National Instruments FieldPoint cFP-RTD-122 User manual

323348B-01 April 2003
©2002–2003 National Instruments Corp. All rights reserved.
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FieldPoint Operating Instructions
FP-RTD-122 and
cFP-RTD-122
Eight-Channel Three-Wire RTD and
Resistance Input Modules
These operating instructions describe how to install and use the
National Instruments FP-RTD-122 and cFP-RTD-122 three-wire
RTD and resistance input modules (referred to inclusively as the
[c]FP-RTD-122). For details on configuring and accessing the
[c]FP-RTD-122 over a network, refer to the user manual for the
FieldPoint network module you are using.
Features
The [c]FP-RTD-122 is a FieldPoint RTD and resistance input
module with the following features:
• Inputs for 100 and 1,000 Ωplatinum RTDs (resistance
temperature detectors)
• Built-in linearization for six TCR (temperature coefficient of
resistance, or alpha) values of RTDs
• Direct resistance measurements in 400 and 4,000 Ωranges
• True three-wire compensation
• 16-bit resolution
• Filtering against 50 and 60 Hz noise
• Hot swappable
• 2,300 Vrms transient overvoltage protection
• –40 to 70 °C operation

FP-RTD-122 and cFP-RTD-122 2 ni.com
Installing the FP-RTD-122
The FP-RTD-122 mounts on a FieldPoint terminal base (FP-TB-x),
which provides operating power to the module. Installing the
FP-RTD-122 onto a powered terminal base does not disrupt the
operation of the bank.
To install the FP-RTD-122, 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-122 module.
2. Align the FP-RTD-122 alignment slots with the guide rails on
the terminal base.
3. Press firmly to seat the FP-RTD-122 on the terminal base.
When the module is firmly seated, the terminal base latch locks
it into place.
Figure 1. Installing the FP-RTD-122
Installing the cFP-RTD-122
The cFP-RTD-122 mounts on a Compact FieldPoint backplane
(cFP-BP-x), which provides operating power to the module.
Installing the cFP-RTD-122 onto a powered backplane does not
disrupt the operation of the bank.
1 I/O Module
2TerminalBase
3 Alignment Slot
4Key
5Latch
6 Guide Rails
1
3
2
4
5
6

© National Instruments Corp. 3 FP-RTD-122 and cFP-RTD-122
To install the cFP-RTD-122, refer to Figure 2 and complete the
following steps:
1. Align the captive screws on the cFP-RTD-122 with the holes
on the backplane. The alignment keys on the cFP-RTD-122
prevent backward insertion.
2. Press firmly to seat the cFP-RTD-122 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-122
1 cFP-RTD-122
2 Captive Screws
3 cFP Controller Module
4 Screw Holes
5 cFP Backplane
2
2
1
3 5
4
4

FP-RTD-122 and cFP-RTD-122 4 ni.com
Wiring the [c]FP-RTD-122
The FP-TB-xterminal bases have connections for each of the eight
input channels on the FP-RTD-122. The cFP-CB-xconnector
blocks provide the same connections for the cFP-RTD-122.
Table 1 lists the terminal assignments for the signals associated
with each channel. The terminal assignments are the same for the
FP-TB-xterminal bases and the cFP-CB-xconnector blocks.
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-122
The [c]FP-RTD-122 has eight input channels. All eight channels
share a common ground reference that is isolated from other
modules in the FieldPoint system. Each channel pulses a 0.25 mA
excitation current out of the EX+ terminal. The excitation current
returns through the COM terminal. The SENSE terminal measures
resistance and compensates 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
EX+ SENSE COM
0 1 2 18
1 3 4 20
2 5 6 22
3 7 8 24
4 9 10 26
5 11 12 28
6 13 14 30
7 15 16 32

© National Instruments Corp. 5 FP-RTD-122 and cFP-RTD-122
Figure 3. [c]FP-RTD-122 Input Circuitry
Taking Measurements from Three-Wire RTDs
Three-wire RTDs often have a wire of one color (usually white,
sometimes red) for positive excitation, and two wires of another
color (usually red, sometimes black). Connect the positive
excitation wire to the EX+ terminal of the module, and connect the
other two wires to the SENSE and COM terminals.
Figure 4. Three-Wire RTD Connections on One Channel
Taking Measurements from Four-Wire RTDs
For the best accuracy, use the NI [c]FP-RTD-124 for input from
four-wire RTDs. Otherwise, leave any one of the RTD wires
unconnected and connect the remaining three as you would for a
three-wire RTD. Refer to Figure 5.
RTD
[c]FP-RTD-122
Pulsed
16-bit
ADC
Input
Circuitry
EX+
SENSE
COM
3-wire RTD
[c]FP-RTD-122
EX+
SENSE
COM

FP-RTD-122 and cFP-RTD-122 6 ni.com
Figure 5. Four-Wire RTD Connections on One Channel
Taking Measurements from Two-Wire RTDs
Connect either wire of a two-wire RTD to the EX+ terminal and the
other wire to the COM terminal, and connect a short jumper wire
between the COM and SENSE terminals.
Figure 6. Two-Wire RTD Connections on One Channel
Measuring Resistance Directly
You can use the [c]FP-RTD-122 to measure resistance in ohms.
In this way, you can take measurements from RTDs of types that
the [c]FP-RTD-122 does not directly support (such as 120 Ωnickel
RTDs) and from resistive devices other than RTDs. You can choose
one of two resistance ranges: 0–400 and 0–4,000 Ω. Resistance
values outside the range you select, including open circuits, result
in an Out of range error for the affected channels. The
[c]FP-RTD-122 ignores any configuration of RTD type for
channels with resistance ranges selected.
4-wire RTD
[c]FP-RTD-122
Leave this wire
unconnected
EX+
SENSE
COM
2-wire RTD
EX+
SENSE
COM
[c]FP-RTD-122
Short Jumper Wire

