Allen-Bradley 1756 ControlLogix Modules: 1756-IRT8I

Technical Dossier

Product Details And Specifications

Allen-Bradley 1756 ControlLogix Series Technical Notes

The Allen-Bradley 1756 ControlLogix platform, developed by Rockwell Automation, is one of the most widely deployed programmable automation controller (PAC) architectures in global heavy industry. Installed across petrochemical refineries, nuclear power generation facilities, offshore oil & gas platforms, pulp and paper mills, and large-scale water treatment plants, the 1756 series functions as the backbone of safety-critical and high-availability control systems. Its modular, scalable chassis architecture supports mixed I/O, redundant controller configurations, and multi-network communication within a single backplane — making it the reference standard for distributed control system (DCS) integration in process industries.

The 1756-IRT8I is an 8-channel isolated RTD/thermocouple input module within this platform, designed for precision temperature measurement in environments where channel-to-channel isolation is mandatory. It supports RTD types (PT100, PT1000, Ni120) and thermocouple types (J, K, E, T, R, S, B, C), with 24-bit resolution per channel and individual channel configuration via Logix Designer software.

The Evolution of 1756 ControlLogix Architecture

The 1756 ControlLogix platform was introduced in the late 1990s as Rockwell Automation's answer to the growing demand for a unified, scalable controller architecture capable of replacing both relay logic panels and legacy PLC-5 and SLC 500 systems. The original 1756-L1 and 1756-L55 controllers established the chassis-based modular concept, where a single 1756-Axx chassis could house controllers, I/O, and communication modules simultaneously on a shared high-speed backplane.

Through successive generations — L6x, L7x, and the current L8x series — the platform evolved to support larger user memory (up to 40 MB on L83E), faster scan times, integrated motion control (CIP Motion), and EtherNet/IP as the primary network fabric. The introduction of the 1756-L7x series brought SD card-based firmware storage and enhanced diagnostics. The L8x generation added cybersecurity hardening, Trusted Slot verification, and native support for CIP Security.

Compatibility across generations is managed through chassis and power supply selection. Older 1756-A7, 1756-A10, and 1756-A13 chassis remain electrically compatible with current modules, though firmware and Logix Designer version alignment is required. Modules from the early L5x era may require firmware downgrade or replacement when integrated into L8x-based systems.

1756 ControlLogix Full Catalog & Functionalities (SKU List)

Controllers

• 1756-L71: ControlLogix 5570 controller, 2 MB user memory, standard processing.
• 1756-L72: ControlLogix 5570 controller, 4 MB user memory, mid-range applications.
• 1756-L83E: ControlLogix 5580 controller, 40 MB memory, EtherNet/IP embedded port, highest performance tier.
• 1756-RM2: ControlLogix redundancy module, enables hot-standby controller pair for high-availability systems.

Analog Input Modules

• 1756-IF16: 16-channel single-ended analog voltage/current input, ±10 V / 0–20 mA, 16-bit resolution.
• 1756-IF8: 8-channel differential analog input, high-resolution 24-bit, for precision process signals.
• 1756-IR6I: 6-channel isolated RTD input module, supports PT100, Ni120, and resistance measurement.
• 1756-IT6I: 6-channel isolated thermocouple input, supports J, K, E, T, R, S, B, C types.
• 1756-IRT8I: 8-channel isolated RTD/thermocouple combination input, 24-bit resolution per channel, full channel-to-channel isolation — the subject SKU of this page.

Analog Output Modules

• 1756-OF8: 8-channel analog output, voltage and current selectable per channel, 16-bit resolution.

Digital Input Modules

• 1756-IB16I: 16-channel isolated 24 VDC digital input, individual channel isolation for high-noise environments.

Digital Output Modules

• 1756-OB16E: 16-channel electronically fused 24 VDC digital output, diagnostic feedback per channel.
• 1756-OW16I: 16-channel isolated relay output, 120/240 VAC or 24 VDC, for high-voltage switching applications.

Communication Modules

• 1756-EN2T: EtherNet/IP communication module, 10/100 Mbps, dual-port, primary network interface for ControlLogix systems.
• 1756-CN2R: ControlNet communication module, redundant coax media, for deterministic network topologies.
• 1756-DNB: DeviceNet scanner module, manages DeviceNet device networks from the ControlLogix backplane.

Power Supplies

• 1756-PA75: 120/240 VAC input power supply, 75 W output, standard chassis power for most configurations.
• 1756-PB75: 18–32 VDC input power supply, 75 W, for DC-powered panel installations.

Chassis

• 1756-A13: 13-slot ControlLogix chassis, standard rack for mid-to-large I/O configurations.
• 1756-A17: 17-slot ControlLogix chassis, maximum slot count for high-density module installations.

Sourcing Hard-to-Find & Obsolete 1756 ControlLogix Parts

A significant portion of installed 1756 ControlLogix systems worldwide operate on hardware that Rockwell Automation has transitioned to end-of-life or discontinued status. Modules such as the 1756-L55, 1756-M02AE, 1756-CN2, and early-generation analog I/O cards are no longer manufactured but remain in active service at facilities with 20+ year operational horizons.

Quality Control for the 1756 ControlLogix Range

The 1756 platform's backplane communication protocol (ControlBus) and module keying system require that quality verification go beyond basic power-on testing. DriveKNMS applies a structured test protocol for all 1756 modules:

• Backplane communication verification: Each module is seated in a live 1756 chassis and confirmed to enumerate correctly in RSLogix 5000 / Logix Designer, with no inhibited or faulted module status.
• Analog channel calibration check: For modules including the 1756-IRT8I, 1756-IF16, and 1756-OF8, each channel is stimulated with a calibrated reference signal and output accuracy is verified against published specifications.
• Isolation integrity test: Isolated-channel modules (1756-IRT8I, 1756-IB16I, 1756-OW16I) undergo channel-to-channel and channel-to-chassis isolation resistance measurement.
• Firmware identification: Module firmware revision is recorded and provided with each shipment to support customer compatibility planning.
• Visual and mechanical inspection: Connector pins, module locking tabs, and keying inserts are inspected for damage prior to shipment.