NI cDAQ-9178 User manual

USER GUIDE AND SPECIFICATIONS
NI cDAQ-9178/9174
For NI cDAQ-9178 (8-Slot) and NI cDAQ-9174 (4-Slot) Chassis
Contents
Introduction............................................................................................. 3
Safety Guidelines .................................................................................... 4
Safety Guidelines for Hazardous Voltages ...................................... 4
Information Resources ............................................................................ 4
Technical Support on the Web......................................................... 4
Training Courses.............................................................................. 4
Installing the Software ............................................................................ 4
Installing Other Software................................................................. 4
Installing the NI cDAQ-9178/9174......................................................... 5
NI cDAQ-9178/9174 Dimensions ................................................... 5
Mounting the NI cDAQ-9178/9174................................................. 6
Setting Up the NI cDAQ-9178/9174 ...................................................... 9
Understanding LED Indications.............................................................. 10
Power LED ...................................................................................... 10
Ready LED ...................................................................................... 10
Active LED ...................................................................................... 10
Using the NI cDAQ-9178/9174 .............................................................. 11
C Series I/O Modules....................................................................... 11
cDAQ Module Interface .................................................................. 11
USB-STC3 ....................................................................................... 12
Analog Input ........................................................................................... 12
Analog Input Triggering .................................................................. 12
Analog Input Timing Signals........................................................... 15
Getting Started with AI Applications in Software........................... 17
Analog Output......................................................................................... 17
Analog Output Data Generation Methods ....................................... 18
Analog Output Triggering ............................................................... 19
Analog Output Timing Signals ........................................................ 21
Minimizing Glitches on the Output Signal ...................................... 22
Getting Started with AO Applications in Software ......................... 22
Digital I/O ............................................................................................... 22
Hardware-Timed Versus Static DIO Modules................................. 22
Static DIO ........................................................................................ 22
Digital Waveform Acquisition (Hardware-Timed Input)................ 23
Digital Input Triggering................................................................... 23
Digital Input Timing Signals ........................................................... 25

NI cDAQ-9178/9174 User Guide and Specifications 2 ni.com
Getting Started with DI Applications in Software ...........................27
Change Detection Event ...................................................................27
Change Detection Acquisition..........................................................27
Digital Output...................................................................................27
Digital Output Triggering.................................................................29
Digital Output Timing Signals .........................................................29
Getting Started with DO Applications in Software..........................31
Digital Input/Output Configuration for NI 9401 ..............................31
PFI ...........................................................................................................32
Counters...................................................................................................32
Counter Timing Engine ....................................................................32
Counter Input Applications ..............................................................33
Counter Output Applications............................................................51
Counter Timing Signals....................................................................59
Default Counter/Timer Routing .......................................................63
Counter Triggering ...........................................................................63
Other Counter Features.....................................................................64
Digital Routing and Clock Generation ....................................................65
Clock Routing...................................................................................66
Specifications...........................................................................................67
Analog Input.....................................................................................67
Analog Output ..................................................................................67
Digital Waveform Characteristics ....................................................67
General-Purpose Counter/Timers.....................................................68
Frequency Generator ........................................................................68
Module PFI Characteristics ..............................................................69
Chassis PFI Characteristics (NI cDAQ-9178 Only).........................69
External Digital Triggers..................................................................69
Module I/O States.............................................................................70
Power Requirements.........................................................................70
Bus Interface.....................................................................................70
Physical Characteristics....................................................................70
Safety................................................................................................71
Environmental ..................................................................................71
Shock and Vibration .........................................................................71
Safety Standards ...............................................................................72
Electromagnetic Compatibility.........................................................72
CE Compliance.................................................................................72
Online Product Certification.............................................................72
Environmental Management ............................................................72
Where to Go for Support .........................................................................73

