National Instruments TBX-68S User manual

INSTALLATION GUIDE
TBX-68S Isothermal Terminal Block
This installation guide describes how to install and connect signals to the
TBX-68S isothermal terminal block for use with the NI 2501 and NI 2503
PXI switch cards.
Contents
Introduction............................................................................................. 1
What You Need to Get Started ............................................................... 2
Signal Connection ................................................................................... 2
Installing Your Terminal Block .............................................................. 3
Rack-Mounting the TBX-68S................................................................. 5
Cold-Junction Temperature Sensor......................................................... 5
Specifications .......................................................................................... 7
Introduction
The TBX-68S is an isothermal DIN-rail mountable terminal block that
consists of a shielded board with screw terminals that connect directly to
the front panel I/O connector of the NI 2501 or NI 2503.
The terminals blocks can easily accommodate thermocouples, resistance
temperature detectors (RTDs), thermistors, and voltage signals. The
terminal blocks feature isothermal construction to minimize the
temperature gradients across the screw terminals and a high-accuracy
thermistor cold-junction temperature sensor for measuring with
thermocouples. Enclosures keep out air currents to maintain an isothermal
environment for the screw terminals and the cold-junction sensor. The
TBX-68S mounts on most European standard DIN EN mounting rails.
The TBX-68S provides screw terminal connections to all channels except
the cold-junction sensor.
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TBX-68S Isothermal Terminal Block 2 ni.com
What You Need to Get Started
You need the following to set up and use your terminal block:
❑One of the following PXI switch cards:
– NI 2501
– NI 2503
❑NI 2501/2503 User Manual
❑TBX-68S Isothermal Terminal Block Installation Guide
❑TBX-68S isothermal terminal block
❑One of the following cable assemblies:
– SH6868S shielded cable (recommended)
– R6868 ribbon cable
❑1/8 in. flathead screwdriver
❑No. 1 Phillips-head screwdriver
❑Wire cutters
❑Wire insulation strippers
Signal Connection
See your NI 2501/2503 User Manual for examples of how to connect your
signals. Refer to Figure 1 as you perform the following steps to connect
your signals to your terminal block:
1. Remove the terminal block cover by unscrewing the four cover screws
in the cover corners using the Phillips-head screwdriver.
2. Use wire cutters and wire insulation strippers to strip the wire ends as
necessary to connect them to screw terminals.
3. Loosen the screws in the screw terminals with a 1/8 in. flathead
screwdriver.
4. Insert the stripped wires into the screw terminals. Tighten the screws
with the 1/8 in. flathead screwdriver.
5. Allow your signal wires to exit through the terminal block cover
opening.
Note The TBX-68S terminal block does not provide strain relief for signal wires. Add
strain relief and insulation for your signal wires, if necessary.

©National Instruments Corporation 3 TBX-68S Isothermal Terminal Block
6. Replace the terminal block cover and tighten the cover screws.
Figure 1. TBX-68S Parts Locator Diagram
Installing Your Terminal Block
Note To minimize the temperature gradient inside the terminal block and thus maintain its
isothermal nature for accurate cold-junction compensation, place the terminal block away
from extreme temperature differentials.
Refer to Figure 2 as you perform the following steps to connect the terminal
block to the NI 2501 or NI 2503 connector:
1. Attach the connector on one end of the cable to the NI 2501 or NI 2503
connector. The two connectors should snap together in place.
2. Attach the connector on the other end of the cable to the terminal block
connector. If you use the SH6868S cable, the two connectors should
snap together. The R6868 ribbon cable has no latches and should
simply join together without a snap.
1CoverScrews
2Cover
3CoverOpening
4 Signal Wires
5 Screw Terminals
2
3
5
1
4

TBX-68S Isothermal Terminal Block 4 ni.com
To disconnect the cable, push the two ejector ears on the backshell of the
cable and gently pull on the backshell. Do not pull the cable.
Caution The connectors of both the switch card and the cable are polarized. You can attach
them in only one way. Do not force the cable when inserting it into or removing it from the
NI 2501 or NI 2503 connector.
Figure 2. Connecting the TBX-68S Cable Assembly
1 NI 2501 or NI 2503
2 SH6868S Shielded Cable (shown)
or R6868 Ribbon Cable
3 Signal Wires
4 TBX-68S
5EjectorEars
1
2
4
3
5

©National Instruments Corporation 5 TBX-68S Isothermal Terminal Block
Rack-Mounting the TBX-68S
When you have completed signal connections and terminal block
installation, you can mount the TBX-68S assembly into your rack. If you
are using the National Instruments TBX Rack-Mount Assembly, refer to
the TBX Rack-Mount Installation Guide for instructions.
If you are not using this rack-mount assembly, mount the TBX-68S directly
by snapping its base onto the DIN rail with a firm push.
To remove the TBX-68S from the DIN rail, place a flathead screwdriver
into the slot above the terminal block base and pry it away from the rail.
Cold-Junction Temperature Sensor
The TBX-68S temperature sensor voltage output varies from 198.54 mV to
19.58 mV over the temperature range 0° to 55° C, respectively, and has an
accuracy of ±0.5° C over the 15° to 35° C temperature range and ±0.9° C
over the 0° to 15° and 35° to 55° C temperature ranges.1
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
VTEMPOUT = output voltage of the temperature sensor
1 Includes the combined effects of the temperature sensor accuracy and 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.
T
K
1
ab R
T
ln() cR
T
ln()
3
++[
]
-------------------------------------------------------------
--
=
R
T
189 KV
TEMPOUT
2.5 V
TEMPOUT
–
----------------------------------
⎝⎠
⎛⎞
=
T ° F () [T(°
C )]9
5
------------------------32+=

TBX-68S Isothermal Terminal Block 6 ni.com
where T(°F) and T(°C) are the temperature readings in degrees Fahrenheit
and degrees Celsius, respectively.
The thermistor resistance varies from 16,305 Ωto 1,492 Ωover a 0° to
55° C temperature range.
Note VTEMPOUT varies from 198.54 mV (at 0° C) to 19.58 mV (at 55° C). For best
resolution, use the maximum gain for this signal range on the analog input channel of your
measurement device.
The 200 mV range is designed to eliminate the necessity of changing a measurement
device’s signal range to measure the cold-junction sensor while scanning thermocouples.
Use an average of a large number of samples to obtain the most accurate reading. Noisy
environments require more samples for greater accuracy.
Figure 3 shows the circuit diagram of the TBX-68S cold-junction
temperature sensor.
Figure 3. Temperature Sensor Circuit Diagram
5 kΩ
at 25˚ C
189 kΩ
0.1%
LM 4040
2.5 V
0.1%
4.7 kΩ
1%
2.5 V
10 μF
16 V 0.1 μF
0.1 μF
1
2
1
2
2
CJSense –
CJSense +
+5 V
+
–†˚

©National Instruments Corporation 7 TBX-68S Isothermal Terminal Block
Specifications
Cold-junction sensor
Accuracy1........................................ 0.5° from 15° to 35° C
0.9° from 0° to 15° C
and 35° to 55° C
Repeatability ................................... 0.2° from 15° to 35° C
Output ............................................. 198.54 mV to 19.58 mV
from 0° to 55° C
Compatible DIN rails ............................. DIN EN 50 022
DIN EN 50 035
Dimensions............................................. 12.7 by 11.2 by 7.62 cm
(5.0 by 4.4 by 3.0 in.)
Max voltage
(signal + common mode) ....................... Each input should remain within
30 Vrms or 60 VDC of ground and
all other channels to eliminate the
possibility of hazardous shock
1 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.

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