National Instruments CFP-TC-120 User manual

373344A-01 October 2002
©2002 National Instruments Corp. All rights reserved.
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FieldPoint Operating Instructions
FP-TC-120 AND CFP-TC-120
8-Channel Thermocouple Input Modules
These operating instructions describe how to install and use the
National Instruments FP-TC-120 and cFP-TC-120 thermocouple
input modules (referred to inclusively as the [c]FP-TC-120). For
details on configuring and accessing the [c]FP-TC-120 over a
network, refer to the user manual for the FieldPoint network
module you are using.
Features
The [c]FP-TC-120 is a FieldPoint thermocouple input module with
the following features:
• Eight thermocouple or millivolt inputs
• Built-in linearization and cold-junction compensation for eight
thermocouple types: J, K, R, S, T, N, E, and B
• Four voltage ranges: ±25, ±50, ±100, and – 20 to 80 mV
• Open-thermocouple detection and indicator LEDs
• 16-bit resolution
• Differential inputs
• Filtering against 50 and 60 Hz noise
• 2,300 Vrms transient overvoltage protection between the
inter-module communication bus and the I/O channels
•250V
rms isolation voltage rating
• –40 to 70 °C operation
• Hot plug-and-play

FP-TC-120 and cFP-TC-120 2 ni.com
Installing the FP-TC-120
The FP-TC-120 mounts on a FieldPoint terminal base (FP-TB-x).
Hot plug-and-play enables you to install the FP-TC-120 onto a
powered terminal base without disturbing the operation of other
modules or terminal bases. The FP-TC-120 receives operating
power from the terminal base.
To install the FP-TC-120, refer to Figure 1 and follow these steps:
1. Slide the terminal base key to either position X (for any
module) or position 1 (for the FP-TC-120).
2. Align the FP-TC-120 alignment slots with the guide rails on
the terminal base.
3. Press firmly to seat the FP-TC-120 on the terminal base.
The latch on the terminal base locks the FP-TC-120 into place
when it is firmly seated.
Figure 1. Installing the FP-TC-120
Installing the cFP-TC-120
The cFP-TC-120 mounts on a FieldPoint backplane (cFP-BP-x).
Hot plug-and-play enables you to install the cFP-TC-120 onto a
powered backplane without disturbing the operation of other
modules or connector blocks. The cFP-TC-120 receives operating
power from the backplane.
To install the cFP-TC-120, refer to Figure 2 and follow these steps:
1. Align the captive screws on the cFP-TC-120 with the holes on
the backplane. The alignment keys on the cFP-TC-120 prevent
backward insertion.
Alignment
Slot
Key
Latch
Guide Rails
Terminal BaseI/O Module

© National Instruments Corp. 3 FP-TC-120 and cFP-TC-120
2. Press firmly to seat the cFP-TC-120 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-TC-120
Wiring the [c]FP-TC-120
The FP-TB-xterminal bases have connections for each of the eight
differential input channels on the FP-TC-120 and a common
(COM) terminal for connecting a shield to each channel. The
cFP-CB-xconnector blocks provide the same connections for the
cFP-TC-120.
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.
1 cFP Backplane
2 cFP Controller Module
3cFP-TC-120
4 Captive Screws
5 Screw Holes
4
4
3
2 1
5
5

FP-TC-120 and cFP-TC-120 4 ni.com
Note On the FP-TB-3 terminal base and the cFP-CB-3
connector block, use the C terminals for COM
connections.
The COM terminals of all the channels are internally connected
and are connected to the terminals labeled C on the terminal base.
You do not need to connect a power source to the V or C terminals
of the terminal base or connector block. The C and COM terminals
are internally connected to the isolated ground reference of the
module and to any shield connections you make, so connecting a
power supply to one of these terminals may create a ground loop
and cause measurements to be misreferenced. Connecting the C
and COM terminals of the [c]FP-TC-120 to the V, C, or COM
terminals of another FieldPoint module defeats the isolation
barriers between the two modules.
Taking Measurements
with the [c]FP-TC-120
The [c]FP-TC-120 has eight differential input channels. All eight
channels share a common ground reference that is isolated from
other modules in the FieldPoint system. Each of the input channels
has a bias resistor to this isolated ground reference on the negative
terminal, IN(–), and a pull-up resistor on the positive terminal,
IN(+), to detect open thermocouples. Each channel has a COM
terminal that connects to the isolated ground reference. Each
channel is filtered, then sampled by a 16-bit analog-to-digital
Table 1. Terminal Assignments
Channel
Terminal Numbers
IN(+) IN(–)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-TC-120 and cFP-TC-120
converter (ADC). Figure 3 shows the input circuitry on one
channel.
Figure 3. [c]FP-TC-120 Analog Input Circuitry on One Channel
Connecting Voltage Input Signals
Connect the positive lead of a millivolt signal to the IN(+) terminal
and the negative lead to the IN(–) terminal. If you are using
shielded wiring, connect one end of the shield to the COM
terminal. Figure 4 shows a millivolt source connected to one
channel of the [c]FP-TC-120.
Figure 4. Connecting a Millivolt Source to the [c]FP-TC-120
The input ranges for the voltage inputs are ±25, ±50, ±100, and
–20 to 80 mV. An input signal outside the selected input range
causes the [c]FP-TC-120 to report an Out of range error for the
affected channel. The [c]FP-TC-120 ignores any configuration of
thermocouple type when you select one of these ranges.
IN(+)
IN(–)
COM
C
+0.7 V
10 MΩ
10 MΩ
Filter 16-Bit
Isolated ADC
[c]FP-TC-120
Millivolt
Source
[c]FP-TC-120
IN(–)
IN(+)
CV
+
–

