PLC Digital Input Types 1, 2 and 3 Explained
Type 1, Type 2 and Type 3 classify the voltage-current behavior of a PLC digital input. They do not identify PNP versus NPN, two-wire versus three-wire, input speed or safety capability. Choose the type only after matching the complete current path and checking the exact module and sensor data.
The fastest practical answer: for common positive-logic 24 V DC sinking inputs, Type 1 uses a 15 V / 2 mA minimum ON reference; Type 2 uses 11 V / 6 mA; and Type 3 uses 11 V / 2 mA. Type 2 draws more field current. Type 3 reduces power, but it is not automatically faster, safer or compatible with every electronic sensor.[2]
These are classification references, not a substitute for the guaranteed thresholds, leakage allowance, filter time and channel-group data of the exact order code.
Which Type 1, Type 2 or Type 3 input fits your device?
Start with the field device and the receiving module, not with a ranking. The table below is a simplified selection view for common positive-logic 24 V DC current-sinking inputs; IEC 61131-2:2017 also addresses negative logic and added Type 3-d.[1]
| Input class | Standard ON region reference | Typical design intent | Main buyer check | Do not assume |
|---|---|---|---|---|
| Type 1 | 15 to 30 V 2 to 15 mA | Electromechanical contacts; compatible three-wire semiconductor outputs may also work after verification. | Does the input still receive its guaranteed ON voltage after output and cable drops? | That every two-wire electronic sensor is compatible. |
| Type 2 | 11 to 30 V 6 to 30 mA | Higher-current semiconductor interfaces, including suitable two-wire devices. | Can the field output supply the required current across voltage and temperature limits? | That the higher current makes the input “better.” |
| Type 3 | 11 to 30 V 2 to 15 mA | Lower-power semiconductor interfaces for higher channel density and lower heat. | Are ON voltage/current and OFF leakage compatible with the exact sensor? | That Type 3 means three-wire, fast or safety-rated. |
Keep it when the exact device reaches the guaranteed ON region, stays in the OFF region and meets the timing need. A newer number alone is not a reason to replace it.
Verify both ON-state series drop and OFF-state residual current. Siemens describes this continuous operating current as a defining two-wire compatibility issue.[4]
Type 3 can reduce per-channel power and heat. Still verify derating, simultaneous active channels and the exact module's configurable modes.
What does an IEC 61131-2 input type prove—and not prove?
The type is one layer of the interface. Treat polarity, wiring, timing, diagnostics and functional safety as separate approval questions.
- The standardized voltage-current operating characteristic for the stated logic and nominal voltage.
- The field current the receiving input may require in its ON region.
- Whether the input characteristic was designed around contact, higher-current semiconductor or lower-power semiconductor behavior.
- A common reference for comparing devices from different suppliers.
- PNP or NPN: this is the output polarity and current-path question.
- Two, three or four wires: this describes device wiring.
- NO or NC logic: this describes the switching function.
- Input speed: filtering, remote-I/O updates and PLC task time remain separate.
- Safety capability: a standard Type 3 channel is not automatically a safety input.
Stop the approval if only “Type 3 compatible” is supplied. Ask for the PLC or remote-I/O order code, input wiring diagram, guaranteed 0/1 signal regions, input current, permitted sensor leakage, delay/filter data and channel grouping. ABB's Type 1 example shows why: its data page states the type, but also separately specifies the 0-signal region, undefined region, 1-signal region, current at several voltages and input delay.[3]
How do you match PNP, NPN and the PLC common?
A PNP sensor normally sources current into a current-sinking input. An NPN sensor normally sinks current to 0 V and needs a current-sourcing input. The names describe opposite sides of one complete path.
If the polarity or COM connection is wrong, the sensor indicator may change while the PLC channel never enters a valid ON region. Selecting Type 1, 2 or 3 cannot repair an open or reversed current path.
- Identify PNP, NPN, push-pull, dry-contact or two-wire output.
- Confirm whether the input is sinking, sourcing or configurable.
- Trace the exact COM for that input group.
- Then compare voltage, current, leakage and timing.
