Allen-Bradley SLC 500 Modules: 1746-NT4 Thermocouple Input Module,

Technical Dossier

Product Details And Specifications

Allen-Bradley SLC 500 Series Technical Notes

The Allen-Bradley SLC 500 series, manufactured by Rockwell Automation, is one of the most widely deployed small-to-mid-range programmable logic controller (PLC) platforms in global heavy industry. Installed across petrochemical refineries, nuclear auxiliary systems, pulp and paper mills, water treatment facilities, and continuous process manufacturing plants, the SLC 500 platform established a benchmark for modular I/O architecture in the 1990s and remains operational in thousands of facilities worldwide as of 2026. The 1746-NT4 is a 4-channel thermocouple and millivolt input module within this platform, designed for precision temperature measurement in high-noise industrial environments.

The Evolution of SLC 500 Architecture

The SLC 500 platform was introduced by Allen-Bradley (now Rockwell Automation) in the late 1980s as a successor to the SLC 100 and SLC 150 fixed controllers. The architecture is built around the 1746 I/O backplane standard, with processors housed in the 1747 family. Early processors such as the 1747-L20 and 1747-L30 supported limited I/O counts and basic ladder logic. Subsequent generations introduced the 1747-L40, 1747-L511, 1747-L514, and the high-capacity 1747-L553, progressively expanding memory, instruction sets, and communication options.

The 1746 I/O family expanded in parallel, covering discrete, analog, specialty, and communication modules. The platform adopted multiple communication standards over its lifecycle: DH-485, DH+, DeviceNet, ControlNet, and Ethernet/IP via add-on adapters. Compatibility between early and late-generation 1746 I/O modules and 1747 processors is generally maintained at the backplane level, though firmware and RSLogix 500 software version constraints apply for advanced specialty modules including the 1746-NT4.

Rockwell Automation placed the SLC 500 platform in a mature/end-of-life status. Replacement migration paths lead to the MicroLogix 1100/1400 for smaller applications and the CompactLogix 5370/5380 (1769 platform) for larger systems. Despite this, the installed base remains extensive, and spare parts demand—particularly for specialty analog and thermocouple modules—remains active in the aftermarket.

SLC 500 Full Catalog & Functionalities (SKU List)

Processors (1747 Family)

• 1747-L20: SLC 5/01 processor, 1K memory, DH-485 port, supports up to 2 I/O chassis.
• 1747-L30: SLC 5/01 processor, 4K memory, DH-485 port, supports up to 3 I/O chassis.
• 1747-L40: SLC 5/01 processor, 4K memory, DH-485 port, supports up to 4 I/O chassis.
• 1747-L511: SLC 5/05 processor, 16K memory, Ethernet/IP + DH-485 dual port.
• 1747-L514: SLC 5/05 processor, 64K memory, Ethernet/IP + DH-485 dual port.
• 1747-L553: SLC 5/05 processor, 64K memory, ControlNet + DH+ dual port, high-capacity.

Analog Input Modules (1746 Family)

• 1746-NI4: 4-channel analog input, voltage/current selectable, 12-bit resolution.
• 1746-NI8: 8-channel analog input, voltage/current selectable, 12-bit resolution.
• 1746-NI16I: 16-channel isolated analog current input, 16-bit resolution.
• 1746-NI16V: 16-channel isolated analog voltage input, 16-bit resolution.
• 1746-NT4: 4-channel thermocouple/millivolt input module, supports J, K, E, T, R, S, B, N type thermocouples; 16-bit resolution; cold junction compensation; open-circuit detection.
• 1746-NT8: 8-channel thermocouple/millivolt input module, same thermocouple type support as NT4, expanded channel count for larger temperature monitoring loops.

Analog Output Modules (1746 Family)

• 1746-NO4I: 4-channel isolated analog current output, 4–20 mA, 12-bit resolution.
• 1746-NO4V: 4-channel isolated analog voltage output, 0–10 VDC, 12-bit resolution.
• 1746-NO8I: 8-channel isolated analog current output, 4–20 mA.

Discrete I/O Modules (1746 Family)

• 1746-IB16: 16-channel 24 VDC sink digital input module.
• 1746-OB16: 16-channel 24 VDC source digital output module.
• 1746-OW16: 16-channel relay output module, Form A contacts, 240 VAC/24 VDC rated.

Communication & Specialty Modules

• 1746-DNET: DeviceNet scanner module for SLC 500 chassis, supports up to 63 nodes.
• 1746-BAS: BASIC module for custom ladder/BASIC hybrid programming.

Sourcing Hard-to-Find & Obsolete SLC 500 Parts

For end-users operating SLC 500 systems beyond the Rockwell-supported lifecycle, DriveKNMS provides long-term maintenance inventory agreements, allowing facilities to reserve critical spare modules—including thermocouple input cards, analog output modules, and processors—against future failure events. This is particularly relevant for chemical processing and power generation facilities where platform migration is constrained by regulatory approval timelines or capital expenditure cycles.

All 1746-NT4 units sourced by DriveKNMS are verified for channel integrity, cold junction compensation accuracy, and backplane communication function prior to dispatch.

Quality Control for the SLC 500 Range

Discrete I/O modules are tested under rated voltage and load conditions. Relay output modules (e.g., 1746-OW16) are cycled a minimum of 500 times under resistive load to verify contact integrity. Communication modules including the 1746-DNET are tested on a live DeviceNet network segment with node enumeration and data exchange verification.