Selecting Sensors for a Sinking PLC Input Module Guide

With a PNP sensor connected to a sinking PLC input, the field device supplies positive voltage to the input when active, producing the required current path without added interface components. Confirm the circuit from the terminal diagrams because sinking and sourcing labels are not used consistently across all product documentation.

Connection Options

Before anything else, obtain the sensor output schematic and the PLC input-channel schematic. The deciding factor is the direction of current through the input, not the wording on the product label.

Recommended Sensor Selection

Select a PNP, or sourcing, sensor for direct connection to a sinking PLC input. When the sensor turns on, its output supplies positive voltage to the input terminal. Current then flows through the input circuit to the input common, which is normally tied to the supply return for this arrangement.

Use the terminal diagrams to confirm all three functions before wiring:

1. Locate the PLC input common and determine whether the internal input circuit returns current to the negative side of the field supply.
2. Locate the sensor supply-positive, supply-return, and switched-output terminals.
3. Confirm that the sensor output sources current when active. A transistor symbol or equivalent output schematic is more reliable than a sinking or sourcing label.
4. Check that the sensor supply range, output voltage, output-current rating, leakage current, and protective functions are compatible with the input-card specifications.

Do not move on until the two diagrams show a complete active-state path: field-supply positive, sensor output transistor, PLC input terminal, PLC input circuit, input common, and field-supply return.

Direct PNP Wiring Procedure

1. Remove field power and identify the input terminal, its associated common terminal, and the sensor terminals from the product drawings. Do not assign terminals from wire color alone.
2. Connect the sensor supply-positive terminal to the positive field supply.
3. Connect the sensor supply-return terminal to the field-supply return.
4. Connect the sinking input group's common terminal as shown in the input-module diagram, normally to the same field-supply return used by the sensor.
5. Connect the sensor switched output to the selected PLC input terminal.
6. Restore power and measure from the input terminal to the input common. With the sensor inactive, the reading must satisfy the module's published OFF-state limit. With the sensor active, it must satisfy the published ON-state voltage and current requirements.
7. Observe the input-channel indicator and the corresponding PLC input status while operating the sensor. Both must change together before the input is used in machine logic.

Current-Path Mechanism

PNP and NPN describe transistor-output behavior. A PNP output switches the positive side of the circuit and sources current into the load. An NPN output switches the negative side and sinks current from the load.

A direct connection works when the sensor and input perform complementary functions. Two sinking devices do not create the intended active-state current path: an NPN sensor pulls toward return, while a sinking input expects the field device to deliver current into it. Likewise, two sourcing devices both act toward the positive side rather than completing the circuit.

Terminology can obscure this relationship because some documentation names a module by its internal action while other documentation emphasizes the field device connected to it. Trace the circuit symbols and common-terminal connection whenever the label and schematic appear to disagree.

NPN Sensor Alternatives

An existing NPN sensor can sometimes operate a sinking input through a pull-up resistor. This is not electrically equivalent to direct PNP wiring. The input becomes ON when the NPN transistor is off and OFF when the transistor is on, so the PLC logic normally requires inversion.

1. Connect the pull-up resistor between the positive field supply and the signal node only after confirming that the sensor output permits that voltage.
2. Connect the signal node to both the NPN output and PLC input.
3. Use the input module's minimum ON voltage or current to calculate the largest acceptable pull-up resistance. If the input is represented by impedance Zin, check Vinput = Vsupply × Zin / (Rpull-up + Zin) against the ON threshold.
4. With the sensor on, check the output transistor's sink current. Approximately (Vsupply - Von) / Rpull-up flows through the resistor, where Von is the sensor's active output voltage.
5. Calculate resistor dissipation for both sensor states and select a component rating from the actual circuit voltages. Do not choose a resistance from convention alone.
6. Measure the signal in both states and compare it with the module's published ON and OFF limits, including leakage-current effects.

A built-in pull-up may remove the need for an external resistor, but it does not prove compatibility. Check its connected voltage, available current, inactive-state voltage, and interaction with the PLC input impedance. If those data are missing or the inverted logic is unacceptable, use a suitable interface device or replace the sensor with a PNP model.

Commissioning Checks and Recurring Faults

Check the physical input indicator separately from the PLC program. A correct terminal voltage with no channel indication points to module wiring, common grouping, or threshold compatibility. A correct channel indication with unexpected machine behavior points downstream to tag mapping, forcing, inversion, or application logic.

Frequently Asked Questions