National Instruments Switch SCXI 1321 User Manual
™
SCXI -1321 OFFSET-NULL AND  
SHUNT-CALIBRATION HIGH-VOLTAGE  
TERMINAL BLOCK INSTALLATION GUIDE  
This guide describes how to install and use the SCXI-1321 offset-null  
and shunt-calibration terminal block with the SCXI-1121 module. You can  
only use the SCXI-1321 terminal block with SCXI-1121 revision C or later  
modules. In addition to the 18 screw terminals, the SCXI-1321 has circuitry  
for offset-null adjustment of Wheatstone bridges, and a socketed shunt  
resistor for strain-gauge shunt calibration. This terminal block was  
primarily designed for Wheatstone bridge transducers such as strain  
gauges, although it can easily accommodate thermocouples, RTDs,  
thermistors, millivolt sources, volt sources, and current-loop receivers.  
Thermocouples have cold-junction compensation (CJC) support.  
Conventions  
The following conventions are used in this guide:  
»
The » symbol leads you through nested menu items and dialog box options  
to a final action. The sequence File»Page Setup»Options directs you to  
pull down the File menu, select the Page Setup item, and select Options  
from the last dialog box.  
This icon denotes a note, which alerts you to important information.  
This icon denotes a caution, which advises you of precautions to take to  
avoid injury, data loss, or a system crash. When this symbol is marked on  
the product, refer to the Read Me First: Safety and Radio-Frequency  
Interference document, shipped with the product, for precautions to take.  
bold  
Bold text denotes items that you must select or click in the software, such  
as menu items and dialog box options. Bold text also denotes parameter  
names.  
LabVIEW™, National Instruments™, NI™, ni.com™, NI-DAQ™, and SCXI™ are trademarks of National Instruments  
Corporation. Product and company names mentioned herein are trademarks or trade names of their respective  
companies. For patents covering National Instruments products, refer to the appropriate location: Help»Patents in your  
software, the patents.txtfile on your CD, or ni.com/patents.  
January 2003  
321924C-01  
ni.com  
© 1998–2003 National Instruments Corp. All rights reserved.  
 
 
Connecting the Signals  
Note Refer to the Read Me First: Safety and Radio-Frequency Interference document  
before removing equipment covers, or connecting or disconnecting any signal wires.  
To connect the signal to the terminal block, complete the following steps,  
referring to Figure 1 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 Disable the offset-nulling circuitry when you are not using a Wheatstone bridge or  
when the excitation channel of the SCXI-1121 is in current mode.  
4. Run the signal wires through the strain-relief opening. You can add  
insulation or padding if necessary.  
5. Prepare the signal wire by stripping the insulation no more than 7 mm  
(0.28 in.).  
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 a short circuit and a  
hardware failure.  
7. Tighten the screws to a torque of 0.57 to 0.79 newton-m (5 to 7 lb-in.).  
8. Connect safety earth ground to the safety-ground lug. Refer to the  
Read Me First: Safety and Radio-Frequency Interference document  
for connection information.  
9. Reinstall the strain-relief bar, and tighten the strain-relief screws.  
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 Installing the Terminal Block section.  
© National Instruments Corporation  
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SCXI-1321 Terminal Block Installation Guide  
 
6
2
7
3
1
4
5
Back View  
3
Front View  
7 Top Cover  
1
2
Strain-Relief Bar  
Safety-Ground Lug  
Mating Connector  
5
6
Thumbscrew  
Strain-Relief Screws  
4
Top-Cover Screws  
Figure 1. SCXI-1321 Parts Locator Diagram  
SCXI-1321 Terminal Block Installation Guide  
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5
6
7
8
9
10  
11  
12  
3
4
2
13  
14  
15  
16  
1
20 19  
18 17  
1
2
3
4
5
6
7
Screw Terminals  
Product Information  
W1 (CH0 Null Enabled)  
W2 (CH1 Null Enabled)  
R3 (CH0 Null Resistor)  
R4 (CH0 Shunt Resistor)  
R5 (CH1 Null Resistor)  
8
R6 (CH1 Shunt Resistor)  
R7 (CH2 Null Resistor)  
15 W4 CH3 Null Enabled)  
16 W3 (CH2 Null Enabled)  
17 R15 (CH3 Null Potentiometer)  
18 R14 (CH2 Null Potentiometer)  
19 R2 (CH1 Null Potentiometer)  
20 R1 (CH0 Null Potentiometer)  
9
10 R8 (CH2 Shunt Resistor)  
11 R9 (CH3 Null Resistor)  
12 R10 (CH3 Shunt Resistor)  
13 W5 (CJC Mode)  
14 Warning Label  
Note: R3 through R10 are socketed.  
Figure 2. SCXI-1321 Circuit Parts Locator Diagram  
Installing the Terminal Block  
To connect the terminal block to the SCXI module front connector,  
complete the following steps:  
1. Connect the module front connector to its connector on the terminal  
block.  
2. Make sure that the module top and bottom thumbscrews do not  
obstruct the rear panel of the terminal block.  
3. Tighten the top and bottom screws on the back of the terminal block to  
hold it securely in place.  
4. Refer to the Performed or Supported Signal Conditioning section for  
information on specific signal conditioning.  
© National Instruments Corporation  
5
SCXI-1321 Terminal Block Installation Guide  
 
