{"id":1392,"date":"2026-09-18T14:10:00","date_gmt":"2026-09-18T06:10:00","guid":{"rendered":"https:\/\/lemaele.com\/blog\/limit-switch-plc-wiring-guide\/"},"modified":"2026-09-18T11:37:30","modified_gmt":"2026-09-18T03:37:30","slug":"limit-switch-plc-wiring-guide","status":"publish","type":"post","link":"https:\/\/lemaele.com\/pt\/blog\/limit-switch-plc-wiring-guide\/","title":{"rendered":"Como ligar um interruptor de limite a uma entrada de PLC"},"content":{"rendered":"

To wire a mechanical limit switch to a PLC input, first read the PLC module\u2019s input-voltage and common-terminal diagram, then identify the switch\u2019s COM, NO and NC terminals with a meter.<\/strong> A dry contact does not create a voltage: it only opens or closes a path from the correct supply rail to the input. For a typical 24 V DC sinking input, the common is connected to 0 V and the limit-switch contact switches +24 V to the input. For a sourcing input, the polarity is reversed. The exact module manual\u2014not a generic color code\u2014decides the connection.<\/p>\n

This guide covers a standard position-indication<\/em> input. A single ordinary limit switch wired to a standard PLC input is not, by itself, a safety-rated emergency-stop or guard-monitoring circuit. Use the machine risk assessment and the approved safety design for those functions.<\/p>\n

Start with the complete input circuit<\/h2>\n

A PLC input reports ON only when current flows through its sensing circuit. The mechanical switch is one element in that loop. Before landing a wire, record the input module model, permitted input voltage, input type, common grouping, power-supply polarity and channel number. The terms sinking<\/em> and sourcing<\/em> describe the current path in a DC circuit; they are not interchangeable labels for every input. AutomationDirect\u2019s input-module explanation<\/a> shows why a contact between supply positive and the input works when the module\u2019s common returns to supply negative. The actual PLC module wiring manual<\/a> remains the authority for a particular installation.<\/p>\n\n\n\n\n\n\n\n\n\n
Item to verify<\/th>\nWhy it matters<\/th>\nWhere to check<\/th>\n<\/tr>\n<\/thead>\n
Rated input voltage and threshold<\/td>\nPrevents a false OFF indication or module damage<\/td>\nPLC input datasheet<\/td>\n<\/tr>\n
Input common polarity and grouping<\/td>\nDetermines the current path for every channel sharing the common<\/td>\nModule terminal diagram<\/td>\n<\/tr>\n
Switch contact identity<\/td>\nCOM\u2013NO and COM\u2013NC change in opposite directions on actuation<\/td>\nSwitch marking and isolated continuity test<\/td>\n<\/tr>\n
Contact rating and minimum load<\/td>\nThe contact must suit the input\u2019s small sensing current as well as its voltage<\/td>\nExact switch datasheet and PLC input current specification<\/td>\n<\/tr>\n
Actuation and release points<\/td>\nConfirms the moving part changes state before mechanical overtravel<\/td>\nMachine drawing and on-machine test<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Identify COM, NO and NC on the actual switch<\/h2>\n

Disconnect power and isolate the switch from the PLC before using a continuity meter. Record the terminal markings, rather than assuming that a screw position or wire color is universal. In the unactuated state, COM to NC is closed and COM to NO is open for a conventional changeover contact. Press the actuator slowly: the readings reverse near the operating point. Release it and check that the original state returns. Some switch assemblies contain more than one contact block, and a product variant may have only one usable pair. If the marking or datasheet disagrees with the expected pattern, stop and identify the exact part number.<\/p>\n

\"LEMA
LEMA limit-switch product references. Verify the markings and terminal diagram of the exact model installed; this photograph is not a pinout.<\/figcaption><\/figure>\n

Choose NO or NC from the signal behavior required by the control program<\/em>. NO is open before the machine reaches the target and closes on actuation. NC is closed before actuation and opens at the target; a broken conductor can then appear as an open circuit, but that alone does not make the installation fail-safe. Diagnostic coverage depends on the full circuit and control logic. Document the normal physical position, the contact chosen, and the expected PLC bit for both positions. Do not use PLC software inversion to conceal an incorrectly wired or unverified contact.<\/p>\n

Example: dry contact into a 24 V DC sinking input<\/h2>\n

Assume that the module manual explicitly identifies a 24 V DC sinking<\/em> input with its common connected to 0 V. With power isolated, connect the supply\u2019s protected +24 V feed to switch COM. Connect the selected NO or NC terminal to the PLC input point. Connect the module\u2019s input common to the same supply\u2019s 0 V, exactly as its manual shows. When the selected contact closes, the path is +24 V \u2192 switch contact \u2192 PLC input sensing circuit \u2192 input common \u2192 0 V. If the chosen contact opens, that current path breaks and the input reads OFF. This is an example only: never transfer it to a module with a different input rating, isolation scheme, internal supply, or common configuration without checking that module\u2019s documentation.<\/p>\n

