A-B 1746-NR4 SLC 4 Point Resistance Input Module

Overview

This chapter describes the four-channel 1746-NR4 RTD/Resistance

Input Module and explains how the SLC controller gathers RTD

(Resistance Temperature Detector) temperature or resistance-initiated

analog input from the module.Included is:

• a general description of the module’s hardware and software features.

• an overview of system operation.

For the rest of the manual, the 1746-NR4 RTD/Resistance Input

Module is referred to as simply the RTD module.

Description

The RTD module receives and stores digitally converted analog data

from RTD units or other resistance inputs such as potentiometers into

its image table for retrieval by all fixed and modular SLC 500

processors. An RTD module consists of a temperature-sensing element

connected by two, three, or four wires that provide input to the RTD

module. The module supports connections from any combination of

up to four RTD units of various types (for example: platinum, nickel,

copper, or nickel-iron) or other resistance inputs.

The RTD module supplies a small current to each RTD unit connected

to the module inputs (up to 4 input channels). The module provides

on-board scaling and converts RTD unit input to temperature (°C, °F)

or reports resistance input in ohms.

Each input channel is individually configurable for a specific input

device. Broken sensor detection (open- or short-circuit) is provided

for each input channel. In addition, the module provides indication if

the input signal is out-of-range.

System Operation

The RTD module has three operational states.

• Cycle power

• Module operation

• Error (module error and channel error)

Cycle Power

When you cycle the module’s power, the RTD module checks its

internal circuits, memory, and basic functions via hardware and

software diagnostics. During this time the module status LED indicator

remains off. If no faults are found during the diagnostics, the module

status LED indicator is on.

After the checks are complete, the RTD module waits for valid

channel configuration data from your SLC ladder logic program

(channel status LED indicators off). After configuration data is written

to one or more channel configuration words and their channel enable

bits are set by the user program, the channel status LED indicators go

on and the module continuously converts the RTD or resistance input

to a value within the range you selected for the enabled channels. The

module is now operating in its normal state.

Each time a channel is read by the module, that data value is tested by

the module for a fault condition, for example, open circuit, short

circuit, over range, and under range. If such a condition is detected, a

unique bit is set in the channel status word and the channel status

LED indicator blinks, indicating a channel error condition.

The SLC processor reads the converted RTD or resistance data from

the module at the end of the program scan or when commanded by

the ladder program. The processor and RTD module determine that

the backplane data transfer was made without error and the data is

used in your ladder program.