National Instruments VXI-TB-1303 User manual

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names of their respective companies.
321567A-01 ©Copyright 1997 National Instruments Corp. All rights reserved. April 1997
VXI-TB-1303 32-CHANNEL ISOTHERMAL
TERMINAL BLOCK
This guide describes how to install and use the VXI-TB-1303 terminal
block with a VXI-SC submodule.
Introduction
The VXI-TB-1303 terminal block is a shielded board with screw
terminals that connect to a VXI-SC submodule. The VXI-TB-1303 has
a high-accuracy thermistor cold-junction temperature sensor, and an
isothermal copper plane to minimize the temperature gradients across
the screw terminals when you measure with thermocouples.
The terminal block has 80 screw terminals for easy connection.
Thirty-two pairs of screw terminals connect to the 32 differential inputs
of the VXI-SC submodule. Four terminals labeled GND connect to the
submodule’s chassis ground pins. The AIREF, AOREF, GUARD,
OUTPUT, OUT0+, OUT0-, OUT1+, OUT1-, OUT2+, OUT2-, OUT3+,
and OUT3- terminals are reserved for use with future VXI-SC
submodules.
The VXI-TB-1303 terminal block has a pull-up resistor connected
between CH+ and +5 V and a ground-reference resistor connected
between CH- and chassis ground. This pull-up resistor helps you detect
open thermocouples by detecting saturation of the VXI-SC submodule
amplifier output. The ground-reference resistor references floating
thermocouples to ground.

2
What You Need to Get Started
To set up and use your VXI-TB-1303, you will need the following:
❑ VXI-TB-1303 terminal block
❑ VXI-TB-1303 32-Channel Isothermal Terminal Block Installation
Guide
❑ One package of four 10 Ωresistor networks
❑ VXI-TB-1000 terminal board carrier and documentation
❑ VXI-SC-1102 or other VXI-SC submodule(s) and documentation
❑ Long-nose pliers
Signal Connection and Installation
Refer to the VXI-TB-1000 Terminal Board Carrier Installation Guide for
instructions on connecting your signals and installing the VXI-TB-1303
terminal block.
Warning: Do not connect hazardous voltage levels (
±
±±
± 42 VAC peak or VDC) to this
product.
Figure 1 shows the VXI-TB-1303 terminal block parts locator diagram.

3
Solder Side
Component Side
3
4
1
2
1 Assembly Number 3 Product Name
2 Pin 1 of RP2 (see note below) 4 Serial Number
Figure 1. VXI-TB-1303 Parts Locator Diagram
Note: Notice that pin 1 is in the same location on each resistor network socket,
and is represented by a square, in Figure 1.

4
Temperature Sensor and Switch Configuration
To enable you to use thermocouples with VXI-SC submodules, the
VXI-TB-1303 terminal block has a thermistor temperature sensor for
cold-junction compensation.
You can connect the temperature sensor to a VXI-SC submodule in either
of two ways:
• Multiplexed Temperature Sensor (MTEMP) mode—Set the
VXI-TB-1303 terminal block switch S1 to the MTEMP position.
This is the factory-default setting.
• Direct Temperature Sensor (DTEMP) mode—Set the VXI-TB-1303
terminal block switch S1 to the DTEMP position. This mode
connects the temperature sensor to a separate DAQ channel via
your VXI-SC submodule. Refer to your VXI-SC submodule
documentation to configure your VXI-SC submodule for DTEMP
mode.
Table 1 shows the terminal block switch settings.
Table 1. Switch S1 Settings
Switch S1 Position Description
MTEMP
DTEMP
MTEMP mode selected; factory-default
setting; preferred mode and parking
position
MTEMP
DTEMP
DTEMP mode selected; connect to a
separate DAQ channel
Note: On the VXI-SC-1102 submodule, the MTEMP and DTEMP modes are
equivalent.