©National Instruments Corporation 3 NI cDAQ-9178/9174 User Guide and Specifications
Introduction
This user guide describes how to use the National Instruments cDAQ-9178/9174 chassis and lists
specifications. For an interactive demonstration of how to install the NI cDAQ-9178/9174, go to
ni.com/info and enter daqinstall.
The NI cDAQ-9178 (eight slots) and NI cDAQ-9174 (four slots) USB chassis are designed for use with
C Series I/O modules. The NI cDAQ-9178/9174 chassis is capable of measuring a broad range of analog
and digital I/O signals and sensors using a Hi-Speed USB 2.0 interface. For module specifications, refer
to the documentation included with your C Series I/O module(s) or go to ni.com/manuals.
Figure 1 shows the NI cDAQ-9178 chassis.
Figure 1. NI cDAQ-9178 Chassis
Figure 2 shows the NI cDAQ-9174 chassis.
Figure 2. NI cDAQ-9174 Chassis
1 USB Connector
2 TRIG 0 (PFI 0) BNC Connector,
TRIG 1 (PFI 1) BNC Connector
3 USB Cable Strain Relief
4 9–30 VDC Power Connector
5 Module Slots
6 Installed C Series Modules
7 Grounding Screw
1 USB Connector
2 USB Cable Strain Relief
3 9–30 VDC Power Connector
4 Module Slots
5 Grounding Screw
12345
INPUT
9-30 V
15 W MAX
POWER READY ACTIVE
V
C
NI cDAQ-9178
NI CompactDAQ
TRIG 0
TRIG 1
1
2
45 6
7
3
4321
INPUT
9-30 V
15 W MAX
POWER READY ACTIVE
V
C
1
34
5
NI cDAQ-9174
NI CompactDAQ
2

NI cDAQ-9178/9174 User Guide and Specifications 4 ni.com
Safety Guidelines
Operate the NI cDAQ-9178/9174 chassis only as described in this user guide.
Note Because some C Series I/O modules may have more stringent certification standards than the
NI cDAQ-9178/9174 chassis, the combined system may be limited by individual component
restrictions. Refer to the Using the NI cDAQ-9178/9174 section of this document for more details.
Caution The NI cDAQ-9178/9174 chassis is not certified for use in hazardous locations.
Hot Surface This icon denotes that the component may be hot. Touching this component may result
in bodily injury.
Safety Guidelines for Hazardous Voltages
If hazardous voltages are connected to the module, take the following precautions. A hazardous voltage
is a voltage greater than 42.4 Vpk or 60 VDC to earth ground.
Caution Ensure that hazardous voltage wiring is performed only by qualified personnel adhering to
local electrical standards.
Caution Do not mix hazardous voltage circuits and human-accessible circuits on the same module.
Caution Make sure that chassis and circuits connected to the module are properly insulated from
human contact.
Caution The NI cDAQ-9178/9174 chassis provides no isolation, but some modules offer isolation.
Follow the safety guidelines for each module when using hazardous voltage.
Information Resources
In addition to the documentation included with your products, check ni.com/manuals for the most
recent hardware and software documentation.
Technical Support on the Web
For additional support, refer to ni.com/support or zone.ni.com.
Training Courses
If you need more help developing an application with NI products, NI offers training courses. For more
information, refer to ni.com/training.
Installing the Software
Install NI-DAQmx. For more information, download the DAQ Getting Started Guide from ni.com/
manuals.
Installing Other Software
If you are using other software, refer to the installation instructions that accompany your software.

©National Instruments Corporation 5 NI cDAQ-9178/9174 User Guide and Specifications
Installing the NI cDAQ-9178/9174
NI cDAQ-9178/9174 Dimensions
Figure 3 shows the dimensions of the NI cDAQ-9178/9174 chassis.
Figure 3. NI cDAQ-9178/9174 with Dimensions in Millimeters (Inches)
8
7654321
19.0 mm
(0.75 in.)
4.1 mm
(0.16 in.)
INPUT
9-30 V
15 W MAX
POWER READY ACTIVE
V
C
19.8 mm
(0.78 in.)
4321
INPUT
9-30 V
15 W MAX
POWER READY ACTIVE
V
C
TRIG 0
TRIG 1
NI cDAQ-9174
NI CompactDAQ
47.8 mm
(1.88 in.)
63.1 mm
(2.49 in.)
25.0 mm
(0.98 in.)
23.7 mm
(0.94 in.)
20.3mm
(0.80 in.)
31.7 mm
(1.25 in.)
23.2 mm
(0.91 in.)
69.9 mm
(2.75 in.)
51.7 mm
(2.04 in.)
36.4 mm
(1.43in.)
88.1 mm
(3.50 in.)
59.6 mm
(2.35 in.)
165.1 mm
(6.50 in.)
NI cDAQ-9178
NI CompactDAQ
53.8 mm
(2.12 in.)
254.0 mm
(10.00 in.)
159.5 mm
(6.28 in.) 44.1 mm
(1.74 in.)
44.1 mm
(1.74 in.)
58.9 mm
(2.32 in.)

