National Instruments SCXI-1321 User manual

LabVIEW ™ , NI-DAQ ™, and SCXI ™ are trademarks of National Instruments Corporation. Product and company names are trademarks or trade names
of their respective companies.
321924A-01 © Copyright 1998 National Instruments Corp. All rights reserved. August 1998
INSTALLATION GUIDE
SCXI-1321 OFFSET-NULL AND
SHUNT-CALIBRATION HIGH-VOLTAGE
TERMINAL BLOCK
This guide describes how to install and use the SCXI-1321 offset-null
and shunt-calibration terminal block with your SCXI-1121 module.
Introduction
You can only use the SCXI-1321 terminal block with SCXI-1121 revision
C and later revisions. The SCXI-1321 terminal block has a shielded board
with supports for connection to the SCXI-1121 input connector. In addition
to the 18 screw terminals, the SCXI-1321 has circuitry for offset-null
adjustment of Wheatstone bridges, and a shunt resistor for strain-gauge
shunt calibration. This terminal block was primarily designed for
bridge-type transducers such as strain gauges. The SCXI-1321 can also
easily accommodate thermocouples, RTDs, thermistors, millivolt sources,
volt sources, and current-loop receivers.
What You Need to Get Started
To set up and use your SCXI-1321, you will need the following items:
❑SCXI-1321 offset-null and shunt-calibration terminal block
❑SCXI-1321 Offset-Null and Shunt-Calibration Terminal Block
Installation Guide
❑SCXI chassis
❑SCXI-1121 module
❑No. 1 and No. 2 Phillips-head screwdrivers
❑1/8in. flathead screwdrivers

SCXI-1321 Installation Guide 2 © National Instruments Corporation
❑Long-nose pliers
❑Wire cutter
❑Wire insulation stripper
Safety Information
Cautions DONOT OPERATE THE DEVICE IN AN EXPLOSIVE ATMOSPHERE OR WHERE THERE MAY BE
FLAMMABLE GASES OR FUMES.
KEEP AWAY FROM LIVE CIRCUITS. Do not remove equipment covers or shields unless
you are trained to do so. If signal wires are connected to the device, hazardous
voltagesmayexist evenwhentheequipment isturned off. Toavoida shockhazard,
do not perform procedures involving cover or shield removal unless you are
qualified to do so and disconnect all field power prior to removing covers or
shields.
Equipment described in this document must be used in an Installation Category
II environment per IEC 664. This category requires local level supply
mains-connected installation.
DONOT OPERATE DAMAGED EQUIPMENT. The safety protection features built into this
device can become impaired if the device becomes damaged in any way. If the
device is damaged, turn the device off and do not use until service-trained
personnel can check its safety. If necessary, return the device to National
Instruments for service and repair to ensure that its safety is not compromised.
Do not operate this equipment in a manner that contradicts the information
specified in this document. Misuse of this equipment could result in a shock
hazard.
Terminals are for use only with equipment that has no accessible live parts.
DONOT SUBSTITUTE PARTS OR MODIFY EQUIPMENT. Because of the danger of
introducing additional hazards, do not install unauthorized parts or modify the
device. Return the device to National Instruments for service and repair to ensure
that its safety features are not compromised.
When using the device with high common-mode voltages, you MUST insulate your
signal wires for the highest input voltage. National Instruments is NOT liable for
any damages or injuries resulting from inadequate signal wire insulation. Use
only 26-14 AWG wire with a voltage rating of 300 V and 60° C for measuring
250 to 300 V.
When connecting or disconnecting signal lines to the SCXI terminal block screw
terminals, make sure the lines are powered off. Potential differences between the
lines and the SCXI ground create a shock hazard while you connect the lines.
!

