Category: A-B

General Description

The thermocouple/mV module receives and stores digitally converted

thermocouple and/or millivolt (mV) analog data into its image table

for retrieval by all fixed and modular SLC 500 processors. The module

supports connections from any combination of up to four

thermocouple or mV analog sensors.

The following tables define thermocouple types and their associated

full scale temperature ranges and also list the millivolt analog input

signal ranges that each 1746-NT4 channel will support. To determine

the practical temperature range your thermocouple supports, refer to

the specifications in Appendix A.

Module Operation

Each input channel consists of an RTD connection, which provides:

· excitation current

· a sense connection, which detects lead-wire resistance

· a return connection, which reads the RTD or resistance value

Each of these analog inputs are multiplexed to an analog converter.

The A/D converter cycles between reading the RTD or resistance value, the

lead wire resistance, and the excitation current. From these readings, an

accurate temperature or resistance is returned to the user program.

The RTD module is isolated from the chassis backplane and chassis ground.

The isolation is limited to 500V ac. Optocouplers are used to communicate

across the isolation barrier. Channel-to-channel common-mode isolation is

limited to ± 5 volts.

LED Status

The illustration below shows the RTD module LED panel consisting of nine

LEDs. The state of the LEDs (for example, off, on, or flashing) depends on the

operational state of the module (see table on page 1-9).

Power-up

At power-up, the RTD module checks its internal circuits, memory, and basic

functions via hardware and software diagnostics. During this time, the module

status LED remains off, and the channel status LEDs are turned on. If no

faults are found during the power-up diagnostics, the module status LED is

turned on, and the channel status LEDs are turned off.

After power-up checks are complete, the RTD module waits for valid channel

configuration data from your SLC ladder logic program (channel status LEDs

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 LEDs 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, overrange,

and under range. If such a condition is detected, a unique bit is set in

the channel status word and the channel status LED flashes, 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.

System Overview

Each individual channel on the RTD module can receive input signals from 2.

3 or 4-wire RTD sensors or from resistance input devices. You configure each

channel to accept either input. When configured for RTD input types, the

module converts the RTD readings into linearized, digital temperature

readings in °C or °F. When configured for resistance inputs, the module

provides a linear resistance value in ohms.

IMPORTANT 

The RTD module is designed to accept input from RTD

sensors with up to 3 wires. When using 4-wire RTD

sensors, one of the 2 lead compensation wires is not used

and the 4-wire sensor is treated like a 3-wire sensor. Lead

wire compensation is provided via the third wire. Refer

to Wiring Considerations on page 2-8 for more information.

System Operation

The RTD module has 3 operational states:

· power-up

· module operation

· error (module error and channel error)

General Diagnostic Features

The RTD module contains diagnostic features that can be used to help you

identify the source of problems that may occur during power up or during

normal channel operation. These power-up and channel diagnostics are

explained in Chapter 6. Module Diagnostics and Troubleshooting.

The RTD module communicates to the SLC 500 processor through the

parallel backplane interface and receives +5V dc and +24V dc power from the

SLC 500 power supply through the backplane. No external power supply is

required. You may install as many RTD modules in your system as the power

supply can support, as shown in the illustration below.

System Overview

Each individual channel on the RTD module can receive input signals from 2.

3 or 4-wire RTD sensors or from resistance input devices. You configure each

channel to accept either input. When configured for RTD input types, the

module converts the RTD readings into linearized, digital temperature

readings in °C or °F. When configured for resistance inputs, the module

provides a linear resistance value in ohms.

Description

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

module inputs (up to 8 input channels). The module provides on-board

scaling and converts RTD 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. For more detail on module functionality, refer to the subsection

entitled System Overview later in this chapter.

General Diagnostic Features

The RTD module contains diagnostic features that can be used to help you

identify the source of problems that may occur during power up or during

normal channel operation. These power-up and channel diagnostics are

explained in Chapter 6. Module Diagnostics and Troubleshooting.

The RTD module communicates to the SLC 500 processor through the

parallel backplane interface and receives +5V dc and +24V dc power from the

SLC 500 power supply through the backplane. No external power supply is

required. You may install as many RTD modules in your system as the power

supply can support, as shown in the illustration below.

Overview

This chapter describes the 8-channel 1746-NR8 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-NR8 RTD/Resistance Input Module is

referred to as simply the RTD module.

The RTD module receives and stores digitally converted analog data from

RTDs or other resistance inputs such as potentiometers into its image table for

retrieval by all fixed and modular SLC 500 processors. An RTD consists of a

temperature-sensing element connected by 2. 3. or 4 wires that provide input

to the RTD module. The module supports connections from any combination

of up to eight RTDs of various types (for example: platinum, nickel, copper, or

nickel-iron) or other resistance inputs.

System Operation

Tle 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.

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.

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.

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.