National Instruments CFP-TC-120 Manuale utente

373344A-01 October 2002

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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

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

2 1

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

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

+0.7 V

10 MΩ

Filter 16-Bit

Isolated ADC

[c]FP-TC-120

Millivolt

Source

[c]FP-TC-120

IN(–)

IN(+)

–

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

To Next

Channel

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.

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

0.5

1.5

2.5

3.5

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)

0.5

1.5

2.5

3.5

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)

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

0.5

1.5

2.5

3.5

4.5

0 300 600 900

Measured Temperature (°C)

Error (°C)

1200 1500 1800

Max Over Temp

Max at Room Temp

Typ Over Temp

Typ at Room Temp

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