NI cDAQ-9178/9174 User Guide and Specifications 6 ni.com
Mounting the NI cDAQ-9178/9174
You can mount the NI cDAQ-9178/9174 chassis using a desktop, 35 mm DIN-Rail, or panel mount
accessory kit. For accessory ordering information, refer to ni.com.
Caution Your installation must meet the following requirements:
• Allows 25.4 mm (1 in.) of clearance above and below the NI cDAQ-9178/9174 chassis for air
circulation.
•Allows at least 50.8 mm (2 in.) of clearance in front of the modules for common connector
cabling such as the 10-terminal detachable screw terminal connector and, as needed, up to
88.9 mm (3.5 in.) of clearance in front of the modules for other types of cabling. For more
information about mechanical dimensions, refer to ni.com/info and enter the info code
rdcrioconn.
NI 9901 Desktop Mounting Kit
The NI 9901 desktop mounting kit includes two metal feet you can install on the sides of the
NI cDAQ-9178/9174 chassis for desktop use. With this kit, you can tilt the NI cDAQ-9178/9174 chassis
for convenient access to the I/O module connectors. When you install the two metal feet, the two existing
screws on the back side and I/O end of the chassis must be removed, as shown in Figure 4. After
removing the screws, replace them with the two longer screws included in the NI 9901 desktop
mounting kit.
Figure 4. NI 9901 Desktop Mounting Kit
NI cDAQ-9178
NI CompactDAQ

©National Instruments Corporation 7 NI cDAQ-9178/9174 User Guide and Specifications
DIN-Rail Mounting Kits
•NI 9915 DIN-Rail Kit—For the NI cDAQ-9178.
•NI 9912 DIN-Rail Kit—For the NI cDAQ-9174.
Each DIN-Rail kit contains one clip for mounting the chassis on a standard 35 mm DIN-Rail. To mount
the chassis on a DIN-Rail, fasten the DIN-Rail clip to the chassis using a number 2 Phillips screwdriver
and two M4 ×17 screws. The screws are included in the DIN-Rail kit. Make sure the DIN-Rail kit is
installed as illustrated in Figure 5, with the larger lip of the DIN-Rail positioned up. When the DIN-Rail
kit is properly installed, the NI cDAQ-9178/9174 chassis is centered on the DIN-Rail.
Caution Remove the I/O modules before removing the chassis from the DIN-Rail.
Figure 5. DIN-Rail Installation on the NI cDAQ-9178
Panel Mount Kits
•NI 9905 Panel Mount Kit—For the NI cDAQ-9178
•NI 9904 Panel Mount Kit—For the NI cDAQ-9174
To mount the chassis on a panel, align the chassis on the panel mount accessory. Attach the chassis to
the panel mount kit using two M4 ×17 screws as shown in Figure 6. National Instruments provides
these screws with the panel mount kit. You must use these screws because they are the correct depth and
thread for the panel. These slots in the panel mount kit can be used with M4, M5, No. 8, or No.10
panhead screws. Figure 6 illustrates the panel dimensions and installation on the NI cDAQ-9178/9174
chassis. Refer to the documentation included with the panel mount kit for more detailed dimensions.

NI cDAQ-9178/9174 User Guide and Specifications 8 ni.com
Figure 6. Panel Mount Dimensions and Installation on the NI cDAQ-9178/9174
87654321
28.1 mm
(1.11 in.)
48.1 mm
(1.90 in.)
27.0 mm
(1.06 in.)
48.4 mm
(1.91 in.)
INPUT
9-30 V
15 W MAX
POWER READY ACTIVE
PFI 0
PFI 1
V
C
4321
INPUT
9-30 V
15 W MAX
POWER READY ACTIVE
V
C
NI cDAQ-9178
NI CompactDAQ
88.1 mm
(3.47 in.)
NI cDAQ-9174
NI CompactDAQ
88.1 mm
(3.47 in.)
234.95 mm
(9.250 in.)
NI cDAQ-9178
NI CompactDAQ
330.2 mm
(13.00 in.)

©National Instruments Corporation 9 NI cDAQ-9178/9174 User Guide and Specifications
Setting Up the NI cDAQ-9178/9174
Complete the following steps to prepare the NI cDAQ-9178/9174 chassis for use:
1. Before connecting the hardware, install the NI-DAQmx software.
Note The NI-DAQmx software is included on the CD shipped with your kit and is available for
download at ni.com/support. The documentation for NI-DAQmx is available after installation
from Start»All Programs»National Instruments»NI-DAQ. Other NI documentation is available
from ni.com/manuals.
2. Make sure the NI cDAQ-9178/9174 power source is not connected.
3. Attach a ring lug to a 14 AWG (1.6 mm) wire. Connect the ring lug to the ground terminal on the
side of the chassis using the ground screw. Attach the other end of the wire to the system safety
ground as shown in Figure 7.
Figure 7. Ring Lug Attached to Ground Terminal
Note Additionally, attach a wire with a ring lug to all other C Series I/O module cable shields. You
must connect this wire to the ground terminal of the chassis using the ground screw.
4. Squeeze both C Series I/O module latches, insert the I/O module into the module slot, and press
until both latches lock the module in place.
Note In order to meet EMC compliance standards, you must apply the ferrite included with your
chassis to the power and USB cables. For more information about attaching the ferrite to the cables,
refer to the Ferrite Installation for NI cDAQ-9178/9174 Cables document, included with your
chassis.
5. Connect the NI cDAQ-9178/9174 chassis with the supplied USB cable to any available USB port
on your computer. Use the jackscrew on the locking USB cable to securely attach the cable to the
chassis.
6. Connect the supplied power source to the NI cDAQ-9178/9174 chassis. The NI cDAQ-9178/9174
chassis requires an external power supply that meets the specifications in the Power Requirements
section.
1 Attached to System Ground
1