 
Note To minimize the temperature gradient inside the terminal block, move the SCXI  
chassis away from any extreme temperature differential.  
Specifications  
All specifications are typical at 25 °C unless otherwise specified.  
Electrical  
Cold-junction sensor  
Accuracy1 ........................................0.9 °C  
Output..............................................10 mV/°C from 0 to 55 °C  
Resistors  
RSHUNT .............................................301 kΩ 1%  
RNULL ...............................................39 kΩ 5%  
RTRIM ...............................................10 kΩ  
Nulling potentiometer  
Range...............................................0 to 10 kΩ  
Step size...........................................infinite (user adjustable)  
Mechanical  
Resistor sockets  
Connecting lead size........................0.023 to 0.026 in.  
Connecting lead length....................0.110 to 0.175 in.  
Lead spacing....................................0.500 in.  
Maximum Working Voltage  
Maximum working voltage refers to the signal voltage plus the  
common-mode voltage.  
Channel-to-earth .....................................300 V, Installation Category II  
Channel-to-channel.................................300 V, Installation Category II  
1
The temperature sensor accuracy includes tolerances in all component values caused by temperature and loading,  
and self-heating.  
SCXI-1321 Terminal Block Installation Guide  
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Environmental  
Operating temperature............................ 0 to 50 °C  
Storage temperature ............................... –20 to 70 °C  
Humidity ................................................ 10 to 90% RH, noncondensing  
Maximum altitude.................................. 2,000 meters  
Pollution Degree (indoor use only)........ 2  
Safety  
The SCXI-1321 is designed to meet the requirements of the following  
standards of safety for electrical equipment for measurement, control and  
laboratory use:  
•
•
•
IEC 61010-1, EN 61010-1  
UL 3111-1, UL61010B-1  
CAN/CSA C22.2 No. 1010.1  
Note For UL and other safety certifications refer to the product label or to ni.com.  
Electromagnetic Compatibility  
Emissions ............................................... EN 55011 Class A at 10 meters;  
FCC Part 15A above 1 GHz  
Immunity................................................ EN 61326:1997 + A2:2001,  
Table 1  
EMC/EMI............................................... CE, C-Tick and FCC Part 15  
(Class A) Compliant  
Note For EMC compliance, operate this device with shielded cabling.  
CE Compliance  
The SCXI-1321 meets the essential requirements of applicable European  
Directives, as amended for CE Marking, as follows:  
Low-Voltage Directive (safety) ............. 73/23/EEC  
Electromagnetic Compatibility Directive (EMC)89/336/EEC  
© National Instruments Corporation  
7
SCXI-1321 Terminal Block Installation Guide  
 
