A-B 1746-NR8 System Overview SLC 500™ RTD/Resistance Input Module

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)

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.

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).

The purpose of the LEDs is as follows:

· Channel Status - One LED for each of the 8 input channels indicates if the

channel is enabled, disabled, or is not operating as configured, due to an error.

· Module Status - If OFF or flashing at any time, other than at powerup, this

LED indicates that non-recoverable module errors (for example, diagnostic

or operating errors) have occurred. The LED is ON if there are no module errors.