Input Unit

Choosing Input Unit components

Input unit selection comes down to the signal type, the channel count, the voltage and current rating per channel, the isolation between channels, and any specialty function (high-speed counting, thermocouple cold-junction compensation, RTD linearization). The wrong input module for the signal cannot be made to work by configuration alone.

Specs to confirm before ordering:

Exact OEM part number with all option codes
Signal type: digital (24 VDC, 120/230 VAC, 48 VDC; sinking vs. sourcing), analog (±10 V, 0–10 V, 4–20 mA, ±20 mA, mV bridge), thermocouple (specific types), RTD (Pt100/Pt1000), weighing (high-resolution bridge), encoder (incremental TTL, differential, SSI), high-speed counter
Channel count and per-channel isolation
Diagnostics: broken-wire detection, short-circuit detection, per-channel LED, module fault to PLC
Compatibility with the rack/backplane (1746 SLC vs. 1756 ControlLogix vs. 1769 CompactLogix vs. 1734 POINT I/O)
Software configuration: device description files (GSDML for Profinet, ESI for EtherCAT, EDS for EtherNet/IP) required by the PLC engineering tool
Update rate / sampling speed (matters for fast analog or counter inputs)
Resolution for analog input modules (12-bit, 14-bit, 16-bit, 24-bit)

Common gotchas: sinking and sourcing digital input modules are not interchangeable; a sinking module wired to a sourcing PLC field-side gives no signal. Analog input modules expect either current OR voltage on a given channel — wired wrong, the module reads zero (current mode with voltage source) or saturates (voltage mode with current source). Thermocouple inputs need the cold-junction compensation enabled and the thermocouple type set in software; mis-configured TC inputs give plausible-looking but wrong readings. Some input cards have channel-grouping isolation (e.g., 16 channels in 4 isolated groups of 4) that limits how field signals can be wired without ground loops.

Typical applications: sensor and switch inputs to PLC programs, process signal inputs for PID and analog logic, motor current monitoring through CT primaries to analog input cards, thermocouple zone temperature feedback on extruders, and encoder feedback on counter cards. On legacy installations, exact-OEM input unit replacement preserves the PLC tag/address layout and the field wiring.

For obsolete input units, send the OEM part number for a sourcing quote.

Do you stock obsolete PLC input units?

Yes. Discontinued Allen-Bradley 1746/1756 input codes, retired Siemens SM 321/SM 331, end-of-life Mitsubishi QX input modules, and earlier Omron CJ1W-IA are sourced through our supplier network.

Sinking vs. sourcing digital input — which do I have?

Sinking input: field device supplies +24 V to the input terminal; input card pulls to common. Sourcing input: input card supplies +24 V to the input terminal; field device pulls to common. Wiring is opposite; both work but match the module to the existing field wiring.

Will the input module need configuration?

Yes — modern PLC input modules need device-description files (GSDML, ESI, EDS) imported into the engineering tool, plus channel configuration (signal range, alarm setpoints, scaling). Out-of-the-box modules typically have default settings that may not match the field signals.

Can I substitute a higher-resolution analog module?

Yes — higher resolution gives finer reading. Verify the PLC tag types and scaling logic accommodate the new bit count. Some upgrades require tag-table changes in the PLC program.

Does the input module need separate power?

Most input modules draw power from the backplane. Some isolated or high-density modules need external 24 V on a dedicated terminal. The OEM datasheet specifies.

What is the warranty?

12-month functional warranty. Damage from over-voltage on inputs (especially analog), short-circuit on input power buses, or miswiring is not covered.

Keyence MU-CB2 Power cable for MU-N Series

Keyence MU-CB4 Power cable for MU-N Series

Keyence MU-CB6 Power cable for MU-N Series

Keyence MU-CB8 Power cable for MU-N Series

Keyence MU-N11 Multi-Sensor Controller, Main unit

Keyence MU-N12 Multi-Sensor Controller, Expansion Unit

Keyence VT5-W07 7″ widescreen TFT color touch panel display

Keyence VT5-W07M 7″ widescreen TFT monochrome touch panel display

Keyence VT5-W10 10″ widescreen TFT color touch panel display

Keyence VT5-X10 10″ TFT color touch panel display

Keyence VT5-X12 12″ TFT color touch panel display

Keyence VT5-X15 15″ TFT color touch panel display

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