NI cDAQ-9178/9174 User Guide and Specifications 10 ni.com
7. Double-click the Measurement & Automation icon, shown at left, on the desktop to open MAX.
8. Expand Devices and Interfaces.
9. Check that your device appears under Devices and Interfaces. If your device does not appear,
press <F5> to refresh the view in MAX. If your device is still not recognized, refer to ni.com/
support/install for troubleshooting information.
10. Right-click your device and select Self-Test. If you need help during the self-test, select Help»
Help Topics»NI-DAQmx and click MAX Help for NI-DAQmx.
When the self-test finishes, a message indicates successful verification or an error. If an error
occurs, refer to ni.com/support/install for troubleshooting information.
Note When in use, the NI cDAQ-9178/9174 chassis may become warm to the touch. This is normal.
Understanding LED Indications
Power LED
The Power LED indicates whether the NI cDAQ-9178/9174 is receiving power.
Ready LED
The Ready LED is lit when the NI cDAQ-9178/9174 chassis is ready for use. The color indicates
whether the USB connection is Full-Speed or Hi-Speed.
Active LED
The Active LED indicates whether the NI cDAQ-9178/9174 chassis is communicating over the
USB bus.
Table 1. Power LED
LED Definition
Green Power supplied
Off No power supplied
Table 2. Ready LED
LED Definition
Amber Hi-Speed (480 Mbit/sec)
Green Full-Speed (12 Mbit/sec)
Off USB connection is not established
Table 3. Active LED
LED Definition
Amber Power is applied, but USB connection is not established
Green USB traffic present
Off No USB traffic present

©National Instruments Corporation 11 NI cDAQ-9178/9174 User Guide and Specifications
Using the NI cDAQ-9178/9174
The cDAQ system consists of three parts: C Series I/O modules, the cDAQ module interface, and the
USB-STC3, as shown in Figure 8. These components digitize signals, perform D/A conversions to
generate analog output signals, measure and control digital I/O signals, and provide signal conditioning.
Figure 8. NI cDAQ-9178/9174 Block Diagram
C Series I/O Modules
National Instruments C Series I/O modules provide built-in signal conditioning and screw terminal,
spring terminal, BNC, D-SUB, or RJ-50 connectors. A wide variety of I/O types are available, allowing
you to customize the cDAQ system to meet your application needs.
C Series I/O modules are hot-swappable and automatically detected by the NI cDAQ-9178/9174 chassis.
I/O channels are accessible using the NI-DAQmx driver software.
Because the modules contain built-in signal conditioning for extended voltage ranges or industrial signal
types, you can usually make your wiring connections directly from the C Series I/O modules to your
sensors/actuators. In most cases, the C Series I/O modules provide isolation from channel-to-earth
ground.
For more information about which C Series I/O modules are compatible with the NI cDAQ-9178/9174
chassis, refer to the KnowledgeBase document, C Series Modules Supported in the NI cDAQ-9178/9174
CompactDAQ. To access this KnowledgeBase, go to ni.com/info and enter the info code rdcdaq.
Hardware-Timed Versus Static DIO Modules
Digital I/O module capabilities are determined by the type of digital signals that the module is capable
of measuring or generating. Static digital I/O modules are designed for signals that change slowly and
are accessed by software-timed reads and writes. Hardware-timed digital I/O modules are for signals
that change rapidly and are updated by either software-timed or hardware-timed reads and writes. For
more information about digital I/O modules, refer to the Digital I/O section.
cDAQ Module Interface
The cDAQ Module Interface manages data transfers between the USB-STC3 and the C Series
I/O modules. The interface also handles autodetection, signal routing, and synchronization.
C Series
I/O Module
C Series
I/O Module
C Series
I/O Module
cDAQ Module
Interface USB-STC3USB 2.0
Chassis
PFI Terminals