5
Configuring the Resistor Networks
The 10 MΩground-reference networks are recommended for use with
the VXI-SC-1102 and are factory installed.
The VXI-TB-1303 terminal block has a pull-up resistor connected
between CH+ and +5 V and has a ground-reference resistor connected
between CH- and chassis ground. Figure 2 shows how the pull-up and
ground-reference resistors are connected to the CH± inputs.
CH+
CH-
CH+
CH- VXI-SC Module
Rpull-up
(RP2, RP3, RP7, RP5)
(in sockets)
Rground-reference
(RP1, RP4, RP8, RP6)
(in sockets)
Screw Terminals
+5 V
Figure 2. Resistor Connections
Table 2 shows the relationship between the channel input signals and the
resistor networks.
Table 2. Channel Input Signals and Resistor Networks
Channel Pull-up Resistor
Network Ground-Reference Resistor Network
0–7 RP2 RP1
8–15 RP3 RP4
16–23 RP7 RP8
24–31 RP5 RP6

6
Table 3 shows which resistor networks to use for your VXI-SC
submodule, signal type, and application.
Table 3. Selecting the Appropriate Resistor Networks
Ground-
Reference
Resistor
Pull-up
Resistor Source Impedance Signal (Floating or Ground-Referenced) Open Thermocoup
le Detection?
Comments
10 M Ω 10 M Ω Low Both Yes Recommended
configuration for the
VXI-SC-1102. Factory-
shipping configuration.
10 Ω10 M Ω Low Floating Yes —
10 ΩNone High or low Floating No —
None None High or low Ground-referenced No —
Low source impedance ≤50
Ω
High
source impedance >50
Ω
A package of 10 Ωground-reference resistor networks is included in the
VXI-TB-1303 kit. These ground-reference resistor networks may be
recommended for future VXI-SC submodules but are not recommended
for the VXI-SC-1102 submodule. You can install them as RP2, RP3,
RP7, and RP5.
Warning: Connecting an external ground-referenced signal with the 10
Ω
ΩΩ
Ω
ground-
reference resistor network in place may cause permanent damage to the
resistor network and the traces on the VXI-TB-1303 printed circuit
board. National Instruments isNOT liable for any damage or injuries
resulting from improper signal connections.
Changing Resistor Networks
Use long-nose pliers to remove or replace the resistor networks in the
sockets; be careful not to damage the network package. Make sure pin 1
of each network is in the correct position in the socket. Refer to Figure 1
for the pin 1 location for each resistor network socket. Notice that pin 1
is in the same location on each resistor network socket, and each one is
represented by a square in the drawing.
Each resistor network is labeled with descriptive numbers on the
left front side, and pin 1 is located directly beneath the darkened
symbol within these numbers. The 10 Ω resistor network is
labeled 100 (10 x 100Ω); the 10 MΩresistor network is labeled
106 (10 x 106Ω). Figure 3 shows examples of these resistors.

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Mfr. code
10x-1-100
Pin 1
a. 10 ΩResistor Network
Mfr. code
10x-1-106
Pin 1
b. 10 MΩResistor Network
Figure 3. Resistor Networks
Open-Thermocouple Detection
The VXI-TB-1303 circuitry helps you detect an open thermocouple.
To detect whether any thermocouple is open, check whether the
corresponding VXI-SC submodule channel is saturated. The
VXI-TB-1303 has pull-up and ground-reference resistors that saturate
the channel by applying +5 V at the input of the open channel. This
will cause saturation to either of the positive or negative rails.
VXI-SC-1102 Submodule
With the 10 MΩground-reference resistor networks, it does not matter
whether your signal is ground-referenced or floating. The channels with
open thermocouples will saturate at all sample rates of the submodule.
Errors Due to Open-Thermocouple Detection Circuitry
Open-thermocouple detection circuitry can cause two types of
measurement errors. These errors are caused by common-mode voltage
at the input of the VXI-SC submodule and current leakage into your
signal leads.
Common-Mode Voltage at the Input of the VXI-SC
Submodule
With 10 MΩpull-up and ground-reference resistor networks, a
common-mode voltage of 2.5 VDC will develop if the thermocouple
is floating. At a gain of 100, the common-mode rejection of the
VXI-SC-1102 submodule is sufficiently high that the resulting offset
voltage is negligible.