© National Instruments Corporation 3 SCXI-1321 Installation Guide
Connections, including power signals to ground and vice versa, that exceed any of
the maximum signal ratings on the SCXI device can create a shock or fire hazard
or can damage any or all of the boards connected to the SCXI chassis, the host
computer, and the SCXI device. National Instruments is NOT LIABLE FOR ANY
DAMAGES OR INJURIES resulting from incorrect signal connections.
If high voltages (≥30 Vrms and 42.4 V peak or 60 VDC) are present, YOU MUST
CONNECT ASAFETY EARTH GROUND WIRE TO THE TERMINAL BLOCK SAFETY GROUND SOLDER
LUG, shown in Figure 1. This complies with safety agency requirements and
protects against electric shock when the terminal block is not connected to the
chassis. To connect the safety earth ground to the safety ground solder lug, run an
earth ground wire in the cable from the signal source to the terminal block.
National Instruments is NOT liable for any damages or injuries resulting from
inadequate safety earth ground connections.
Donot loosen orre-orient thesafetyground solderlug hardwarewhenconnecting
the safety ground wire; to do so reduces the safety isolation between the high
voltage and safety ground.
Temperature Sensor Output and Accuracy
The temperature sensor outputs 10 mV/°C and has an accuracy of ±0.9° C
over the 0° to 55° C temperature range. To determine the temperature, use
the following formulas:
T (°C) = 100 ×VTEMPOUT
T (°F) = T (°C) ×9/5 + 32
where VTEMPOUT is the temperature sensor output voltage, and T (°F) and
T (°C) are the temperature readings in degrees Fahrenheit and degrees
Celsius, respectively.
Note Use the average of a large number of samples to obtain the most accurate reading.
Noisy environments require averaging for greater accuracy.
Using the Nulling Circuitry and the Shunt Calibration
SCXI-1321 Nulling Circuitry
The nulling circuitry operates with full-bridge, half-bridge, quarter-bridge,
and strain-gauge configurations. Each channel has its own nulling circuitry
and its own trimming potentiometer as listed in Table 1.

SCXI-1321 Installation Guide 4 © National Instruments Corporation
To null the static offset voltage of the bridge, use the following procedure:
1. Hook up your bridge configuration to the selected channel.
2. Select and read the channel output.
3. While monitoring the output, rotate the trimmer wiper with a flathead
screwdriver until you reach 0 V.
You have nulled your bridge and are ready for a measurement.
The nulling range that is provided with your terminal block is ±2.5 mV,
assuming that you have a 120 Ωstrain gauge quarter-bridge configuration
and 3.333 V excitation voltage. You can change this range by replacing the
nulling resistor with a resistor your choice of another value. Each channel
has an independent nulling resistor. You can therefore mix your ranges to
accommodate each channel requirement. Table 2 lists the nulling resistors
and their corresponding channels.
Thevalueofall the nullingresistors onyour terminalblock is 39 kΩ. Notice
that these resistors are socketed for easy replacement. These sockets best fit
a 1/4W resistor lead size.
To determine your nulling range, use the following formula (refer to
Figure 1 for visual help):
Table 1. Trimmer Potentiometers and Corresponding Channels
Channel Number Trimmer Potentiometer
0 R1
1 R2
2R14
3 R15
Table 2. Nulling Resistors and Corresponding Channels
Channel Number Nulling Resistor
0 R3
1 R5
2 R7
3 R9
Vnulling range Vexc
2
----------- VexcRdRnull Rg
+
()
R
nullRgRdRnull Rg
+
()
+
---------------------------------------------------------------–
±
=

© National Instruments Corporation 5 SCXI-1321 Installation Guide
where
Rgis the nominal strain gauge resistance value.
Rdis either a completion resistor or a second strain gauge nominal
resistance.
Rnull is the nulling resistor value.
Vexc is the excitation voltage (3.333 V or 10 V).
For example, assuming:
Vexc = 3.333 V
Rg= 120 Ω
Rd= 120 Ω
Rnull = 39 kΩ
Vnulling range = ±2.56 mV
Assuming a strain gauge range with a gauge factor of GF = 2 and a
quarter-bridge configuration, this range corresponds to ±1,498 µε as given
by the strain formula for a quarter-bridge strain gauge configuration:
where
ε4Vr
–
GF 1 2Vr
+
()
--------------------------------=
Vrstrained voltage static unstrained voltage– Vexc
------------------------------------------------------------------------------------------------------=

SCXI-1321 Installation Guide 6 © National Instruments Corporation
Figure 1. Nulling Circuit
Note Rcstands for the completion resistors.
Using the SCXI-1321 with RTDs and Thermistors
When using this terminal block with RTDs or thermistor-type transducers
and with the SCXI-1121 excitation set in the Current mode, you must
disable the nulling circuit of the channel of interest. You can do this in two
steps:
1. Place the enable/disable jumper in position D (disable) as shown in
Table 3.
2. Remove the nulling resistor from its sockets.
Table 3. Jumper Settings of the Nulling Circuits
Jumper Position Description
W1 Nulling circuit of Channel 0
is enabled; factory setting
Nulling circuit of Channel 0
is disabled
Trimmer
Potentionmeter
EX–
CH+
CH–
EX+
SCXI-1321
Rnull
RgRc
RdRc
D
E
D
E