Note Refer to the Declaration of Conformity (DoC) for this product for any additional  
regulatory compliance information. To obtain the DoC for this product, click Declarations  
of Conformity Information at ni.com/hardref.nsf/.  
Performed or Supported Signal Conditioning  
This section provides information on types of signal conditioning  
performed by the SCXI-1321 or is supported by it.  
Offset Nulling  
Offset nulling is a hardware nulling procedure used with  
Wheatstone-bridge transducers that have an initial offset error. Correcting  
this error improves measurement accuracy. The nulling circuitry operates  
with quarter-bridge, half-bridge, and full-bridge strain-gauge  
configurations. Each channel has its own nulling circuitry and its own  
trimming potentiometer as listed in Table 1.  
Table 1. Trimmer Potentiometers and Corresponding Channels  
Channel Number  
Trimmer Potentiometer  
0
1
2
3
R1  
R2  
R14  
R15  
To null the static offset voltage of the bridge, complete the following steps:  
1. Connect the 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 the bridge and are ready for a measurement.  
The nulling range that is provided with the terminal block is 2.5 mV,  
assuming that you have a 120 Ω strain gauge quarter-bridge configuration  
and 3.3 V excitation voltage. You can change this range by replacing the  
nulling resistor with a resistor of another value. Each channel has an  
independent socketed nulling resistor. Therefore, you can mix the ranges to  
accommodate each channel requirement. Table 2 lists the nulling resistors  
and their corresponding channels.  
SCXI-1321 Terminal Block Installation Guide  
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Table 2. Nulling Resistors and Corresponding Channels  
Channel Number  
Nulling Resistor  
0
1
2
3
R3  
R5  
R7  
R9  
The factory default value of all the nulling resistors on the terminal block  
is 39 kΩ.  
Note These resistors are socketed for easy replacement. These sockets best fit a 1/4 W  
resistor lead size.  
To determine the nulling range, use the following formula while referring  
to Figures 3 through 5:  
Vexc  
--------- – ----------------------------------------------------------  
RnullRg + Rd(Rnull + Rg)  
VexcRd(Rnull + Rg)  
Vnulling range  
=
2
where  
Vexc is the excitation voltage (3.3 V or 10 V)  
Rd is either a completion resistor or a second strain-gauge nominal  
resistance  
Rnull is the nulling resistor value  
(range of trim potentiometer + nulling resistor)  
Rg is the nominal strain-gauge resistance value  
For example,  
Vnulling range = 2.56 mV  
Vexc = 3.3 V  
Rd = 120 Ω  
Rnull = 39 kΩ  
Rg = 120 Ω  
© National Instruments Corporation  
9
SCXI-1321 Terminal Block Installation Guide  
 
 
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:  
–4Vr  
ε = -------------------------------  
GF(1 + 2Vr)  
where  
strained voltage – static unstrained voltage  
Vr = -------------------------------------------------------------------------------------------------------  
Vexc  
SCXI-1121  
SCXI-1321  
Transducer  
RL  
CH+  
CH+  
RSCAL  
+
–
301 kΩ  
CH–  
EX+  
CH–  
EX+  
EX+  
R4(gauge)  
RL  
R1  
RNull  
CH+  
4.5 kΩ  
4.5 kΩ  
+
–
RTrim  
10 kΩ  
VEX  
39.1 kΩ  
RL  
R2  
EX–  
EX–  
R3(dummy)  
RTrim  
(Screw Adjusts Potentiometer)  
Note: R1 and R2 are completion resistors.  
Figure 3. Quarter-Bridge Nulling Circuit  
SCXI-1321 Terminal Block Installation Guide  
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SCXI-1121  
SCXI-1321  
Transducer  
RL  
CH+  
CH+  
RSCAL  
+
–
301 kΩ  
CH–  
EX+  
CH–  
EX+  
EX+  
R4(gauge)  
RL  
R1  
R2  
RNull  
CH+  
4.5 kΩ  
4.5 kΩ  
+
RTrim  
10 kΩ  
VEX  
–
39.1 kΩ  
RL  
R3(gauge)  
EX–  
EX–  
EX–  
RTrim  
(Screw Adjusts Potentiometer)  
Note: R1 and R2 are completion resistors.  
Figure 4. Half-Bridge Nulling Circuit  
© National Instruments Corporation  
11  
SCXI-1321 Terminal Block Installation Guide  
 
