PLC Digital Input Module Wiring: Sourcing and Sinking Configuration, Voltage Rating, and Input Filter Settings

# PLC Digital Input Module Wiring: Sourcing and Sinking Configuration, Voltage Rating, and Input Filter Settings

Introduction

Programmable Logic Controllers (PLCs) are the backbone of modern industrial automation systems, enabling precise control and monitoring of machinery and processes. One of the most critical aspects of PLC implementation is the wiring and configuration of digital input modules. These modules receive signals from various sensors and switches, translating them into digital data that the PLC can process. Proper wiring, including sourcing and sinking configurations, voltage ratings, input current requirements, and filter settings, is essential for system reliability and performance. This article delves into these technical aspects, providing practical insights and recommendations. We will also compare popular PLC platforms and discuss troubleshooting common issues. By understanding these concepts, engineers and technicians can ensure seamless integration and optimal operation of their automation systems.

Sourcing vs. Sinking Configuration

Sourcing (PNP) Configuration

In a sourcing (PNP) configuration, the PLC digital input module provides a current path to the common (COM) terminal. The sensor or switch connects between the positive voltage supply (+24V) and the input terminal of the PLC. When the sensor is activated, it allows current to flow from the +24V supply, through the sensor, into the PLC input, and finally to the COM terminal.

Key Characteristics:

• The PLC input module acts as the current source.
• Commonly used in European and Asian PLC systems.
• Siemens S7-1200/1500 series use sourcing by default.

Sinking (NPN) Configuration

In a sinking (NPN) configuration, the PLC digital input module sinks current from the input terminal to the COM terminal. The sensor or switch connects between the input terminal and the positive voltage supply (+24V). When the sensor is activated, it allows current to flow from the +24V supply, through the sensor, into the PLC input, and finally to the COM terminal.

Key Characteristics:

• The PLC input module acts as the current sink.
• Commonly used in North American PLC systems.
• Mitsubishi FX series use sinking by default.
• Allen-Bradley CompactLogix/ControlLogix modules are typically configurable for both sourcing and sinking.

Comparison Table

Voltage Rating and Input Types

24VDC Inputs

The most common voltage rating for PLC digital inputs is 24VDC, with an acceptable range typically between 5V and 30V. This voltage level is suitable for most industrial applications and is compatible with a wide range of sensors and switches.

Advantages:

• Lower power consumption compared to higher voltage inputs.
• Reduced risk of electrical hazards.
• Compatible with standard industrial power supplies.

120VAC and 240VAC Inputs

For applications involving legacy machinery or high-power systems, 120VAC and 240VAC inputs are used. These inputs are typically opto-isolated to provide galvanic isolation between the field wiring and the PLC backplane.

Considerations:

• Higher voltage inputs require proper isolation and safety measures.
• Commonly used in power systems and high-voltage applications.
• Verify voltage compatibility between the sensor output and the PLC input card.

Voltage Compatibility

It is crucial to ensure that the sensor output voltage matches the PLC input voltage rating. For example, using a 24VDC sensor with a 120VAC input module will result in improper operation and potential damage to the components.

Input Current Requirements

Typical Input Impedance

For 24VDC inputs, the typical input impedance ranges from 3kΩ to 10kΩ. This impedance determines the current drawn by the input module when the sensor is activated.

Minimum ON Current

The minimum current required to activate the input is typically between 2mA and 5mA. This threshold ensures reliable detection of the input signal.

Maximum OFF Leakage Current

The maximum leakage current when the input is off is typically between 0.1mA and 1.5mA. This leakage current should be minimal to prevent false triggering of the input.

Example Calculation

For a 24VDC input with a 5kΩ impedance:

\[ \text{Current} = \frac{\text{Voltage}}{\text{Impedance}} = \frac{24V}{5kΩ} = 4.8mA \]

This current is sufficient to activate the input, assuming the minimum ON current is 2mA.

Input Filter Time Constant

Purpose of Input Filters

Input filters are used to reject electrical noise and prevent false triggering of digital inputs. They work by adding a delay to the input signal, which helps to filter out high-frequency noise spikes.

Adjustable Filter Settings

Most PLC digital input modules have adjustable filter settings, typically ranging from 0.2ms to 20ms. The appropriate filter setting depends on the application:

• High-Speed Counting: Use a short filter time (e.g., 0.5ms) to allow for fast response.
• Standard Limit Switches: Use a moderate filter time (e.g., 3-5ms) to balance noise rejection and response time.
• Noisy Environments (e.g., Near VFDs): Use a longer filter time (e.g., 10-20ms) to effectively filter out noise.

Example Product

The Filter Hengst 1005124B R928022312 2.0130 PWR10-B00-0-M can be used to provide additional filtering for PLC inputs in noisy environments.

Wiring Practices

Shielded Twisted Pair Cable

For both analog and digital signals, especially near Variable Frequency Drives (VFDs), it is recommended to use shielded twisted pair cable. This type of cable helps to reduce electromagnetic interference (EMI) and ensures reliable signal transmission.