FP-TC-120 and cFP-TC-120 6 ni.com
Connecting Thermocouple Input Signals
Connect the positive lead of a thermocouple to the IN(+) terminal
and the negative lead to the IN(–) terminal. If you are using
shielded wiring, connect one end of the shield to the COM
terminal. Thermocouple wires are color coded. The color code
depends on the thermocouple type and the country of manufacture.
If you are unsure which of the thermocouple leads is the positive
and which is the negative, check the thermocouple documentation
or the thermocouple wire spool. The [c]FP-TC-120 supports
thermocouple types J, K, R, S, T, N, E, and B. Figure 5 shows
a shielded thermocouple connected to one channel of the
[c]FP-TC-120.
Figure 5. Connecting a Shielded Thermocouple to the [c]FP-TC-120
The [c]FP-TC-120 linearizes the thermocouple voltage and returns
a reading in units of temperature. The available ranges are
0 to 2,048 K, –270 to 1,770 °C, and – 454 to 3,218 °F.
Temperatures outside these ranges result in an Out of range
error for the affected channels.
Note You must configure each channel of the
[c]FP-TC-120 for the type of thermocouple connected
to it.
The algorithms in the [c]FP-TC-120 linearize measurements
according to the National Institute of Standards and Technology
NIST-175 standard for thermocouple characteristics, based on the
ITS-90 International Temperature Scale. These linearization
algorithms are typically accurate to within ±0.05 °C (0.03 °F) of
the NIST standard over the entire range of temperatures defined
by the NIST for the supported thermocouple types.
Thermocouple
[c]FP-TC-120
IN(–)
IN(+)
Shield
CV
To Next
Channel

© National Instruments Corp. 7 FP-TC-120 and cFP-TC-120
Detecting Open Thermocouples
The [c]FP-TC-120 can detect open thermocouples. When you
select a temperature range for a channel, open-thermocouple
detection for that channel is enabled. When the [c]FP-TC-120
detects an open thermocouple, it reports an Open thermocouple
error for that channel and lights the corresponding red LED.
Open-thermocouple detection works in part by using the resistors
shown in Figure 3. This circuit produces a 35 nA input current at
IN(–), a 35 nA output current at IN(+), and an input impedance of
20 MΩ. The open-thermocouple detection circuitry normally
results in negligible errors when used with thermocouples. Other
voltage sources with higher source impedance can introduce more
significant errors. The following gain and offset errors result from
this circuit:
• Offset error: 0.035 µV per ohm of source resistance
• Gain error: 0.05 ppm (parts per million) per ohm of source
resistance
Thus, a voltage source with a source impedance of 1 kΩhas
an additional offset error of 35 µV and an additional gain error
of 50 ppm.
Cold-Junction Compensation
Connecting thermocouple leads to the terminal base or connector
block creates new thermocouple junctions between the
thermocouple wires and the terminals. These cold junctions affect
the measured readings of the thermocouple. The [c]FP-TC-120
automatically enables cold-junction compensation when you select
a temperature range but not when you select a voltage range.
The [c]FP-TC-120 reads the temperature of the terminals from
temperature-measuring elements built into all FieldPoint terminal
bases and connector blocks. The module uses this temperature data
to compensate for cold junctions and reports it as
CJ Temperature in software. If the FP-TC-120 senses a
temperature of less than –50 °C or more than 85 °C, the
FP-TC-120 reports Out of range for the cold-junction
temperature and for all of the channel temperatures.
For optimal cold-junction compensation, NI recommends that you
use the FP-TB-3 isothermal terminal base with the FP-TC-120 and
the cFP-CB-3 isothermal connector block with the cFP-TC-120.

