives a sinking input. A sourcing input pushes current out to the field device, which must sink it to common. A dry contact works with either because it carries current in either direction. A transistor sensor works with one, and a mismatch reads permanently off or permanently on. The panel standard chooses one convention, states it on the drawings, and buys sensors to match; the exceptions get an interposing relay.
Fail-safe
An alarm contact that closes to alarm cannot tell the controller about a broken wire, a blown fuse, or a dead sensor; the input reads off, which looks like no alarm. An alarm contact that opens to alarm, held closed when healthy, alarms on every one of those failures. The same applies to run feedbacks, permissives, and safety signals: the healthy state carries current. NAMUR sensors take it further by distinguishing off from broken. The convention is on the drawing, and an input configured the wrong way round is an alarm that never comes.
Contacts at low current
A 24 volt input draws a few milliamperes. A contact designed for amperes at 120 volts has a film of oxide that a small current cannot break through, and it reads open intermittently after a year in a damp panel. Contacts for controller inputs are rated for low-level switching, gold-plated or bifurcated, and float switches and limit switches are specified with that rating. A mechanical contact also bounces for milliseconds on closure, which a counter or a fast input reads as several closures unless it is debounced in the input filter or the program.
Wiring rules
- Discrete field wiring at 24 volts in its own wireway and conduit, away from power.
- One common per input group, from the panel supply, not from the field device supply.
- Wetted signals from other panels through input relays or isolated inputs.
- Two-wire sensor leakage checked against the input off-state threshold; a burden resistor where it is not.
- Long runs to sensitive inputs with shielded cable and a filter delay in the input configuration.
- Every discrete point on the I/O list with its type, its fail-safe convention, and its debounce.
Frequently asked questions
- The input stays on with the sensor off.
- A two-wire sensor leaking enough current to hold the input on, a sourcing sensor on a sourcing input wired to the supply, or a wire shorted to positive. Measure the off-state voltage at the input; a burden resistor across the input or the right input type fixes the leakage case.
- Should alarms be normally open or normally closed?
- Closed when healthy, opening on alarm, so that a broken wire alarms. Normally open alarm contacts are used only where a spurious alarm is worse than a missed one, and that is rare in a water plant.
- Can I run a 120 volt discrete signal to a 24 volt input?
- Not directly. Use an input relay with a 120 volt coil and a dry contact to the input, or an isolated 120 volt input module. A 120 volt wire landed on a 24 volt input destroys the channel.
- How much debounce?
- A few milliseconds for a clean contact, ten to fifty for a float or a bouncy limit switch, set in the input filter or a timer in the program, and never so long that a genuine short pulse is missed. Document the value beside the point.
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.
- Panel I/OArranging the I/O in a controller panel: module types for the field devices, sinking, sourcing, and isolated inputs, fused and relay outputs, the path from field terminal to module, spare capacity, and addressing that follows the I/O list.
- Float SwitchesThe oldest level instrument and still the one that saves a station: how tilt and stem floats work, the wiring conventions, mounting and spacing in a wet well, classified-area ratings, and how to use them alongside a continuous instrument.
- Inputs Not ReadingA switch closes in the field and the program never sees it. How the wetting voltage, the common, sinking and sourcing, and the wiring each lose the signal, and how to find which one did.
- Pulse SignalsPulse outputs for totalizing: flowmeters, energy meters, rain gauges, and feeders, output types, scaling in volume per pulse, counter inputs, pulse width and rate, debouncing reeds, totalizer retention and rollover, and verifying against the register.
- Alarms in the ControllerGenerating alarm conditions in the controller rather than in the graphics: why the PLC should own the alarm state, a standard alarm block with setpoints, deadband, and delays, equipment alarms, and packing alarm bits for SCADA.
Direct contact
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