For wiring terminology and diagrams, use the published PLC sinking and sourcing guide and NPN versus PNP sensor output guide.
Will the sensor reliably reach both ON and OFF?
Compatibility is a circuit result. Check the worst-case ON state and OFF state separately, then test the exact combination when margins are narrow.
Calculate ON-state voltage and current margin
Use minimum field supply, maximum sensor residual voltage and maximum cable/terminal drop. The receiving input must still be inside its guaranteed ON region.
Vinput,min = Vsupply,min − Voutput,max − Vcable,maxAlso confirm that the sensor output can supply the module input current plus any other loads without exceeding its rated output current. A nominal 24 V label does not prove either condition.
Check OFF-state leakage for two-wire sensors
A two-wire electronic sensor needs operating current even when “off.” Add the sensor's maximum residual current to leakage from suppression devices, diagnostic circuits and parallel paths.
Ileak,total = Isensor + Isuppression + IparallelThe total must remain within the exact module's guaranteed OFF allowance, and the terminal voltage must stay inside its 0-signal region.[4]
Use the IEC values only for orientation. Replace every prefilled limit with the guaranteed value from the exact input and sensor data sheets before approving hardware.
The tool will compare terminal voltage and output-current capacity with the entered requirements.
The result is a current screen. Confirm OFF-state terminal voltage on the real circuit as a separate measurement.
Given: 20.4 V minimum field supply, 2.0 V maximum sensor output drop, 1.0 V cable/terminal drop and an 11 V module ON minimum.
20.4 V − 2.0 V − 1.0 V = 17.4 V at the inputScreening result: voltage margin is 6.4 V, so the entered ON-voltage condition passes. If the sensor output is rated 100 mA and the input requires 2 mA, the current-capacity screen also passes. This is not final approval: verify the exact order codes, supply tolerance, temperature limits, leakage, COM wiring and timing, then test the installed circuit.
Can the PLC capture the signal fast enough?
Input type describes voltage-current behavior, not total response time. A valid electrical pulse can still disappear in sensor response, input filtering, remote-I/O update, network transfer or PLC task timing.
ABB, for example, lists Type 1 electrical behavior and a separately configurable input delay on the same module page.[3] This separation is the correct model: calculate the entire chain against the shortest valid process event.
Approval rule: the minimum valid pulse must exceed the worst-case combined detection and update path with an engineering margin. Use maximum values, not typical values, whenever the process can create a missed-event hazard or production loss.
What should you verify in the exact input-module datasheet?
Tie every value to the manufacturer, full order code, hardware revision and configured input mode. A family brochure or bare “IEC 61131-2 Type 3” claim is not enough.
Order code, channel number, COM group, nominal voltage, positive/negative logic and sinking/sourcing mode.
0-signal, undefined and 1-signal voltage-current characteristics—not only typical switching voltage.
Input current or resistance, two-wire compatibility, permitted OFF leakage and any diagnostic/test current.
ON/OFF delay, debounce/filter options, pulse stretching, event capture and remote-I/O update behavior.
Isolation boundaries, simultaneous active channels, derating, supply range, cable rules, EMC and temperature.
If the signal is safety-related, use the safety manual, certified architecture and validation process. Ordinary input type is not safety evidence.
Recommended evidence pack: save the module data page, sensor data sheet, terminal diagram, configured parameter export and the calculation or bench-test record together. That makes a future replacement or fault review traceable.
How should you approve the exact sensor and input combination?
Use the IEC type for screening, then release the circuit only after the exact order codes, configuration and worst-case operating conditions have been checked together.
1. Review the exact documents
Match the sensor and input-module suffixes to their data sheets. Record PNP/NPN topology, COM wiring, 0/1 regions, input current, output drop, OFF leakage and timing limits.
2. Test the realistic worst case
Use production-length cable, minimum field supply and the intended filter settings. Switch the real target or contact repeatedly, then confirm the terminal reaches the guaranteed ON and OFF regions and every required pulse is captured.
3. Save release evidence
Keep the wiring diagram, parameter export, measured values, test conditions and accepted order codes together. If a module, sensor suffix, cable or filter changes, repeat the affected checks before substitution.