NI cDAQ-9178/9174 User Guide and Specifications 12 ni.com
USB-STC3
The USB-STC3 features independent high-speed data streams; flexible AI, AO, and DIO sample timing;
triggering; PFI signals for multi-device synchronization; flexible counter/timers with hardware gating;
digital waveform acquisition and generation; and static DIO.
AI, AO, and DIO Sample Timing
The USB-STC3 contains advanced AI, AO, and DIO timing engines. A wide range of timing and
synchronization signals are available through the PFI lines. Refer to the Analog Input Timing Signals,
Analog Output Timing Signals, Digital Input Timing Signals, and Digital Output Timing Signals
sections for more information about the configuration of these signals.
Triggering Modes
The NI cDAQ-9178/9174 supports different trigger modes, such as start trigger, reference trigger, and
pause trigger with analog, digital, or software sources. Refer to the Analog Input Triggering, Analog
Output Triggering, and Digital Output Triggering sections for more information.
Independent Data Streams
The NI cDAQ-9178/9174 supports seven independent high-speed data streams, allowing for up to
seven simultaneous hardware timed tasks, such as analog input, analog output, buffered counter/timers,
and hardware-timed digital input/output.
PFI Signals
The PFI signals provide access to advanced features such as triggering, synchronization, and
counter/timers. PFI signals are available through hardware-timed digital input and output modules
installed in up to two chassis slots and through the two PFI terminals provided on the NI cDAQ-9178
chassis. Refer to the PFI section for more information.
Flexible Counter/Timers
The NI cDAQ-9178/9174 includes four general-purpose 32-bit counter/timers that can be used to count
edges, measure pulse-widths, measure periods and frequencies, and perform position measurements
(encoding). In addition, the counter/timers can generate pulses, pulse trains, and square waves with
adjustable frequencies. You can access the counter inputs and outputs using hardware-timed digital I/O
modules installed in up to two slots, or by using the two chassis PFI terminals (NI cDAQ-9178 only).
Refer to the Counters section for more information.
Analog Input
To perform analog input measurements, insert a supported analog input C Series I/O module into any
slot on the cDAQ chassis. The measurement specifications, such as number of channels, channel
configuration, sample rate, and gain, are determined by the type of C Series I/O module used. For more
information and wiring diagrams, refer to the documentation included with your C Series I/O modules.
The NI cDAQ-9178/9174 has three AI timing engines, which means that three analog input tasks can be
running at a time on a chassis. An analog input task can include channels from multiple analog input
modules. However, channels from a single module cannot be used in multiple tasks.
Multiple timing engines allow the NI cDAQ-9178/9174 to run up to three analog input tasks
simultaneously, each using independent timing and triggering configurations. The three AI timing
engines are ai, te0, and te1.
Analog Input Triggering
A trigger is a signal that causes an action, such as starting or stopping the acquisition of data. When you
configure a trigger, you must decide how you want to produce the trigger and the action you want the

©National Instruments Corporation 13 NI cDAQ-9178/9174 User Guide and Specifications
trigger to cause. The NI cDAQ-9178/9174 chassis supports internal software, external digital triggering,
and analog triggering.
Three triggers are available: Start Trigger, Reference Trigger, and Pause Trigger. An analog or digital
trigger can initiate these three trigger actions. Up to two C Series hardware-timed digital input modules
can be used in any chassis slot(s) to supply a digital trigger. To find your module triggering options, refer
to the documentation included with your C Series I/O modules. For more information about using digital
modules for triggering, refer to the Digital I/O section.
AI Start Trigger Signal
Use the Start Trigger signal to begin a measurement acquisition. A measurement acquisition consists of
one or more samples. If you do not use triggers, begin a measurement with a software command. Once
the acquisition begins, configure the acquisition to stop in one of the following ways:
• When a certain number of points has been sampled (in finite mode)
• After a hardware reference trigger (in finite mode)
• With a software command (in continuous mode)
An acquisition that uses a start trigger (but not a reference trigger) is sometimes referred to as a
posttriggered acquisition. That is, samples are measured only after the trigger.
When you are using an internal sample clock, you can specify a default delay from the start trigger to
the first sample.
Using a Digital Source
To use the Start Trigger signal with a digital source, specify a source and an edge. Use the following
signals as the source:
• Any PFI terminal
• Counter nInternal Output
The source also can be one of several other internal signals on your NI cDAQ-9178/9174 chassis. Refer
to the Device Routing in MAX topic in the NI-DAQmx Help or the LabVIEW Help for more information.
The NI-DAQmx Help is available after installation from Start»All Programs»National Instruments»
NI-DAQ»NI-DAQmx Help. To view the LabVIEW Help, select Help»Search the LabVIEW Help in
LabVIEW. Alternately, to download the LabVIEW Help, go to ni.com/manuals.
You also can specify whether the measurement acquisition begins on the rising edge or falling edge of
StartTrigger.
Using an Analog Source
Some C Series I/O modules can generate a trigger based on an analog signal. In NI-DAQmx, this is
called the Analog Comparison Event. When you use an analog trigger source for StartTrigger, the
acquisition begins on the first rising edge of the Analog Comparison Event signal.
Routing AI Start Trigger to an Output Terminal
You can route the Start Trigger signal to any output PFI terminal. The output is an active high pulse.
AI Reference Trigger Signal
Use Reference Trigger to stop a measurement acquisition. To use a reference trigger, specify a buffer of
finite size and a number of pretrigger samples (samples that occur before the reference trigger). The
number of posttrigger samples (samples that occur after the reference trigger) desired is the buffer size
minus the number of pretrigger samples.

