National Instruments FP-RTD-124, cFP-RTD-124 User Manual

FieldPoint Operating Instructions

FP-RTD-124

AND CFP-RTD-124

Eight-Channel Four-Wire RTD and Resistance Input Modules

These operating instructions describe how to install and use the National Instruments FP-RTD-124 and cFP-RTD-124 three-wire RTD and resistance input modules (referred to inclusively as the [c]FP-RTD-124). For details on configuring and accessing the [c]FP-RTD-124 over a network, refer to the user manual for the FieldPoint network module you are using.

Features

The [c]FP-RTD-124 is a FieldPoint RTD and resistance input module with the following features:

• Inputs for 100 Ω platinum RTDs (resistance temperature detectors)

• Built-in linearization for six TCR (temperature coefficient of resistance, or alpha) values of RTDs

• Direct resistance measurements with a 400 Ω range

• Four-wire measurement for high accuracy

• 16-bit resolution

• Filtering against 50 and 60 Hz noise

• 2,300 V

• –40 to 70 °C operation

• Hot plug-and-play

transient overvoltage protection

rms

FieldPoint™, National Instruments™, NI™, and ni.com™ are trademarks of National Instruments Corporation. Product and company names mentioned herein are trademarks or trade names of their respective companies. For patents covering National Instruments products, refer to the appropriate location: Help»Patents in your software, the

patents.txt file on your CD, or ni.com/patents.

323349A-01 October 2002

Installing the FP-RTD-124

The FP-RTD-124 mounts on a FieldPoint terminal base (FP-TB-x). Hot plug-and-play enables you to install the FP-RTD-124 onto a powered terminal base without disturbing the operation of other modules or terminal bases. The FP-RTD-124 receives operating power from the terminal base.

To install the FP-RTD-124, refer to Figure 1 and complete the following steps:

1. Slide the terminal base key to either position X, used for any module, or position 1, used for the FP-RTD-124 module.

2. Align the FP-RTD-124 alignment slots with the guide rails on the terminal base.

3. Press firmly to seat the FP-RTD-124 on the terminal base. When the module is firmly seated, the terminal base latch locks it into place.

Key

Latch

Alignment

Slot

Guide Rails

Terminal BaseI/O Module

Figure 1. Installing the FP-RTD-124

Installing the cFP-RTD-124

The cFP-RTD-124 mounts on a Compact FieldPoint backplane (cFP-BP-x). Hot plug-and-play enables you to install the cFP-RTD-124 onto a powered backplane without disturbing the operation of other modules or connector blocks. The cFP-RTD-124 receives operating power from the backplane.

FP-RTD-124 and cFP-RTD-124 2 ni.com

To install the cFP-RTD-124, refer to Figure 2 and complete the following steps:

1. Align the captive screws on the cFP-RTD-124 with the holes on the backplane. The alignment keys on the cFP-RTD-124 prevent backward insertion.

2. Press firmly to seat the cFP-RTD-124 on the backplane.

3. Using a number 2 Phillips screwdriver with a shank of at least 64 mm (2.5 in.) length, tighten the captive screws to 1.1 N ⋅ m (10 lb ⋅ in.) of torque. The nylon coating on the screws prevents them from loosening.

2 1

1 cFP Backplane 2 cFP Controller Module

Figure 2. Installing the cFP-RTD-124

3 cFP I/O Module 4Captive Screws

5 Screw Holes

Wiring the [c]FP-RTD-124

The FP-TB-x terminal bases have connections for each of the eight input channels on the FP-RTD-124. The cFP-CB-x connector blocks provide the same connections for the cFP-RTD-124.

Table 1 lists the terminal assignments for the signals associated with each channel.

Table 1. Terminal Assignments

Terminal Numbers

Channel

If you are using shielded wiring, you can reduce input signal noise by connecting one end of the shield to the COM terminal. Do not connect the shield to any of the wires at the signal end.

SENSE+ SENSE– EXCITE+ COM

0 1 2 17 18

1 3 4 19 20

2 5 6 21 22

3 7 8 23 24

4 9 10 25 26

5 11 12 27 28

6 13 14 29 30

7 15 16 31 32

Taking RTD Measurements with the [c]FP-RTD-124

The [c]FP-RTD-124 has eight input channels. All eight channels share a common ground that is isolated from other modules in the FieldPoint system. Each channel pulses a 2 mA excitation current out of the EXCITE+ terminal. The excitation current returns through the COM terminal. The SENSE+ and SENSE– terminals measure resistance and compensate for lead resistance errors. Each channel is filtered, then sampled by a 16-bit analog-to-digital converter.

FP-RTD-124 and cFP-RTD-124 4 ni.com

Figure 3 shows the input circuitry on one channel.

Pulsed 2 mA

EXCITE+

Four-Wire

RTD

SENSE+

SENSE–

COM

[c]FP-RTD-124

Figure 3. FP-RTD-124 Input Circuit

Amplifier

16-bit

ADC

Taking Measurements from Four-Wire RTDs

Four-wire RTDs generally have two wires of one color and two of another color (usually red and white). Connect the wires of one color to the positive terminals (EXCITE+ and SENSE+). Connect the wires of the other color to the negative terminals (COM and SENSE–).

EXCITE+

SENSE+

Four-Wire

RTD

SENSE–

COM

[c]FP-RTD-124

Figure 4. Four-Wire RTD Connections on One Channel

Taking Measurements from Three-Wire RTDs

NI recommends the [c]FP-RTD-122 for three-wire RTD measurements because it includes special sensing circuitry to compensate for up to 95% of the lead resistance of the third wire. Most three-wire RTDs have two wires of one color (usually red, sometimes black) and one wire of another color (usually white, sometimes red). Connect the two wires of the same color to the

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