© National Instruments Corporation 7 SCXI-1321 Installation Guide
SCXI-1121 Shunt Calibration
Shunt calibration circuits are independent from each other but are
controlled together. In other words, when SCAL is set to 1 on the
SCXI-1121, all the shunt switches close; when SCAL is cleared to 0, all the
switches open. At startup or reset, all switches are open. This shunt
calibrationcircuitry configurationplaces a shuntingresistor in parallelwith
the strain gauge, as shown in Figure 2.
W2 Nulling circuit of Channel 1
is enabled; factory setting
Nulling circuit of Channel 1
is disabled
W3 Nulling circuit of Channel 2
is enabled; factory setting
Nulling circuit of Channel 2
is disabled
W4 Nulling circuit of Channel 3
is enabled; factory setting
Nulling circuit of Channel 3
is disabled
Table 3. Jumper Settings of the Nulling Circuits (Continued)
Jumper Position Description
D
E
D
E
D
E
D
E
D
E
D
E

SCXI-1321 Installation Guide 8 © National Instruments Corporation
Figure 2. Shunt Circuit
The shunting resistors RSCAL are socketed so thatyou can replace themwith
a resistor of another value to achieve the required changes. The RSCAL
provided on your terminal block have a 301 kΩ±1% value.
Assuming a quarter-bridge strain-gauge configuration with a gauge factor
of GF = 2, the equivalent strain change introduced by the RSCAL shunting
resistor is –199 µε.
Use the following formula to determine the change due to this shunting
resistor:
Next, using the appropriate strain-gauge strain formula, and assuming that
you have no static voltage, determine the equivalent strain the RSCAL should
provide. For example, RSCAL = 301 kΩand a quarter-bridge 120 Ωstrain
gauge with a gauge factor of GF = 2 and Vexc = 3.333 V and R = 120Ω
produces the following:
Vchange = 0.3321 mV
Replacing the strained voltage by Vchange in the quarter-bridge strain
equation produces an equivalent –199 µε of change.
SCAL
EX–
CH+
CH–
EX+
SCXI-1321
RSCAL RgRc
RdRc
Vchange VexcRdRSCAL Rg
+
()
R
SCAL RdRSCAL Rg
+
()
+
----------------------------------------------------------------- Vexc
2
-----------–=

© National Instruments Corporation 9 SCXI-1321 Installation Guide
Using Shunt Calibration with National Instruments
Software
If you are using LabVIEW for Windows 2.5.2 or LabVIEW 3.0, use
the channel string shunt
x to enable shunt calibration. If you are
using NI-DAQ or LabVIEW 2.2.1 or earlier versions, use the
SCXI_Calibrate_Setup function or VI. If you are using register-level
code to program the SCXI-1121 and the SCXI-1321, you must write to the
SCAL register yourself.
Signal Connection
Note Refer to the Safety Information section before removing equipment covers or
connecting or disconnecting any signal wires.
When connecting your signals to the SCXI-1321, follow the labeling on the
SCXI-1321 for the appropriate module, as indicated in Figure 2.
To connect the signal to the terminal block, perform the following steps,
referring to Figures 1 and 2 as necessary:
1. Unscrew the top cover screws and remove the cover.
2. Loosen the strain-relief screws and remove the strain-relief bar.
3. Enable or bypass each of the nulling circuits, depending on the signal
you are measuring.
Note For correct operation of the SCXI-1121 excitation channels, disable the nulling
circuit on the excitation channels when you are using the SCXI-1321 with the
SCXI-1121.
4. Run the signal wires through the strain-relief opening. You can add
insulation or padding if necessary.
5. Prepare your signal wire by stripping the insulation no more than
7mm.
6. Connect the wires to the screw terminals by inserting the stripped end
of the wire fully into the terminal. No bare wire should extend past the
screw terminal. Exposed wire increases the risk of shorting and
causing a failure.
7. Tighten the screws to a torque of 5–7 in.-lb.
8. Connect safety earth ground to the safety ground solder lug. Refer to
the Safety Information section for connection information.
9. Reinstall the strain-relief bar and tighten the strain-relief screws.

SCXI-1321 Installation Guide 10 © National Instruments Corporation
10. Reinstall the top cover and tighten the top cover screws.
11. Connect the terminal block to the module front connector as explained
in the Installation section later in this guide.
Figure 3 shows the SCXI-1321 terminal block parts locator diagram.
Figure 3. SCXI-1321 Parts Locator Diagram
1 Strain Relief Bar
2 Strain Relief Screws 3 Safety Ground Solder Lug 4 Mating Connector 5 Thumbscrew 6 Top Cover Screws 7 Top Cover
2
7
3
6
4
5
1
Back View Front View