NI cDAQ-9178/9174 User Guide and Specifications 14 ni.com
Once the acquisition begins, the NI cDAQ-9178/9174 chassis writes samples to the buffer.
After the NI cDAQ-9178/9174 chassis captures the specified number of pretrigger samples, the
NI cDAQ-9178/9174 chassis begins to look for the reference trigger condition. If the reference trigger
condition occurs before the NI cDAQ-9178/9174 captures the specified number of pretrigger samples,
the NI cDAQ-9178/9174 ignores the condition.
If the buffer becomes full, the NI cDAQ-9178/9174 continuously discards the oldest samples in the
buffer to make space for the next sample. This data can be accessed (with some limitations) before the
NI cDAQ-9178/9174 chassis discards it. Refer to the KnowledgeBase document, Can a Pretriggered
Acquisition be Continuous?, for more information. To access this KnowledgeBase, go to ni.com/
info and enter the info code rdcanq.
When the reference trigger occurs, the NI cDAQ-9178/9174 continues to write samples to the buffer
until the buffer contains the number of posttrigger samples desired. Figure 9 shows the final buffer.
Figure 9. Reference Trigger Final Buffer
Using a Digital Source
To use Reference Trigger with a digital source, specify a source and an edge. Either PFI or one of several
internal signals on the NI cDAQ-9178/9174 chassis can provide the source. Refer to the Device Routing
in MAX topic in the NI-DAQmx Help or the LabVIEW Help for more information.
The NI-DAQmx Help is available after installation from Start»All Programs»National Instruments»
NI-DAQ»NI-DAQmx Help. To view the LabVIEW Help, select Help»Search the LabVIEW Help in
LabVIEW. Alternately, to download the LabVIEW Help, go to ni.com/manuals.
You also can specify whether the measurement acquisition stops on the rising edge or falling edge of the
Reference Trigger.
Using an Analog Source
Some C Series I/O modules can generate a trigger based on an analog signal. In NI-DAQmx, this is
called the Analog Comparison Event.
When you use an analog trigger source, the acquisition stops on the first rising or falling edge of the
Analog Comparison Event signal, depending on the trigger properties.
Routing the Reference Trigger Signal to an Output Terminal
You can route ReferenceTrigger to any output PFI terminal. Reference Trigger is active high by default.
AI Pause Trigger Signal
You can use the Pause Trigger to pause and resume a measurement acquisition. The internal sample
clock pauses while the external trigger signal is active and resumes when the signal is inactive. You can
program the active level of the pause trigger to be high or low.
Reference Trigger
Pretrigger Samples
Complete Buffer
Posttrigger Samples