© National Instruments Corp. 7 FP-RTD-122 and cFP-RTD-122
Converting Resistance Measurements
to Temperature Measurements
The [c]FP-RTD-122 has built-in linearization algorithms for
platinum RTDs of either 100 or 1,000 Ωnominal resistance,
and for six 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-122 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-122 for the RTD type 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-122 can directly
measure the temperature of platinum RTDs of either 100 or
1,000 Ωnominal resistance. These RTDs are commonly referred to
as PT100 or PT1000 RTDs, respectively.
Different types of platinum RTDs have different TCRs. The
[c]FP-RTD-122 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.
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-122 are
appropriate for nearly all applications.

FP-RTD-122 and cFP-RTD-122 8 ni.com
The [c]FP-RTD-122 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, 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-122 supports. If you have a
nonstandard RTD that does not match one of these linearization
curves, measure the resistance with the [c]FP-RTD-122 and
convert the resistance to temperature in the manner suggested by
the RTD vendor.
Three-Wire Compensation of Lead
Resistance Errors
The [c]FP-RTD-122 uses a three-wire compensation technique to
compensate for the lead resistances. The SENSE lead measures the
resistance of the return COM lead. If the EX+ lead has the same
resistance as the COM lead, the [c]FP-RTD-122 corrects for the
Table 2. Callendar-Van Dusen Coefficients Used by the [c]FP-RTD-122
TCR
mΩ/Ω/°C
A
(°C)–1
B
(°C)–2
C
(°C)–4
3.750a3.81 ×10–3 –6.02 ×10–7 –6.0 ×10–12
3.851b3.9083 ×10–3 –5.775 ×10–7 –4.183 ×10–12
3.911c3.9692 ×10–3 –5.8495 ×10–7 –4.233 ×10–12
3.916d3.9739 ×10–3 –5.870 ×10–7 –4.4 ×10–12
3.920e3.9787 ×10–3 –5.8686 ×10–7 –4.167 ×10–12
3.928f3.9888 ×10–3 –5.915 ×10–7 –3.85 ×10–12

© National Instruments Corp. 9 FP-RTD-122 and cFP-RTD-122
effects of the leads. The only residual errors are those caused by
mismatching the EX+ and COM leads. Most RTDs have lead
resistances within 5% of each other, so the compensation of the
[c]FP-RTD-122 corrects for 95% or more of the errors introduced
by lead resistances. This is a more accurate method than the typical
bridge completion methods described in many reference books.
The bridge methods not only have the same sensitivity to lead
resistance mismatch, but also are effective only for temperatures
very near those at which the bridge is balanced (usually 0 °C).
The temperature measurement accuracy specifications for the
[c]FP-RTD-122 at the end of these instructions include the effects
of a typical application using 10 m of 22 gauge copper wire
(approximately 0.5 Ωper lead), with 5% mismatch in the lead
resistances. If you are using leads with greater resistances, the
additional errors are approximately 3 °C per Ωof mismatch in the
lead resistances for 100 ΩRTDs, and 0.3 °C per Ωof mismatch in
the lead resistance for 1,000 ΩRTDs. For example, for 2 Ωleads
matched to 5% of each other, the lead resistance mismatch is
5% ×2 Ω= 0.1 Ω, which would cause 0.3 °C of error in
measurements of a 100 ΩRTD.
If you are using the [c]FP-RTD-122 with two-wire RTDs, the
errors due to lead resistances are much greater because three-wire
compensation is not used. With two-wire RTDs, the additional
errors are approximately 3 °C per Ωof the sum of the lead
resistances for 100 ΩRTDs, and 0.3 °C per Ωof the sum of lead
resistances for 1,000 ΩRTDs. For example, a 1,000 Ωtwo-wire
RTD with 2 Ωleads has a total lead resistance of 4 Ω(2 Ωper lead),
which causes 1.2 °C of error.
Status Indicators
The [c]FP-RTD-122 has two green status LEDs, POWER and
READY. After you insert the [c]FP-RTD-122 into a terminal base
or backplane and apply power to the connected network module,
the green POWER indicator lights and the [c]FP-RTD-122
informs the network module of its presence. When the network
module recognizes the [c]FP-RTD-122, it sends initial
configuration information to the [c]FP-RTD-122. After the
[c]FP-AI-111 receives this initial information, the green READY
indicator lights and the module is in normal operating mode.

FP-RTD-122 and cFP-RTD-122 10 ni.com
Upgrading the FieldPoint Firmware
You may need to upgrade the FieldPoint firmware when you add
new I/O modules to the FieldPoint system. For information on
determining which firmware you need and how to upgrade your
firmware, go to ni.com/info and enter fpmatrix.
Isolation and Safety Guidelines
Caution Read the following information before
attempting to connect the [c]FP-RTD-122 to any circuits
that may contain hazardous voltages.
This section describes the isolation of the [c]FP-RTD-122 and its
compliance with international safety standards. The field wiring
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-122 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.
•Donot 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.