liminfoPLC I/O Reference
Free reference guide: PLC I/O Reference
About PLC I/O Reference
The PLC I/O Reference is a searchable guide to programmable logic controller input/output modules, covering digital inputs (DI) with 24V DC specifications, digital outputs (DO) with transistor sink/source and relay types, NPN vs PNP sensor wiring, and 2-wire/3-wire/4-wire connection methods for industrial sensors.
Analog I/O sections detail 4-20mA and 0-10V signal scaling formulas with PLC raw value conversion (Siemens 27648 and Allen-Bradley 32767 ranges), analog output wiring for current/voltage modes, input filtering techniques, and I/O diagnostics for wire break and overload detection.
Specialized topics include RTD (Pt100) temperature measurement with 2/3/4-wire compensation, thermocouple types (K, J, T, E, R, S) with cold junction compensation (CJC), high-speed counter inputs for encoder connections, PWM output for proportional control, safety I/O with SIL 3/PL e compliance, and comprehensive wiring best practices for industrial PLC installations.
Key Features
- Digital I/O module specs with 24V DC input levels, transistor/relay output types, and response times
- NPN (sink) vs PNP (source) sensor connection diagrams with regional usage conventions
- 4-20mA and 0-10V analog scaling formulas with Siemens and Allen-Bradley PLC raw value ranges
- RTD (Pt100) 2/3/4-wire wiring methods and thermocouple types K/J/T/E/R/S with CJC compensation
- High-speed counter configuration for encoder A/B/Z phase connections up to 1 MHz
- Safety I/O modules with dual-channel monitoring, cross-check, and SIL 3 / PL e compliance details
- Output protection circuits with flyback diodes, RC snubbers, and varistors for inductive loads
- Sensor power supply design with capacity calculations and noise filtering recommendations
Frequently Asked Questions
What is the difference between NPN and PNP sensors for PLC inputs?
NPN (sink) sensors output 0V when active, requiring the PLC common terminal connected to +24V. PNP (source) sensors output +24V when active, with the common terminal at 0V. Korea and Europe predominantly use PNP sensors, while Japan favors NPN. The 3-wire color convention is brown (+24V), blue (0V), and black (signal output).
How do I scale a 4-20mA signal to engineering units in a PLC?
Use the formula: EU = EUmin + (Raw - 4mA) / (20mA - 4mA) x (EUmax - EUmin). For Siemens PLCs, 4mA maps to raw value 0 and 20mA maps to 27648, so EU = Raw / 27648 x (EUmax - EUmin) + EUmin. For example, a 0-10 bar pressure transmitter at 12mA reads: 0 + (12-4)/(20-4) x 10 = 5.0 bar.
What are the differences between transistor and relay digital outputs?
Transistor outputs (sink or source) switch DC loads with fast response (~0.1ms) and unlimited mechanical life, suitable for high-frequency switching. Relay outputs handle both AC and DC loads with higher current capacity (typically 2A at 250VAC) but slower response (~10ms) and limited life (100K-1M cycles). Inductive loads require surge protection with either type.
How do I wire an RTD (Pt100) temperature sensor to a PLC?
Pt100 reads 100 ohms at 0 degrees C with an alpha of 0.00385. Use 2-wire for basic measurements (includes wire resistance error), 3-wire for standard applications (compensates lead wire resistance, most common), or 4-wire for high-precision measurement. Class A accuracy is plus/minus 0.15 degrees C at 0 degrees C, Class B is plus/minus 0.30 degrees C.
What thermocouple types are available and when should I use each?
Type K (Chromel-Alumel, -200 to 1260 degrees C) is the most common general-purpose thermocouple. Type J (-210 to 760 degrees C) suits reducing atmospheres. Type T (-270 to 370 degrees C) offers high precision at low temperatures. Type E provides the highest EMF output. Types R and S (Platinum, 0 to 1600 degrees C) are for high-temperature, high-precision applications.
How do I configure a high-speed counter for an encoder?
Connect the encoder A phase to HSC Channel A, B phase to Channel B, and Z phase to the reset input. High-speed counters support frequencies from 100 kHz to 1 MHz with 1-phase counting (A only), 2-phase counting (A+B for direction detection), and x1/x2/x4 multiplication modes. These are typically built into the CPU or available as dedicated modules.
What are the 4-20mA diagnostic thresholds for wire break and short circuit?
Standard 4-20mA diagnostic ranges are: below 3.6mA indicates sensor wire break, above 21mA indicates sensor short circuit. The range 3.8-4.0mA is flagged as Under Range and 20.0-20.5mA as Over Range. These diagnostics help detect field wiring faults before they cause process upsets. Most modern PLC analog input modules support these diagnostic functions natively.
What are the wiring best practices for PLC I/O installations?
Key rules include: separate power and signal wiring in different cable trays, use shielded twisted pair cables for analog signals with shield grounded at one end only, install flyback diodes for DC inductive loads and RC snubbers for AC inductive loads, protect each output group with fuses, use dedicated Class D grounding (100 ohms or less) separate from power ground, and supply sensor power from a dedicated filtered SMPS.