©National Instruments Corporation 15 NI cDAQ-9178/9174 User Guide and Specifications
Using a Digital Source
To use the Pause Trigger, specify a source and a polarity. The source can be either from PFI or one of
several other internal signals on your NI cDAQ-9178/9174 chassis. Refer to the Device Routing in MAX
topic in the NI-DAQmx Help or the LabVIEW Help for more information.
The NI-DAQmx Help is available after installation from Start»All Programs»National Instruments»
NI-DAQ»NI-DAQmx Help. To view the LabVIEW Help, select Help»Search the LabVIEW Help in
LabVIEW. Alternately, to download the LabVIEW Help, go to ni.com/manuals.
Using an Analog Source
Some C Series I/O modules can generate a trigger based on an analog signal. In NI-DAQmx, this is
called the Analog Comparison Event.
When you use an analog trigger source, the internal sample clock pauses when the Analog Comparison
Event signal is low and resumes when the signal goes high (or vice versa).
Note Pause triggers are only sensitive to the level of the source, not the edge.
Analog Input Timing Signals
Sample Clock
A sample consists of one reading from each channel in the AI task. SampleClock signals the start of a
sample of all analog input channels in the task. SampleClock can be generated from external or internal
sources as shown in Figure 10.
Figure 10. Sample Clock Timing Options
Routing the Sample Clock to an Output Terminal
You can route SampleClock to any output PFI terminal. SampleClock is an active high pulse by default.
AI Sample Clock Timebase
The AI Sample Clock Timebase signal is divided down to provide a source for SampleClock.
SampleClockTimebase can be generated from external or internal sources. SampleClockTimebase is not
available as an output from the chassis.
AI Convert Clock Behavior For Analog Input Modules
Scanned
Scanned C Series analog input modules contain a single A/D converter and a multiplexer to select
between multiple input channels. When the cDAQ Module Interface receives a Sample Clock pulse, it
begins generating a Convert Clock for each scanned module in the current task. Each Convert Clock
Programmable
Clock
Divider
ai/SampleClock
Timebase
PFI
Analog Comparison Event
Ctr nInternal Output
ai/SampleClock
Sigma-DeltaModule Internal Output
Analog Comparison
Event
20 MHz Timebase
80 MHz Timebase
PFI
100 kHz Timebase

NI cDAQ-9178/9174 User Guide and Specifications 16 ni.com
signals the acquisition of a single channel from that module. The Convert Clock rate depends on the
module being used, the number of channels used on that module, and the system Sample Clock rate.
The driver chooses the fastest conversion rate possible based on the speed of the A/D converter for each
module and adds 10 µs of padding between each channel to allow for adequate settling time. This
scheme enables the channels to approximate simultaneous sampling. If the AI Sample Clock rate is too
fast to allow for 10 µs of padding, NI-DAQmx selects a conversion rate that spaces the AI Convert Clock
pulses evenly throughout the sample. NI-DAQmx uses the same amount of padding for all the modules
in the task. To explicitly specify the conversion rate, use the ActiveDevs and AI Convert Clock Rate
properties using the DAQmx Timing property node or functions.
Simultaneous Sample-and-Hold
Simultaneous sample-and-hold (SSH) C Series analog input modules contain multiple A/D converters
or circuitry that allows all the input channels to be sampled at the same time. These modules sample
their inputs on every Sample Clock pulse.
Sigma-Delta
Sigma-delta C Series analog input modules function much like SSH modules, but use A/D converters
that require a high-frequency oversample clock to produce accurate, synchronized data. Sigma-delta
modules in the cDAQ chassis automatically share a single oversample clock to synchronize data from
all sigma-delta modules when they all share the same task. The NI cDAQ-9178/9174 supports a
maximum of two synchronization pulse signals configured for your system. This limits the system to
two tasks with sigma-delta modules.
The oversample clock is used as the AI Sample Clock Timebase. While most modules supply a common
oversample clock frequency (12.8 MHz), some modules, such as the NI 9234, supply a different
frequency. When sigma-delta modules with different oversample clock frequencies are used in an analog
input task, the AI Sample Clock Timebase can use any of the available frequencies; by default, the
fastest available is used. The sampling rate of all modules in the system is an integer divisor of the
frequency of the AI Sample Clock Timebase.
When one or more sigma-delta modules are in an analog input task, the sigma-delta modules also
provide the signal used as the AI Sample Clock. This signal is used to cause A/D conversion for other
modules in the system, just as the AI Sample Clock does when a sigma-delta module is not being used.
When sigma-delta modules are in an AI task, the chassis automatically issues a synchronization pulse
to each sigma-delta modules that resets their ADCs at the same time. Because of the filtering used in
sigma-delta A/D converters, these modules usually exhibit a fixed input delay relative to
non-sigma-delta modules in the system. This input delay is specified in the C Series I/O module
documentation.
Slow Sample Rate Modules
Some C Series analog input modules are specifically designed for measuring signals that vary slowly,
such as temperature. Because of their slow rate, it is not appropriate for these modules to constrain the
AI Sample Clock to operate at or slower than their maximum rate. When using such a module in the
cDAQ chassis, the maximum Sample Clock rate can run faster than the maximum rate for the module.
When operating at a rate faster than these slow rate modules can support, the slow rate module returns
the same point repeatedly, until a new conversion completes. In a hardware-timed task, the first point is
acquired when the task is committed. The second point is acquired after the start trigger as shown in
Figure 11.