Cable Separation

Signal cables should be separated from power cables by a minimum distance of 200mm. This separation helps to prevent EMI from power cables affecting the signal cables.

Ferrite Cores

Using ferrite cores on cables entering the control panel can further reduce EMI. Ferrite cores act as inductors, attenuating high-frequency noise.

Shield Grounding

The shield of the cable should be connected at one end only, typically at the panel side, to avoid ground loops. Ground loops can introduce unwanted currents and noise into the system.

Example Product

The Parker G04277 Filter Element for Hydraulic Systems can be used to filter hydraulic system noise that may affect PLC inputs.

Common PLC Platforms Comparison

Siemens S7-1200 SM 1221 DI

• Points: 8/16
• Configuration: Sourcing (PNP) by default
• Features: Compact design, integrated PROFINET, scalable
• Filtering: Per channel configurable

Siemens S7-1500 DI Modules

• Points: Various configurations available
• Configuration: Sourcing (PNP) with configurable filtering per channel
• Features: Advanced diagnostics, high performance, integrated safety
• GE Fanuc IC695CPL410-ACAN CPU Controller for PACSystems can be used with these modules for enhanced control.

Allen-Bradley 1769-IA16

• Points: 16
• Configuration: 120VAC input
• Features: Compact I/O, high-density packaging, easy installation

Allen-Bradley 1756-IB16D

• Points: 16
• Configuration: 24VDC sink/source selectable with diagnostics
• Features: Integrated diagnostics, high-speed counting, advanced diagnostics

Mitsubishi FX3U-16EYR

• Points: 16
• Configuration: Relay output
• Features: Built-in positioning, high-speed counters, expandable

Mitsubishi FX5U Series

• Points: Various configurations available
• Configuration: SD/GS input capability
• Features: High-speed processing, integrated Ethernet, advanced motion control

Example Product

The YASKAWA YCP21-E CPU Unit for Factory Automation Systems can be used with these PLC platforms to provide enhanced processing power and connectivity.

Troubleshooting Common Faults

LED Indicator ON but No Software State Change

• Cause: Incorrect wiring to the COM terminal or mismatch in sourcing/sinking configuration.
• Solution: Verify wiring connections and ensure the sourcing/sinking configuration matches the sensor and PLC specifications.

All Inputs Reading ON

• Cause: 0V common connection issue or possible short circuit.
• Solution: Check the 0V common connection and inspect for any short circuits in the wiring.

Intermittent Faults

• Cause: Incorrect input filter setting, poor cable shielding, or loose terminal screw.
• Solution: Adjust the input filter setting, improve cable shielding, and check for loose connections.

Use of Multimeter

• Solution: Use a multimeter to verify the voltage at the terminal block to ensure proper signal levels.

Example Product

The PLC Signal Protection Phoenix Contact F-MS12ST 2817990 can be used to protect PLC inputs from voltage spikes and surges.

High-Speed Input Considerations

Standard Digital Inputs

Standard digital inputs are typically limited to a switching frequency of 10-50Hz. This limitation is due to the scan cycle time of the PLC and the input filter settings.

High-Speed Counter Modules

For applications requiring higher switching frequencies, such as encoder feedback, high-speed counter modules should be used. These modules can support up to 100kHz or more, depending on the PLC platform.

Hardware Interrupt Inputs

Hardware interrupt inputs provide a way to respond to fast-changing signals without the delay of the PLC scan cycle. They allow for immediate response to critical events.

Grounding and Isolation

Opto-Isolator

The opto-isolator provides galvanic isolation between the field wiring and the PLC backplane. This isolation typically has a rating of 500V-1500V, ensuring safety and reliability.

Panel Grounding

Proper panel grounding is essential for system safety and noise reduction. The DIN rail should be bonded to earth to provide a stable ground reference.

Ground Planes

Separate analog and digital ground planes should be used to prevent noise coupling between them. This separation helps to maintain signal integrity and reduces the risk of interference.

Safety-Related Digital Inputs

SIL 2/3 Rated Input Modules

For safety-critical applications, such as emergency stop and safety interlock circuits, SIL 2/3 rated input modules should be used. These modules meet stringent safety standards and provide reliable operation.

Dual-Channel Input with Cross-Fault Detection

Dual-channel inputs with cross-fault detection per IEC 62061 and ISO 13849 ensure that any faults in the safety circuit are detected. This feature is crucial for maintaining safety integrity.

Test Pulse Function

The test pulse function is used to detect wiring faults in safety circuits. It periodically checks the integrity of the circuit and ensures that the safety system is functioning correctly.

Conclusion

Proper wiring and configuration of PLC digital input modules are critical for the reliable operation of industrial automation systems. Understanding the sourcing and sinking configurations, voltage ratings, input current requirements, and filter settings is essential for system design and implementation. By following best practices and using high-quality components, such as those available on EDV-Automation, engineers and technicians can ensure the safety, efficiency, and longevity of their automation systems.

Call to Action

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