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However, if your application demands extremely high accuracy, you
can eliminate this offset error by calibrating your system. You can also
remove the pull-up resistor network, giving up the open-thermocouple
detection feature in the process, or use the 10 Ωground-reference resistor
networks, which will bring the common-mode voltage down to nearly
0 VDC.
Current Leakage
The open-thermocouple detection circuitry causes a small current leakage
into the thermocouple. The magnitude of this current depends on whether
your thermocouple is grounded. The following calculation uses a
20-ft 24 AWG J-type thermocouple as an example. This example
thermocouple has a resistance of 8.78 Ω per lead, based on the equation:
Rlead = 0.439 Ω/ft x 20 ft = 8.78 Ω per lead
If your thermocouple is floating, as shown in Figure 4, a leakage current
of approximately 0.25 µA (5 V /20 MΩ) will flow through both leads of
your thermocouple. The resultant measurement error will be 4.4 µV for
the example thermocouple, based on the equation:
2 leads x 8.78 Ω/lead x 0.25 µA = 4.4 µV
This corresponds to an error of 0.09° C.
2.2 µV 0.25 µA
2.2 µV
10 MΩ
10 MΩ
+5 V
+
-
-+
+- -
+
Vthermocouple Vmeas
Rlead
Rlead
Figure 4. Floating Thermocouple

9
If your thermocouple is grounded on the negative lead, as shown in
Figure 5, or grounded on the positive lead, as shown in Figure 6, a
leakage current of approximately 0.5 µA (5 V/10 MΩ) will flow through
your thermocouple. The resultant measurement error will be 4.4 µV for
the example thermocouple, based on the equation:
1 lead x 8.78 Ω/lead x 0.5 µA = 4.4 µV
This corresponds to an error of 0.09° C.
4.39 µV 0.5 µA10 MΩ
10 MΩ
+5 V
+
-
-+
-
+
Vthermocouple Vmeas
Rlead
Rlead
Figure 5. Thermocouple with Negative Lead Grounded
4.39 µV 0.5 µA10 MΩ
10 MΩ
+5 V
+
-
-+
-
+
V
thermocouple
V
meas
R
lead
R
lead
Figure 6. Thermocouple with Positive Lead Grounded

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You can make similar calculations when you use a 10 Ω
ground-reference resistor instead of a 10 MΩresistor.
If your application demands very high accuracy, you can eliminate
this error by removing the appropriate pull-up resistor network or by
calibrating the system offset.
Temperature Sensor Output and Accuracy
The VXI-TB-1303 temperature sensor voltage output varies from 1.91 to
0.58 V over the temperature range, as shown in Table 4.
Table 4. Temperature Sensor Voltage Output Accuracy
Temperature Range Voltage Output Accuracy*
0° to 15° C ±1° C
15° to 35° C ±0.65° C
35° to 55° C ±1° C
* Includes the combined effects of the temperature sensor accuracy and the
temperature difference between the temperature sensor and any screw terminal. The
temperature sensor accuracy includes tolerances in all component values, the effects
caused by temperature and loading, and self-heating.
To select and read the temperature sensor, refer to your LabVIEW,
LabWindows/CVI, NI-DAQ, or other software documentation for
programming information.
Alternatively, you can use the following formulas to convert the
cold-junction sensor voltage to cold-junction temperature:
T(°C) = TK- 273.15
where TKis the temperature in kelvin
a = 1.295361 x 10-3
b = 2.343159 x 10-4
c = 1.018703 x 10-7
RT= resistance of the thermistor in Ω
R5,000 V
2.5 V
TTEMPOUT
TEMPOUT
=−





Ω
VTEMPOUT = output voltage of the temperature sensor
T( F) = [T( C)] +
°°9
532