National Instruments Fieldpoint cFP-RTD-124 User manual

323349A-01 October 2002
©2002 National Instruments Corp. All rights reserved.
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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 Ωplatinum 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 Ωrange
• Four-wire measurement for high accuracy
• 16-bit resolution
• Filtering against 50 and 60 Hz noise
• 2,300 Vrms transient overvoltage protection
• –40 to 70 °C operation
• Hot plug-and-play

FP-RTD-124 and cFP-RTD-124 2 ni.com
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 terminalbase.
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.
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.
Alignment
Slot
Key
Latch
Guide Rails
Terminal BaseI/O Module

© National Instruments Corp. 3 FP-RTD-124 and cFP-RTD-124
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.
Figure 2. Installing the cFP-RTD-124
1 cFP Backplane
2 cFP Controller Module
3 cFP I/O Module
4CaptiveScrews
5 Screw Holes
4
4
3
2 1
5
5

FP-RTD-124 and cFP-RTD-124 4 ni.com
Wiring the [c]FP-RTD-124
The FP-TB-xterminal bases have connections for each of the eight
input channels on the FP-RTD-124. The cFP-CB-xconnector
blocks provide the same connections for the cFP-RTD-124.
Table 1 lists the terminal assignments for the signals associated
with each 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.
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.
Table 1. Terminal Assignments
Channel
Terminal Numbers
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
511 12 27 28
613 14 29 30
715 16 31 32

© National Instruments Corp. 5 FP-RTD-124 and cFP-RTD-124
Figure 3 shows the input circuitry on one channel.
Figure 3. FP-RTD-124 Input Circuit
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–).
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
SENSE+
[c]FP-RTD-124
SENSE–
COM
EXCITE+
Amplifier
Pulsed 2 mA
Four-Wire
RTD
16-bit
ADC
SENSE+
[c]FP-RTD-124
SENSE–
COM
EXCITE+
Four-Wire
RTD

FP-RTD-124 and cFP-RTD-124 6 ni.com
negative terminals (COM and SENSE–). Connect the third wire to
the positive terminal (SENSE+). Use a short jumper wire to
connect the EXCITE+ terminal to the SENSE+ terminal.
Figure 5. Three-Wire RTD Connections on One Channel
Taking Measurements from Two-Wire RTDs
Connect one wire of a two-wire RTD to the SENSE+ terminal and
the other wire to the SENSE–terminal. Use a short jumper wire to
connect the SENSE+ terminal to the EXCITE+ terminal. Use
another short jumper wire to connect the SENSE–terminal to the
COM terminal.
Figure 6. Two-Wire RTD Connections on One Channel
SENSE+
[c]FP-RTD-124
SENSE–
COM
EXCITE+
Three-Wire
RTD
Short Jumper Wire
SENSE+
[c]FP-RTD-124
SENSE–
COM
EXCITE+
Two-Wire
RTD
Short
Jumper
Wire
Short
Jumper
Wire

© National Instruments Corp. 7 FP-RTD-124 and cFP-RTD-124
Measuring Resistance
You can use the [c]FP-RTD-124 to measure resistance in ohms.
In this way, you can take measurements from RTDs of types that
the [c]FP-RTD-124 does not directly support (such as 120 Ωnickel
RTDs) and from resistive devices other than RTDs. You can
measure resistances up to 400 Ω. Resistance values greater than
400 Ω, including open circuits, result in an Out of range error
for the affected channels. The [c]FP-RTD-124 ignores any
configuration of RTD type for channels with resistance selected.
Converting Resistance Measurements
to Temperature Measurements
The [c]FP-RTD-124 has built-in linearization algorithms for
platinum RTDs of either 100 or 1,000 Ωnominal resistance,
and for 6 TCR (or alpha, α) values. The TCR is the average
temperature coefficient of resistance of an RTD from 0 to 100 °C.
This document specifies TCR in units of mΩ/Ω/°C.
The [c]FP-RTD-124 linearizes resistance values and returns
readings in units of temperature. The available ranges are 73 to
1,123 K, –200 to 850 °C, and –328 to 1,562 °F. You can configure
each channel independently, so you can connect different types of
RTDs to each channel.
Note You must configure each channel of the
[c]FP-RTD-124 for the type of RTD connected to it.
The module does not automatically recognize RTD types.
RTD Types
RTD types are specified by material composition, nominal
resistance at 0 °C, and TCR. The [c]FP-RTD-124 can directly
measure the temperature of platinum RTDs of 100 Ωnominal
resistance. These RTDs are commonly referred to as PT100 RTDs.
Different types of platinum RTDs have different TCRs. The
[c]FP-RTD-124 supports the following TCRs: 3.750, 3.851, 3.911,
3.916, 3.920, and 3.928 mΩ/Ω/°C. The most common TCR for
RTDs is 3.851 mΩ/Ω/°C. It is defined in international standards
such as IEC-751, DIN 43760, BS 1904, and ASTM E1137. The
TCR of 3.928 mΩ/Ω/°C is used in the reference function for
platinum thermometers in the International Temperature Scale
of 1990 (ITS-90) for high-accuracy metrology applications.

