1771-NB4T | Allen-Bradley Combo Module

The 1771-NB4T is a high-resolution analog combination module used in Allen-Bradley’s PLC-5 systems to acquire and generate precise analog signals. It includes 2 input channels for thermocouple or millivolt (mV) measurements and 2 current output channels, typically in the 4–20 mA range. This makes it suitable for industrial environments where temperature monitoring and analog control are critical, such as chemical processing, food manufacturing, and energy systems. The thermocouple input allows direct sensor connectivity without external signal conditioners. Its integration with the PLC-5 chassis enables seamless data exchange and real-time process control.

The 1771-NB4T is classified as a high-resolution analog module, meaning it offers significantly finer measurement detail than standard analog modules in the PLC-5 family. This enhanced resolution allows it to detect smaller changes in analog signals, which is essential for precise control and monitoring. For temperature or low-voltage applications, high resolution ensures better accuracy in trend analysis and feedback control. It is particularly useful in critical systems where minor fluctuations in temperature or voltage can affect product quality or safety. While the exact bit resolution isn't always specified publicly, it outperforms conventional analog input/output modules.

The 1771-NB4T provides analog current outputs in the range of 4 to 20 mA, which is the industry standard for control signal transmission. This signal range allows it to interface reliably with a variety of actuators, such as control valves, variable-speed drives, or indicators. Current signals are less susceptible to electrical noise and voltage drop, making them ideal for transmission over long cable runs. The module can dynamically adjust its output based on control instructions from the PLC-5 processor. This supports closed-loop control systems where sensor inputs are used to regulate output behavior.

The 1771-NB4T must be installed in a PLC-5 compatible chassis, specifically from the 1771 series of I/O racks. These racks support the backplane communication and power supply requirements for the module. The module slides into a designated slot and locks into place with side clips. It communicates with the PLC-5 processor over the rack’s backplane bus. When planning an upgrade or replacement, ensure the rack provides sufficient space and meets power supply conditions. Only PLC-5 systems can host this module; it is not compatible with ControlLogix or CompactLogix chassis.

No, the 1771-NB4T is a legacy module and is no longer actively manufactured by Allen-Bradley. It was part of the now-discontinued PLC-5 platform, which has been succeeded by the more modern ControlLogix and CompactLogix systems. However, it remains widely used in legacy systems, and spare parts are available through Rockwell Automation’s aftermarket services and authorized resellers. For new projects, migration to newer platforms with similar analog capabilities is recommended. Still, in existing installations, the 1771-NB4T continues to perform reliably when maintained properly.