Allen-Bradley 1756-IRT8I ControlLogix Temperature Input Module

Product Overview

The Allen-Bradley 1756-IRT8I is a high-density, channel-isolated temperature input module designed for the flagship ControlLogix PLC platform. Engineered by Rockwell Automation, this advanced hardware supports up to 8 isolated input channels, allowing direct connection to Resistance Temperature Detectors (RTDs), Thermocouples, and low-voltage millivolt (mV) linear signals. By implementing strict channel-to-channel electrical isolation, the module eliminates ground loops and systemic electrical noise, making it highly reliable for harsh process manufacturing plants.

Weighing a rigid and substantiative 0.34 kg (approx. 0.75 lbs) net, the robust build profile of this module reflects its industrial-grade internal shielding and isolation components. Featuring a premium 24-bit analog-to-digital resolution (0.01 µV per count) and a pinpoint accuracy rating of ±0.3°C, the 1756-IRT8I delivers exceptional measurement depth. Its rapid 1ms scan time allows for immediate loop updates, which makes it ideal for highly sensitive thermal environments like pharmaceutical batch processing, complex chemical reactors, and industrial furnace control chambers where minor temperature fluctuations impact product safety and quality boundaries.

Technical Specifications

Advanced Engineering Capabilities

Cold Junction Compensation (CJC)

To deliver precise thermocouple metrics, the 1756-IRT8I requires specific terminal housing blocks embedded with **Dual CJC sensors**. These dedicated sensors monitor localized temperature fluctuations at the physical connection blocks, allowing the hardware logic to continuously cross-reference thermal differentials and hold the system accuracy stable at ±0.3°C.

Automated Calibration & Fault Detection

The module eliminates the need for manual, scheduled tool calibrations by initiating an autonomous **auto-calibration routine** on every power cycle. Furthermore, built-in **wire break detection** active diagnostics sample line resistance profiles continuously. If a field sensor burns out or suffers a snapped lead wire, the hardware flags a channel fault tag within 2 to 5 seconds, prompting operators before control values drift dangerously.

Broad Multi-Sensor Compatibility

The 1756-IRT8I simplifies inventory footprint management by supporting an array of industry sensors out of a single slot:

• Supported RTD Elements: Platinum (100 Ω, 200 Ω, 500 Ω, 1000 Ω), Nickel (100 Ω, 120 Ω, 200 Ω, 500 Ω), and Copper (10 Ω).
• Supported Thermocouple Types: Specialized and high-temp classes including Types B, C, D, E, J, K, N, R, S, T, and TXK/XK.

Installation Requirements

Please note that the 1756-IRT8I utilizes a unique high-density 36-pin Removable Terminal Block (RTB) connector scheme. It is completely incompatible with standard 20-pin RTB units used by lower-density analog modules. Due to the 36-pin block profile and the 0.34 kg internal component mass, ensure the RTB side screws are firmly anchored to prevent mechanical sagging. Installers must order the 1756-TBCH (extended depth screw-clamp block) or the 1756-TBS6H (extended depth spring-clamp block) to establish safe and proper field terminations.

Technical FAQs

Q1: What is the benefit of the 24-bit conversion resolution over typical 16-bit analog input modules?

A1: A 24-bit resolution provides up to 0.01 µV per binary count, enabling the module to read subtle thermal shifts that 16-bit systems ignore. This grain-level tracking results in tight loop stability, allowing the PID loops within the ControlLogix processor to react to fractional variations before process trends deviate.

Q2: Why does the module draw power across both the 5.1V and 24V backplane current buses?

A2: The module splits its power duties to isolate signal paths cleanly. The 200 mA current draw on the 5.1V rail powers the internal microprocessors, A/D converters, and digital backplane communication links. The 150 mA current draw on the 24V rail runs the analog input conditioning blocks and maintains the high-potential electrical isolation barrier between channels.

Q3: Can I mix RTDs and Thermocouples on the same 1756-IRT8I module simultaneously?

A3: Yes. Because every single channel is completely isolated and possesses independent configuration parameters, you can customize each point individually in Studio 5000 Logix Designer. Channel 0 can track a Type K thermocouple while Channel 1 is configured to process a Pt100 RTD without any cross-talk or impedance issues.