FP-RTD-124 and cFP-RTD-124 8 ni.com
Unfortunately, not all TCR values are as well defined by standards
organizations, and the behavior of RTDs with the same TCR value
may vary from vendor to vendor. The variations are usually small,
and the built-in linearization algorithms of the [c]FP-RTD-124 are
appropriate for nearly all applications.
The [c]FP-RTD-124 uses a linearization curve known as the
Callendar-Van Dusen equation to measure the temperature of
RTDs. The equation is as follows:
Temperatures below 0 °C:
RT= R0[1 +A × T+B × T2+C × T3× (T–100 °C)]
Temperatures above 0 °C:
RT= R0[1 +A × T+B × T2]
T= temperature in °C
RT= RTD resistance at temperature T
R0= RTD nominal resistance at 0 °C
A, B, and C are coefficients given in Table 2.
Table 2 lists the coefficients used in this equation for each of the
TCR values that the [c]FP-RTD-124 supports. If you have a
nonstandard RTD that does not match one of these linearization
curves, measure the resistance with the [c]FP-RTD-124 and
convert the resistance to temperature in the manner suggested by
the RTD vendor.
Table 2. Callendar-Van Dusen Coefficients Used by the [c]FP-RTD-124
TCR
mΩ/Ω/°C
A
(°C)–1
B
(°C)–2
C
(°C)–4
3.750 3.81 ×10–3–6.02 ×10–7–6.0 × 10–12
3.851 3.9083 ×10–3–5.775 ×10–7–4.183 ×10–12
3.911 3.9692 ×10–3–5.8495 ×10–7–4.233 × 10–12
3.916 3.9739 × 10–3–5.870 ×10–7–4.4 × 10–12
3.920 3.9787 × 10–3–5.8686 ×10–7–4.167 ×10–12
3.928 3.9888 × 10–3–5.915 ×10–7–3.85 ×10–12

© National Instruments Corp. 9 FP-RTD-124 and cFP-RTD-124
Effects of Lead Resistance
The [c]FP-RTD-124 measures resistance by driving a current
through a circuit and sensing the resulting voltage. Any resistance
in the lead wires that connect to the RTD can add errors to your
readings if the measured voltage includes the drop across these
wires.
For example, consider a two-wire RTD element connected as
previously shown in Figure 2. The resistance of the lead wires is
indistinguishable from the resistance of the RTD element. In a
typical application using 10 m of 22 AWG copper wire, each of the
two leads adds approximately 0.5 Ωof error to the measurement.
The 0.5 Ωof error corresponds to a temperature error of about
2.6 °C. Using the three-wire RTD connection in Figure 2 causes
the inclusion of only one of the leads in the sensed measurement.
The resultant error caused by lead resistances is half that of the
error in the two-wire example.
The most accurate way to measure temperature from RTDs is the
four-wire method shown in Figures 2 and 3. With this method, one
pair, the EXCITE and COM leads, carries the current through the
RTD. The other pair, the SENSE leads, measures the voltage
developed by the RTD element independent of the lead resistance.
Status Indicators
The [c]FP-RTD-124 has two green status LEDs, POWER and
READY. After you insert the [c]FP-RTD-124 into a terminal base
or backplane and apply power to the connected network module,
the green POWER indicator lights and the [c]FP-RTD-124
informs the network module of its presence. When the network
module recognizes the [c]FP-RTD-124, it sends initial
configuration information to the [c]FP-RTD-124. After receiving
this initial information, the green READY indicator lights and the
[c]FP-RTD-124 is in normal operating mode.
Isolation and Safety Guidelines
Caution Read the following information before
attempting to connect the [c]FP-RTD-124 to any circuits
that may contain hazardous voltages.
This section describes the isolation of the [c]FP-RTD-124 and its
compliance with international safety standards. The field wiring

FP-RTD-124 and cFP-RTD-124 10 ni.com
connections are isolated from the backplane and the inter-module
communication bus. The isolation is provided by the module,
which has optical and galvanic isolation barriers designed and
tested to protect against transient fault voltages of up to 2,300 Vrms.
Follow these guidelines to ensure a safe total system:
•The [c]FP-RTD-124 has a safety isolation barrier between the
I/O channels and the inter-module communication bus. There
is no isolation between channels unless otherwise noted. If any
of the channels on a module are wired at a hazardous potential,
make sure that all other devices or circuits connected to that
module are properly insulated from human contact.
•Do not share the external supply voltages (the V and C
terminals) with other devices (including other FieldPoint
devices), unless those devices are isolated from human contact.
•For Compact FieldPoint, you must connect the protective earth
(PE) ground terminal on the cFP-BP-xbackplane to the system
safety ground. The backplane PE ground terminal has the
following symbol stamped beside it: . Connect the
backplane PE ground terminal to the system safety ground
using 14 AWG (1.6 mm) wire with a ring lug. Use the 5/16 in.
panhead screw shipped with the backplane to secure the ring
lug to the backplane PE ground terminal.
•As with any hazardous voltage wiring, make sure that all
wiring and connections meet applicable electrical codes and
commonsense practices. Mount terminal bases and backplanes
in an area, position, or cabinet that prevents accidental or
unauthorized access to wiring that carries hazardous voltages.
•Operate the [c]FP-RTD-124 only at or below Pollution
Degree 2. Pollution Degree 2 means that only nonconductive
pollution occurs in most cases. Occasionally, however, a
temporary conductivity caused by condensation must be
expected.
•Refer to the FieldPoint product label for regulatory
certification under hazardous location standards. If the
FieldPoint product is not certified for operation in hazardous
locations, do not operate it in an explosive atmosphere or
where there may be flammable gases or fumes.