FP-TC-120 and cFP-TC-120 8 ni.com
Alternative Cold-Junction Compensation
with the cFP-TC-120
If you are using an external terminal block or a cable backshell to
connect to the cFP-TC-120, you can compensate for cold junctions
by connecting a thermistor between pins 36 and 37. You must use
a thermistor with a resistance of 5 kΩat 25 °C. Refer to the
Specifications section for information about the accuracy of the
thermistor in the cFP-CB-xconnector block.
Software Cold-Junction Compensation
with the [c]FP-TC-120
The default setting is for the [c]FP-TC-120 to enable cold-junction
compensation for all temperature measurements. To disable this
feature and use a software algorithm instead, set the CJC Source
attribute of the CJ Temperature channel to either 0 or 25 °C.
Selecting one of these settings causes the [c]FP-TC-120 to assume
a cold-junction temperature of either 0 or 25 °C for all temperature
readings. Regardless of the setting of the CJC Source attribute, CJ
Temperature continues to reflect the actual measured temperature
of the terminal base or connector block. This method of software
cold-junction compensation is sufficient for applications in which
temperatures are fairly stable and uniform across the system.
Determining Measurement Accuracy
and Minimizing Errors
It is difficult to define temperature measurement errors in simple
terms because thermocouples are very nonlinear. The errors
depend in part on the thermocouple type, the cold-junction
temperature, the temperature being measured, and the accuracy
of the thermocouple. Also, it is important to distinguish between
absolute accuracy and resolution. Absolute accuracy (referred
to in this document as accuracy) is a measure of how far off a
measurement of the [c]FP-TC-120 is from the correct value.
Absolute accuracy includes all gain and offset errors, differential
and integral nonlinearity, quantization errors, noise errors, errors
in linearization algorithms, and errors in measurement of the
cold-junction temperature. Resolution refers to the smallest
measurable change in a value.

© National Instruments Corp. 9 FP-TC-120 and cFP-TC-120
The following charts show the typical and maximum errors for the
different thermocouple types, both with the [c]FP-TC-120 at room
temperature (15 to 35 °C) and over the full temperature range
(–40 to 70 °C). These charts allow for a 0.2 °C temperature
gradient across the terminal base or connector block and do not
include the accuracy of the thermocouple itself.
Figure 6. Type R and S Errors
Figure 7. Type J, K, N, T, and E Errors
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 300 600 900 1200 1500 1800–300
Max Over Temp
Max at Room Temp
Typ Over Temp
Typ at Room Temp
Measured Temperature (°C)
Error (°C)
0
0.5
1
1.5
2
2.5
3
3.5
4.5
5
0 300 600 900 1200 1500 1800–300
4
Max Over Temp
Max at Room Temp
Typ Over Temp
Typ at Room Temp
Measured Temperature (°C)
Error (°C)

FP-TC-120 and cFP-TC-120 10 ni.com
Figure 8. Type B Errors
Cold-Junction Temperature
Measurement Accuracy
Heat dissipated by adjacent modules (or other nearby heat sources)
can cause errors in thermocouple measurements by heating up the
terminals so that they are at a different temperature than the sensor
used to measure the cold junction. The thermal gradient generated
across the terminals can cause the terminals of different channels
to be at different temperatures, so the resulting measurement
creates errors not only in absolute accuracy but also in the relative
accuracy between channels. The accuracy specifications for the
FP-TC-120 include the errors caused by a 0.2 °C (0.36 °F)
gradient. The actual gradient you can expect to encounter depends
on the terminal base or connector block you use and the details of
your installation. The following sections provide guidelines for
estimating and minimizing thermal gradients when using the
FP-TB-xterminal bases. If you are using the cFP-CB-xconnector
block, referto the cFP-CB-xCompact FieldPointConnector
Blocks Operating Instructions.
Estimating Thermal Gradients across the FP-TB-3
Isothermal Terminal Base
The FP-TB-3 is designed with isothermal construction to keep the
terminals at the same temperature. NI recommends that you use the
FP-TB-3 to ensure the best accuracy of thermocouple
measurements. Adjacent FieldPoint modules (either network
0
0.5
1
1.5
2
2.5
3
3.5
4.5
5
0 300 600 900
Measured Temperature (°C)
Error (°C)
1200 1500 1800
4
Max Over Temp
Max at Room Temp
Typ Over Temp
Typ at Room Temp