Product Overview

The ABB Bailey NTDI21 Infi 90 Digital I/O Termination Unit is a wiring-interface component used to organize digital input and output connections within an Infi 90 control system. It provides a defined termination point for discrete field circuits before they connect with the associated control-system I/O hardware.

Digital input and output signals are used for many basic control functions, including equipment running status, switch positions, permissive conditions, interlocks, alarms, and discrete commands. The NTDI21 helps keep these circuits structured at the cabinet interface, making individual signal paths easier to identify during installation and maintenance.

In a typical Infi 90 installation, the termination unit sits between field wiring and the active digital I/O circuitry. Input signals travel from field devices toward the control system, while output commands travel from the control system toward relays, solenoids, contactors, or other discrete loads.

The NTDI21 is a termination component rather than a programmable control module. Its value within the system comes from maintaining an organized and serviceable connection between field circuits and the Infi 90 control hardware.

Technical Specifications

Product Features

• Infi 90 digital I/O termination unit
• Supports digital input and output circuit connections
• Provides an organized field-wiring interface
• Suitable for discrete monitoring and control signals
• Simplifies circuit identification and tracing
• Supports cabinet-level wiring and maintenance
• Suitable for legacy ABB Bailey Infi 90 control systems
• Can be used in control-system replacement and refurbishment work

Working Principle

The NTDI21 establishes the physical termination point for discrete input and output circuits. It does not interpret the process logic itself; instead, it maintains the electrical path between field wiring and the appropriate Infi 90 I/O circuitry.

For an input signal, a field device changes its electrical state and the resulting signal passes through the wiring and NTDI21 toward the digital input circuitry.

Field Switch / Contact → Field Wiring → NTDI21 → Infi 90 Digital Input → Controller

For an output signal, the direction is reversed. The controller activates the appropriate digital output, and the resulting electrical signal passes through the termination arrangement toward the connected field device.

Controller → Infi 90 Digital Output → NTDI21 → Field Wiring → Relay / Solenoid / Device

This arrangement makes the NTDI21 a convenient inspection point when tracing a discrete signal. A fault can be narrowed down by checking the field device, wiring, termination connection, I/O channel, and controller logic in sequence.

Role in Infi 90 Control Architecture

The NTDI21 belongs to the field-interface layer of an ABB Bailey Infi 90 digital I/O architecture.

Field Devices ↔ Field Wiring ↔ NTDI21 ↔ Digital I/O Hardware ↔ Infi 90 Controller ↔ Operator Interface

The termination unit does not replace the digital I/O hardware or controller. It provides the physical connection structure required for those active system components to communicate with field circuits.

Industrial Applications

Installation and Maintenance

1. Install the NTDI21 at the designated Infi 90 digital I/O termination location.
2. Verify the cabinet mounting arrangement before connecting field conductors.
3. Identify each input and output circuit against the applicable electrical drawings.
4. Confirm terminal assignments before transferring wiring from an existing unit.
5. Connect each conductor to its specified termination point.
6. Check that all terminal connections are properly secured.
7. Maintain clear identification between incoming input circuits and outgoing output circuits.
8. Inspect wiring for loose connections, damaged insulation, or incorrect termination.
9. Check continuity when a digital input does not reflect the expected field condition.
10. Trace the output path from the controller through the termination point when a field device does not respond.
11. Inspect the termination area for corrosion, contamination, overheating, or mechanical damage.
12. After replacement or rewiring, test all affected digital I/O circuits before returning the system to normal operation.

Compatible System Components

Recommended Related Models

Key Advantages

• Provides a structured connection point for Infi 90 digital I/O circuits
• Supports both discrete input and output wiring
• Makes field circuits easier to identify during troubleshooting
• Provides a practical interface between field wiring and active I/O hardware
• Helps reduce unnecessary disturbance to controller and I/O modules during wiring maintenance
• Suitable for legacy Infi 90 system maintenance and refurbishment

Technical FAQs

What is the main difference between the NTDI21 and a digital input termination unit?

The NTDI21 is intended for digital I/O connections, covering both input and output circuits. A digital input termination unit is focused on incoming discrete signals from field devices.

Where should troubleshooting begin when an NTDI21-connected input shows the wrong status?

Start at the field device and work toward the controller. Check the field contact, wiring, NTDI21 termination point, corresponding digital input channel, and finally the control-system configuration. This helps determine whether the problem is physical or logic-related.

What should be checked when an output reaches the NTDI21 but the field device does not operate?

Verify the controller output command and I/O channel first, then inspect the NTDI21 connection, outgoing field wiring, interposing relay or interface device, and final load. A termination connection alone cannot confirm that the downstream device is receiving the correct operating signal.

Why is the NTDI21 useful in an older Infi 90 installation?

Legacy control systems often contain extensive field wiring accumulated over years of operation and modification. A dedicated digital I/O termination point helps maintain a traceable connection structure, which is valuable during troubleshooting, component replacement, and system refurbishment.