Allen-Bradley 1746-NI4

The Allen-Bradley 1746-NI4 is a high-density analog input module designed for the SLC 500 PLC platform. It provides four high-resolution input channels capable of processing both voltage and current signals from various industrial sensors like pressure transducers, flow meters, and thermocouples (via transmitters).

1. Key Technical Specifications

The “NI4” is a non-isolated module, meaning all four channels share a common signal return.

2. Hardware Configuration (Jumper Settings)

Unlike modern modules configured entirely in software, the 1746-NI4 requires you to set physical jumpers on the circuit board to define the input type (Current vs. Voltage) for each channel.

• To configure: You must remove the module from the chassis and locate the jumpers labeled J1 through J4 (corresponding to channels 0 through 3).

• Current Positioning: Place the jumper on the “Current” pins (typically the bottom two pins).

• Voltage Positioning: Place the jumper on the “Voltage” pins (typically the top two pins).

Warning: Applying a voltage signal to a channel jumpered for “Current” can damage the internal 250 $\Omega$ precision resistor.

3. Wiring and Connection

The module features a removable terminal block. Because it is a differential input module, each channel has a positive (+) and negative (-) terminal.

• Shielding: To prevent electrical noise from causing “jitter” in your readings, use shielded twisted-pair cable. Connect the shield to the ANL GND terminal on the module.

• Loop Power: The 1746-NI4 does not provide loop power for 2-wire sensors. You must use an external 24V DC power supply to power the current loop.

4. Data Representation and Scaling

In the SLC 500 (programmed via RSLogix 500), the data from this module is represented as a 16-bit signed integer.

• Raw Range: Typically -32,768 to +32,767.

• Scaling: To convert these raw numbers into engineering units (like PSI or Bar), you use the SCL (Scale) or SCP (Scale with Parameters) instruction in your ladder logic.

   

  • Example for 4-20mA: 3,277 (at 4mA) to 16,384 (at 20mA) depending on the specific configuration bit settings.

5. Common Troubleshooting Tips

• Input Data is 0: Check the jumper settings. If the jumper is missing or in the wrong position, the A/D converter will not see the signal.

• Module Status LED Flashing: This usually indicates a configuration error or that the module is not receiving 24V DC power from the SLC backplane. Ensure your power supply (e.g., 1746-P1 or P2) is healthy.

• Channel Interference: Since this is a non-isolated module, a ground loop on one sensor can affect the readings on all four channels. If you see “bleeding” between channels, you may need an external signal isolator.