On a sourcing DC input, the module\u2019s common and field contact path differ: the contact normally completes a path toward the negative rail rather than feeding positive into a sinking point. Some modules allow either arrangement, but only within the rules for a shared common bank. AutomationDirect\u2019s sourcing\/sinking lesson<\/a> illustrates both current directions. A two-wire mechanical contact is not an NPN or PNP transistor sensor; the terminology describes the PLC input circuit and how its supply is wired.<\/p>\n

Commissioning sequence at the cabinet and machine<\/h2>\n
    \n
  1. Isolate and label.<\/strong> Lock out the relevant energy sources under the site procedure. Identify the field cable and input point; verify that the circuit is de-energized with a suitable tester.<\/li>\n
  2. Check the switch separately.<\/strong> With wires disconnected as required, meter COM\u2013NO and COM\u2013NC before, during and after actuation. A bouncing, intermittent or non-returning reading calls for mechanical inspection or replacement.<\/li>\n
  3. Compare the drawings.<\/strong> Trace supply positive, supply negative, common and the chosen input terminal against the module\u2019s actual diagram. Confirm that adjacent inputs on the same common bank are wired consistently.<\/li>\n
  4. Terminate and protect.<\/strong> Use the correct conductor size, ferrules where specified, strain relief and enclosure protection. Route low-voltage input wiring away from high-energy motor conductors as the PLC manual instructs.<\/li>\n
  5. Energize and observe.<\/strong> Restore power safely, then record the input-channel indicator and live PLC bit at the normal position and at full switch actuation. Confirm that the state changes in the intended direction.<\/li>\n
  6. Check mechanical margin.<\/strong> Approach the actuator at operating speed and from the actual direction of travel. The input must switch before a damaging end stop, and the device must not be overdriven.<\/li>\n<\/ol>\n
    \"LEMA
    Inspect the installed actuator travel and terminal identification; a bench continuity result does not confirm correct on-machine actuation.<\/figcaption><\/figure>\n

    Why a wired switch may still show the wrong PLC state<\/h2>\n

    If the input is permanently OFF, first measure the supply at the module and contact with the circuit energized only where safe and authorized. The selected contact may be open in the current machine position, the common may be on the wrong rail, the channel may be disabled, or the supply may not meet the input threshold. If it is permanently ON, check for a shorted cable, an unexpected NC selection, another path through a shared terminal, or a mechanically held actuator. If the input flickers, investigate loose terminations, actuator chatter, vibration, contact condition and the configured input filter. A filter can suppress brief noise, but it cannot repair a damaged switch or a badly placed actuator.<\/p>\n

    Use a meter to distinguish the field circuit from the control program. An input LED that changes while the software tag does not may indicate the wrong address or program mapping. A tag that changes but fails to trigger the expected machine action points to logic, interlocks or downstream outputs. Record both the electrical observation and the program observation; \u201cthe switch does not work\u201d is not enough to diagnose the fault.<\/p>\n

    Choose a switch that fits the input and motion<\/h2>\n

    PLC inputs often draw only a small current. A switch specified only for a high-power load may not automatically be suitable for a low-level signal; ask for the exact model\u2019s minimum applicable load and contact material rather than assuming a rating printed in amperes answers the question. Select the actuator\u2014roller lever, plunger or other form\u2014from the direction and speed of travel. Confirm operating force, pretravel, differential travel, overtravel and repeatability in the supplier drawing. The enclosure and cable exit also need to suit the dust, moisture and impact conditions at the mounting point.<\/p>\n

    Compare LEMA limit-switch models<\/a> and the HL-series compact limit switch<\/a> against the exact electrical and mechanical requirements; do not select from an image alone. For actuator geometry, see our limit-switch actuator guide<\/a>. If the signal is intended as part of a safety function, use a separately validated safety architecture, including the required device, monitoring and performance level; a standard PLC input tutorial is not a safety design.<\/p>\n

    FAQ<\/h2>\n

    Should a limit switch use NO or NC with a PLC?<\/h3>\n

    Either can provide position information. Pick the contact whose normal and actuated states match the documented logic, then verify both states at the PLC. NC can reveal some open-wire faults but is not independently a safety rating.<\/p>\n

    Does a mechanical limit switch need a separate power wire?<\/h3>\n

    A plain dry-contact switch has no sensor electronics to power. The PLC input loop still needs a supply or an internal excitation source according to the input module manual.<\/p>\n

    Can I connect the switch straight across the PLC input and common?<\/h3>\n

    Only if that is exactly the connection shown by the input module\u2019s manual. Many DC inputs need an external or internal supply in the loop; connecting two terminals without understanding the current path may produce no signal or an unsafe fault.<\/p>\n

    Why does the input LED work but the PLC program not react?<\/h3>\n

    Confirm the software address, input mapping, scan logic, and any filter or debounce setting. The LED confirms an electrical condition at a module, not that the program uses the intended tag.<\/p>\n

    Video: DC sinking and sourcing input paths<\/h2>\n

    The following independent training video explains how DC input common polarity changes the current path. Apply the concept only after checking your own PLC\u2019s wiring diagram.<\/p>\n