Allen Bradley 1746-NI8 Analog Input Module Application Case

The single ended input signal is connected positively to CHx (+) and negatively to CHx (-), and the multi-channel common terminal can be short circuited to reduce wiring.

The common mode voltage of the differential input signal source should be ≤± 10.5V. It is forbidden to connect the common terminal of the signal source to the module to avoid grounding loops.

The power module does not provide sensor power and needs to be separately matched with a DC power supply of the transmitter specifications (such as 24V DC).

Run basic configuration

（1） Module identification and addressing

ID code setting: The corresponding ID code (Class 1:3526, Class 3:12726) needs to be entered in the programming software. Early versions of RSLogix 500 only supported Class 1 and need to be upgraded to v1.30+to support Class 3.

Memory Mapping:

Class 1: Output image O: e.0-O: e.7 consists of 8 channel configuration words, while input image I: e.0-I: e.7 consists of 8 channel data words;

Class 3: The output image contains an additional O: e.8-O: e.11 (2 sets of scaling upper and lower limits), and the input image I: e.8-I: e.15 consists of 8 channel status words.

（2） Key parameter calculation

Module update time: The total sampling conversion time for all enabled channels is 0.75ms for a single channel and 6.0ms for 8-channel full configuration (independent of filtering frequency).

Channel response time:

Start time (enable until status word update): 101-107ms;

Shutdown time (from disabled to data reset): 1-7ms;

Reconfigure time (modify configuration): 101-107ms (excluding data format modifications).

Filtering and Anti aliasing:

The filtering frequency determines the step response (such as achieving 1% accuracy in 730ms for 1Hz filtering, and only 0.5ms without filtering);

Aliasing frequency=1/(number of enabled channels x 0.00075), channel 1 is 1333Hz, and channel 8 is 167Hz. It is necessary to ensure that the signal frequency is lower than the aliasing frequency.

Channel configuration and data processing

（1） Detailed Explanation of Configuration Words

Each channel configuration word (16 bits) defines the channel working mode, and the key bits have the following meanings:

Description of Range Function Options

The voltage (± 10V/1-5V/0-5V/0-10V) and current (0-20mA/4-20mA/± 20mA/0-1mA) of the 0-2 input type should be consistent with the DIP switch

3-5 data format engineering unit (1mV/1 μ A step) Scaled-for-PID（0-16383）、 Proportional counting (-32768-32767), NI4 compatible format, Class 3 user-defined

6-7 open circuit state is only valid for 4-20mA, with optional zero value, upper limit value, and lower limit value

8-10 filtering frequency 000=no filtering, 001=75Hz, 010=50Hz, 011=20Hz, 100=10Hz, 101=5Hz, 110=2Hz, 111=1Hz

11 channel enable 1=enabled (participating in sampling), 0=disabled (data reset)

12-15 reservation needs to be set to 0

（2） Data scaling

The digital output of the module needs to be converted into actual engineering units (such as temperature and pressure), supporting multiple scaling methods. Examples are as follows:

Engineering unit: directly corresponding to physical quantities (such as 4-20mA corresponding to 100-500 ℃, data word 5500 → 247.5 ℃);

Scaled for PID: adapted to SLC PID instructions, 0 corresponds to lower limit, 16383 corresponds to upper limit (e.g. 4-20mA corresponds to 212-932 ° F, data word 5500 → 453.71 ° F);

User defined (Class 3): Set two sets of zoom ranges through O: e.8-O: e.11 and map them proportionally (e.g. 0-10V corresponds to 0-200psi, data word 16600 → 166psi).

（3） Status word monitoring (Class 3)

The input images I: e.8-I: e.15 are channel status words, and the key bits provide feedback on the operating status

Position 0-2: Current input type;

Position 3-5: Current data format;

Bit 11: Channel enable status (1=enabled);

Bit 12: Open circuit error (1=detected);

Bit 13: Over range error (1=input exceeds upper limit);

Bit 14: Under range error (1=input exceeds lower limit);

Bit 15: Configuration error (1=invalid configuration word).

Example of trapezoidal logic

Provide ladder diagram examples of typical programming scenarios, including:

Initialization configuration: Use the first scan bit (S: 1/15) to copy the preset configuration word (stored in the N10 file) to the module output image area (such as O: 3.0-O:3.7) through the COP instruction, and batch enable 8 channels.

PID interface: Set the channel data to Scaled for PID format and directly use it as a process variable (PV) for PID instructions without intermediate scaling (e.g. I: 3.0 → PID control block N11:23).

Fault alarm: Monitor the open circuit error bit of the Class 3 status word (such as I: 3.8/12), trigger the output point (such as O: 2.0) to drive the alarm light.

Diagnosis and troubleshooting

（1） LED status interpretation

Solution to the cause of module status light channel status light malfunction

Always on, always on. Normal operation without operation required

Constant flashing, open circuit/over range/configuration error 1. Check wiring and sensors; 2. Confirm that the configuration word is valid; 3. Check the status word to locate the error type

Extinguish module fault/not powered on 1. Check the power supply of the backplane; 2. Re plug and unplug the module; 3. Contact technical support