Do not approve yet if either state enters the undefined region, the shortest pulse is undocumented, the supplier evidence covers a different suffix, or a safety function depends on an ordinary input channel.
How do you troubleshoot a PLC input that will not switch correctly?
Start from the PLC terminal, not only the sensor LED. Measure under load with the production cable and the real receiving input connected.
An open-circuit meter can show a convincing voltage even when the source cannot deliver the required current. A high-impedance meter can also display voltage created by a small leakage current.
Safe troubleshooting boundary: follow the machine's electrical safety procedure and the equipment manuals. Do not defeat safety test pulses or add a bleed resistor/interface relay until the exact failure mode, ratings and consequences are understood.
| Symptom | Likely causes | First useful check | Approval boundary |
|---|---|---|---|
| Sensor LED changes; PLC stays OFF | Wrong PNP/NPN topology, wrong COM, excessive voltage drop or insufficient current. | Trace the current path; measure voltage at the input while ON. | Do not approve until the terminal enters the guaranteed ON region. |
| Input stays ON when a two-wire sensor is OFF | Residual current or another leakage path keeps the input outside its OFF region. | Measure OFF current and terminal voltage under load. | Use only a manufacturer-supported remedy for the exact combination. |
| Works on the bench; fails on the machine | Supply tolerance, long cable, connectors, noise, grounding or temperature changed the margin. | Repeat at minimum field supply and full production cable length. | Recreate worst-case conditions before releasing the design. |
| PLC misses a short pulse | Sensor response, input filter, update or scan time is too long. | Capture the pulse at the terminal; audit every timing stage. | Do not infer speed from the Type 1/2/3 label. |
What information should engineering and procurement send?
A supplier cannot verify compatibility from “24 V sensor” or “Type 3 input.” Send the receiving module, field device and operating conditions as one circuit data pack.
- Manufacturer and exact order code
- Input channel and COM group
- Type and sinking/sourcing mode
- 0/1 voltage-current data
- Leakage allowance and filter settings
- Exact sensor/switch model
- PNP, NPN, two-wire or dry contact
- Maximum ON voltage drop
- Maximum OFF leakage current
- Output rating and response time
- Field supply tolerance
- Cable length and conductor size
- Shortest valid pulse
- Temperature, noise and grounding
- Safety or diagnostic requirements
Useful supplier request: ask the supplier to confirm compatibility against both exact order codes, identify every assumption and return the supporting data-sheet or manual pages. A general statement such as “works with 24 V PLCs” is not enough for approval.
Which assumptions about PLC input types cause selection errors?
Is Type 3 the same as a three-wire input?
No. Type 3 is an IEC voltage-current input characteristic. Three-wire describes a field device with separate supply-positive, supply-negative and output conductors. A three-wire sensor must still match the input's PNP/NPN topology, COM wiring, voltage, current, leakage and timing.
Is Type 3 always better than Type 1 or Type 2?
No. Type 3 reduces input power compared with Type 2 and can support denser I/O, but the correct choice is the type that matches the exact device and application. A Type 1 or Type 2 installation can be fully suitable when its electrical and timing margins are verified.
Can a PNP sensor connect to any Type 3 input?
No. A PNP sensor normally needs a current-sinking receiving input and the correct COM connection. You must also verify worst-case terminal voltage, input current, sensor output rating, OFF leakage and timing. The Type 3 label alone does not confirm polarity or wiring.
Why can a two-wire sensor keep a PLC input ON?
A two-wire electronic sensor needs operating current while OFF. If its residual current plus other leakage paths exceeds the module's guaranteed OFF allowance, the channel can remain ON or enter the undefined region. Measure both current and terminal voltage with the real module connected.
Does Type 3 make an input fast or safety-rated?
No. Type 3 describes electrical voltage-current behavior. Input delay, filtering, I/O update and PLC task time are separate speed specifications. Safety capability requires a suitable safety architecture, diagnostics, certification, wiring and validation; an ordinary Type 3 input is not safety evidence.