Chapter 9 SCXI—Signal Conditioning

the SCXI amplifiers and multiplexers to settle between channel switching before sampling the next channel.

LabVIEW calculates the delay for you. If you set a scan rate that is too fast to allow for the default interchannel delay, LabVIEW shrinks the interchannel delay and returns a warning from the AI Start or

AI Control VIs. Refer to your hardware manuals for SCXI settling times.

You can open the advanced-level AI Clock Config VI to retrieve the channel clock selection. Set the which clock control to channel clock 1, and set the clock frequency to –1.00 (no change). Now run the VI. The actual clock rate specification cluster is on the right side of the panel.

Note When using NI 406X devices with SCXI, you cannot use the external triggering feature of the NI 406X device.

Common SCXI Applications

Now that you have your SCXI system set up and you are aware of the special SCXI programming considerations, you should learn about some common SCXI applications. This section covers example VIs for analog input, analog output, and digital modules. For analog input, you will learn how to measure temperature (with thermocouples and RTDs) and strain (with strain gauges) using the SCXI-1100, SCXI-1101, SCXI-1102, SCXI-1112, SCXI-1121, SCXI-1122, SCXI-1125, SCXI-1141, SCXI-1142, SCXI-1143, and SCXI-1520 modules. If you are not measuring temperature or pressure, you still can gain basic conceptual information on how to measure voltages with an analog input module.

Four other analog input modules, the SCXI-1140, SCXI-1520, SCXI-1530, and SCXI-1531, are simultaneous sampling modules. All the channels acquire voltages at the same time, which means you can preserve interchannel phase relationships. After all channel voltages are sampled by going into hold mode, the software reads one channel at a time. When a scan of channels is done, the module returns to track mode until the next scan period. Both of these operations are performed by the analog input VIs. You can use any of the DAQ VIs, available in the examples\daq\ anlogin\anlogin.llb, or the Getting Started Analog Input VI, available in the examples\daq\run_me.llb, to acquire data from the SCXI-1140, SCXI-1520, SCXI-1530, or SCXI-1531 module.

For analog output, you will learn how to output voltage or current values using the SCXI-1124 module. For digital I/O, you will learn how to input values on the SCXI-1162/1162HV modules and output values on the

Chapter 9 SCXI—Signal Conditioning

SCXI-1160, SCXI-1161, and SCXI-1163/1163R modules. For switching, you will learn how to open, close, or make other channel connections on the SCXI-1127, SCXI-1128, SCXI-1150, and SCXI-1151.

Analog Input Applications for Measuring Temperature and Pressure

Two common transducers for measuring temperature are thermocouples and RTDs. A common transducer for measuring pressure is strain gauges. Read the following sections on special measuring considerations needed for each transducer.

If you use the DAQ Channel Wizard to configure your analog input channels, you can simplify the programming needed to measure your channels. This section describes methods of measuring data using named channels configured in the DAQ Channel Wizard and using the conventional method.

Measuring Temperature with Thermocouples

If you want to measure the temperature of the environment, you can use the temperature sensors in the terminal blocks. If you want to measure the temperature of an object away from the SCXI chassis, you must use a transducer, such as a thermocouple. A thermocouple is a junction of

two dissimilar metals that gives varying voltages based on the temperature. However, when using thermocouples, you need to compensate for the thermocouple voltages produced at the screw terminal because the junction with the screw terminals itself forms another thermocouple. You can use the resulting voltage from the temperature sensor on the terminal block for cold-junction compensation (CJC). The CJC voltage is used when linearizing voltage readings from thermocouples into temperature values.

The SCXI modules used to measure temperature in this section are the SCXI-1100, SCXI-1101, SCXI-1102, SCXI-1112, SCXI-1120, SCXI-1120D, SCXI-1121, SCXI-1122, SCXI-1125, SCXI-1127, SCXI-1141, SCXI-1142, and SCXI-1143. Most of the terminal blocks used with these modules have temperature sensors that you can use for CJC.

In addition, the SCXI-1100, SCXI-1112, SCXI-1122, SCXI-1125, SCXI-1141, SCXI-1142, and SCXI-1143 offer a way for you to ground the module amplifier inputs so you can read the amplifier offset. You can subtract the amplifier offset value to determine the actual voltages.

Chapter 9 SCXI—Signal Conditioning

Temperature Sensors for Cold-Junction Compensation

The temperature sensors in the terminal blocks for the analog input modules can be used for CJC. If you are operating your SCXI modules in multiplexed mode, leave the cold-junction sensor jumper on the terminal block in the mtemp (factory default) position. If you are using parallel mode, you can use the dtemp jumper setting.

Note The SCXI-1101, SCXI-1102, SCXI-1112, and SCXI-1127 use the cjtemp string only in multiplexed mode. The SCXI-1125 also uses the cjtemp string when the temperature sensor is configured in mtemp mode.

To read the temperature sensor, use the standard SCXI string syntax in the channels array with mtemp substituted for the channel number, as shown in the following table.

Chapter 9 SCXI—Signal Conditioning

If you want to read the cold-junction temperature sensor in dtemp mode, you can read the following onboard channels for these modules.

For example, you can run the Getting Started Analog Input VI, available in the examples\daq\run_me.llb, with the channel string ob0!sc1!md1!mtemp to read the temperature sensor on the terminal block connected to the module in slot 1 of SCXI chassis 1.

SCXI terminal blocks have two different kinds of sensors: an Integrated Circuit (IC) sensor or a thermistor. For terminal blocks that have IC sensors, such as the SCXI-1300 and the SCXI-1320, you can multiply the voltage read from the IC sensor by 100 to get the ambient temperature in degrees Centigrade at the terminal block. For terminal blocks that have thermistors, such as the SCXI-1303, SCXI-1322, SCXI-1327,

and SCXI-1328, use the Convert Thermistor VI, available on the

Functions»Data Acquisition»Signal Conditioning palette, to convert the raw voltage data into units of temperature.

You cannot sample other SCXI channels from the same module while you are sampling the mtemp sensor. However, if you are in parallel mode, you can sample the dtemp sensor along with other channels on the same module at the same time because you are not performing any multiplexing on the SCXI module. You also can sample the cjtemp sensor along with other channels on the SCXI-1101, SCXI-1102, SCXI-1112, SCXI-1125, and SCXI-1127. For the SCXI-1102, cjtemp must be the first channel in the channel list. The SCXI-1112 and SCXI-1125 can sample cjtemp channels in any order in the channel list.

For greater accuracy, take several readings from the temperature sensor and average those readings to yield one value. If you do not want to average several readings, take a single reading using the Easy Analog Input VI, AI Sample Channel.