PLC Input and Output Modules Selection Guide
Engineering guide to choosing PLC digital I/O, analog 0–10 V / 4–20 mA, RTD/thermocouple temperature, load cell and fieldbus communication expansion modules.
What are PLC input and output (I/O) modules? PLC I/O expansion modules attach to a local PLC CPU rack or remote fieldbus coupler to add digital channels, analog inputs/outputs, direct temperature/weighing measurement, high-speed counting, and industrial communication ports.
This guide covers selection criteria for products in the PLC Input and Output Modules category. For main controller units, see the PLC Selection Guide.
Subcategories and applications
- Digital input modules, digital output modules, and digital I/O modules: 8-, 16-, and 32-channel 24 V DC sink/source input modules and relay or PNP/NPN transistor output modules.
- Analog input modules, analog output modules, and analog I/O modules: 0–10 V, ±10 V, and 0/4–20 mA modules for transmitters, drives, and proportional valves.
- Mixed analog and digital I/O modules: Space-saving expansion cards combining DI/DO and AI/AO channels in one housing.
- Temperature modules and load cell modules: Dedicated RTD (PT100/PT1000), thermocouple (J, K, T, S), and 24-bit strain-gauge weighing modules.
- Communication modules and remote I/O modules: RS-485/Modbus, Ethernet, CANopen, and distributed remote I/O adapters.
- High-speed I/O modules, power modules, and connection modules: High-speed counter/pulse-train modules, bus power supplies, and terminal breakout boards.
PLC I/O module comparison table
| Module Type | Signal Standard | Output / Resolution | Primary Application |
|---|---|---|---|
| Digital Input (DI) | 24 V DC (Sink / Source) | 8 / 16 / 32 Channels | Reading push buttons, limit switches and proximity sensors |
| Digital Output (DO) – Relay | 24 V DC / 230 V AC (2 A) | Dry contact | Switching contactors, solenoids and mixed-voltage loads |
| Digital Output (DO) – Transistor | 24 V DC (PNP Source / NPN Sink) | High-frequency solid-state | Step/servo pulse trains (PTO), PWM and rapid cycling |
| Analog I/O (AI / AO) | 0–10 V, ±10 V, 0/4–20 mA | 12-bit / 14-bit / 16-bit | Pressure/flow transmitters and VFD speed references |
| Temperature / Load Cell | PT100, J/K TC, mV/V | 0.1 °C / 24-bit ADC | Multi-zone PID heating and batch weighing systems |
Frequently asked questions
When should I choose transistor (PNP/NPN) outputs instead of relay outputs on a PLC module?
Choose relay outputs for switching 230 V AC coils or low-frequency 24 V DC loads up to 2 A. Choose transistor outputs (PNP source or NPN sink) for high-speed pulse output (stepper/servo control), PWM control, or applications with frequent switching where mechanical relay contacts would wear out.
Can a PT100 temperature sensor be wired directly to a standard 4–20 mA analog input module?
No. Standard analog input modules read voltage (0–10 V) or current (4–20 mA). To connect PT100 RTDs or thermocouples directly without external head transmitters, select a dedicated PLC temperature (RTD/TC) module.
Are PLC expansion modules interchangeable across different series of the same brand?
No. Each PLC family (such as Delta DVP-S vs. AS series, GMT GLC series, or Schneider Modicon families) uses its own backplane bus connector and internal protocol. Always match the expansion module series to the host CPU family.