SCXI-1121  
SCXI-1321  
Transducer  
RL  
CH+  
CH–  
CH+  
CH–  
RSCAL  
+
301 kΩ  
–
RL  
EX+  
R1  
(gauge)  
R4  
(gauge)  
RL  
RL  
EX+  
EX–  
EX+  
EX–  
CH– CH+  
RNull  
+
–
RTrim  
10 kΩ  
R2  
(gauge)  
R3  
(gauge)  
VEX  
39.1 kΩ  
RTrim  
(Screw Adjusts Potentiometer)  
Figure 5. Full-Bridge Nulling Circuit  
Shunt Calibration  
Shunt calibration provides a method of adjusting for gain error to improve  
accuracy. The shunt-calibration circuitry configuration places a shunting  
resistor in parallel with the resistive element connected between EX+ and  
CH+ (element R4) of the Wheatstone bridge gauge, as shown in Figure 5.  
Shunt calibration circuits of each channel are independent from each other,  
although they are controlled together in software; therefore, when SCAL is  
engaged on a channel, all the shunt switches of the channels are closed.  
When SCAL is disengaged, all the switches are open. At startup or reset,  
all switches are open by default. You can control SCAL with the NI-DAQ  
function SCXI_Calibrate_Setup. Set the Cal_Opparameter to 2for  
engaged or 0for disengaged.  
You can use LabVIEW to take measurements from channels with the shunt  
resistors connected by using the SCXI channel string  
obx ! scy ! mdz ! shuntw.  
where  
x is the onboard channel number (0 for single chassis systems)  
y is the chassis ID (1 by default)  
SCXI-1321 Terminal Block Installation Guide  
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z is the module slot of the SCXI-1121  
w is the channel of the module that you want to engage the shunt and  
take measurements  
For example, if you want to measure the voltage at channel 0 with the shunt  
resistor enabled, use the SCXI channel string  
ob0 ! sc1 ! md1 ! shunt0.  
You also can specify a list of channels by using  
ob0 ! sc1 ! md1 ! shunt0:w, for example. Refer to the LabVIEW  
Measurements Manual for information on using SCXI channel strings.  
The shunting resistors RSCAL are socketed so that you can replace them with  
a resistor of another value to achieve the required nulling range for your  
application. The sockets and corresponding channels are shown in Table 3.  
The factory installed RSCAL provided on the terminal block have a 301 kΩ  
1% value.  
Table 3. Socket to Channel Relationship  
Channel  
Shunt Resistor Socket  
0
1
2
3
R4  
R6  
R8  
R10  
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.  
Refer to the Traditional NI-DAQ User Manual for more information on  
strain-gauge bridge configurations and formulas.  
Use the following formula to determine the change due to this shunting  
resistor:  
VexRd(RSCAL + Rg)  
Vchange = ----------------------------------------------------------------- – -------  
RgRSCAL + Rd(RSCAL + Rg)  
Vex  
2
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 kΩ and a quarter-bridge 120 Ω strain  
© National Instruments Corporation  
13  
SCXI-1321 Terminal Block Installation Guide  
 
 
gauge with a gauge factor of GF = 2, Vexc = 3.3 V, and R = 120 Ω 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. Refer to the NI-DAQ  
User Manual for more information on voltage to strain conversion  
equations.  
Cold-Junction Compensation  
CJC is used only with thermocouples and provides improved accuracy of  
temperature measurements. The CJC temperature sensor, mounted in the  
SCXI-1321, 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 (°C) × 9  
T (°F) = ------------------------- + 32  
5
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. You cannot use virtual  
channels to take the readings.  
You can enable the CJC sensor in one of two ways depending on the input  
mode configuration of the SCXI-1121. Jumper W5 switches the  
temperature sensor output between MTEMP (multiplexed mode) and  
DTEMP (parallel mode) modes. In MTEMP mode, you must scan the  
cold-junction temperature independently of the other AI channels on the  
SCXI-1121 using the LabVIEW Getting Started Analog Input VI, available  
in examples\daq\run_me.llb, with the channel string  
ob0 ! sc1 ! md1 ! mtemp. This reads the temperature sensor on the  
terminal block connected to the module in slot 1 of SCXI chassis 1.  
You then can average several measurements of the cold-junction  
temperature and use this average to compensate for the cold junction of the  
thermocouple. Using this averaging method compensates for temperature  
variations during the measurement period and makes the CJC temperature  
more accurate.  
SCXI-1321 Terminal Block Installation Guide  
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You can read a temperature at the beginning of the test and use that value  
with the data that follows. Using this method assumes that there are no  
temperature variations during the measurement period. If there are  
temperature variations, the measurements can be less accurate. You also  
can read the cold-junction temperature once per scan of the thermocouple  
channels.  
Use the DTEMP mode if you are operating the SCXI-1121 in parallel  
mode. DTEMP uses CH4 analog input, therefore CH3 is not available.  
RTD and Thermistor Excitation  
By properly setting the excitation, you can configure the SCXI-1321 on  
a per channel basis for RTD and thermistor measurements. 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 4.  
2. Remove the nulling resistor from its sockets.  
Table 4. Jumper Settings of the Nulling Circuits  
Jumper  
Position  
Description  
W1  
Nulling circuit of Channel 0  
is enabled (factory default setting)  
D
E
Nulling circuit of Channel 0  
is disabled  
D
E
W2  
Nulling circuit of Channel 1  
is enabled (factory default setting)  
D
E
Nulling circuit of Channel 1  
is disabled  
D
E
© National Instruments Corporation  
15  
SCXI-1321 Terminal Block Installation Guide  
 
 
Table 4. Jumper Settings of the Nulling Circuits (Continued)  
Jumper  
Position  
Description  
W3  
Nulling circuit of Channel 2  
is enabled (factory default setting)  
D
E
Nulling circuit of Channel 2  
is disabled  
D
E
W4  
Nulling circuit of Channel 3  
is enabled (factory default setting)  
D
E
Nulling circuit of Channel 3  
is disabled  
D
E
*321924C-01*  
321924C-01  
Jan03  
 

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