©National Instruments Corporation 17 NI cDAQ-9178/9174 User Guide and Specifications
Figure 11. Sample Clock Timing Example
For example, if running an AI task at 1 kHz using a module with a maximum rate of 10 Hz, the slow
module returns 100 samples of the first point, followed by 100 samples of the second point, etc. Other
modules in the task will return 1,000 new data points per second, which is normal. When performing a
single-point acquisition, no points are repeated. To avoid this behavior, use multiple AI timing engines,
and assign slow sample rate modules to a task with a rate at or slower than their maximum rate.
Refer to the KnowledgeBase document, C Series Modules Supported in the NI cDAQ-9178/9174
CompactDAQ, for more information. To access this KnowledgeBase, go to ni.com/info and enter the
info code rdcdaq.
Getting Started with AI Applications in Software
You can use the NI cDAQ-9178/9174 chassis in the following analog input applications:
• Single-Point
•Finite
• Continuous
For more information about programming analog input applications and triggers in software, refer to the
NI-DAQmx Help or the LabVIEW Help for more information.
The NI-DAQmx Help is available after installation from Start»All Programs»National Instruments»
NI-DAQ»NI-DAQmx Help. To view the LabVIEW Help, select Help»Search the LabVIEW Help in
LabVIEW. Alternately, to download the LabVIEW Help, go to ni.com/manuals.
Analog Output
To generate analog output, insert an analog output C Series I/O module in any slot on the
NI cDAQ-9178/9174 chassis. The generation specifications, such as the number of channels, channel
configuration, update rate, and output range, are determined by the type of C Series I/O module used.
For more information, refer to the documentation included with your C Series I/O modules.
You can run one hardware-timed (waveform) analog output task at a time on the NI cDAQ-9178/9174
chassis. At the same time, you can also run one or more software-timed (single-point or immediate)
tasks.
StartTrigger
Datafrom
A/D Conversion
(Slow Module)
SampleClock
DataReturned
to AI Task
1st A/D Conversion 2nd A/D Conversion 3rd A/D Conversion
ABC
AA ABB BC

NI cDAQ-9178/9174 User Guide and Specifications 18 ni.com
For each analog output module, you can either:
• Assign all of the channels on the module to the hardware-timed task.
• Assign all of the channels on the module to one or more software-timed tasks.
On a single AO module, you cannot assign some channels to a hardware-timed task and other channels
(on the same module) to a software-timed task.
Analog Output Data Generation Methods
When performing an analog output operation, you either can perform software-timed or hardware-timed
generations. Hardware-timed generations must be buffered.
Software-Timed Generations
With a software-timed generation, software controls the rate at which data is generated. Software sends
a separate command to the hardware to initiate each DAC conversion. In NI-DAQmx, software-timed
generations are referred to as on-demand timing. Software-timed generations are also referred to as
immediate or static operations. They are typically used for writing out a single value, such as a constant
DC voltage.
The following considerations apply to software-timed generations:
• If any AO channel on a module is used in a hardware-timed (waveform) task, no channels on that
module can be used in a software-timed task.
• You can configure software-timed generation to simultaneously update.
• Only one simultaneous update task can run at a time.
• Simultaneous update is not restricted to 16 channels.
• A hardware-timed AO task and a simultaneous update AO task cannot run at the same time.
Hardware-Timed Generations
With a hardware-timed generation, a digital hardware signal controls the rate of the generation. This
signal can be generated internally on the chassis or provided externally.
Hardware-timed generations have several advantages over software-timed acquisitions:
• The time between samples can be much shorter.
• The timing between samples is deterministic.
• Hardware-timed acquisitions can use hardware triggering.
Hardware-timed AO operations on the NI cDAQ-9178/9174 chassis must be buffered.
Buffered Analog Output
A buffer is a temporary storage in computer memory for generated samples. In a buffered generation,
data is moved from a host buffer to the NI cDAQ-9178/9174 onboard FIFO before it is written to the
C Series I/O modules.
One property of buffered I/O operations is sample mode. The sample mode can be either finite or
continuous:
• Finite—Finite sample mode generation refers to the generation of a specific, predetermined
number of data samples. After the specified number of samples is written out, the generation stops.

©National Instruments Corporation 19 NI cDAQ-9178/9174 User Guide and Specifications
• Continuous—Continuous generation refers to the generation of an unspecified number of samples.
Instead of generating a set number of data samples and stopping, a continuous generation continues
until you stop the operation. There are three different continuous generation modes that control
how the data is written. These modes are regeneration, onboard regeneration, and
non-regeneration:
– In regeneration mode, you define a buffer in host memory. The data from the buffer is
continually downloaded to the FIFO to be written out. New data can be written to the host
buffer at any time without disrupting the output. There is no limitation on the number of
waveform channels supported by regeneration mode.
– With onboard regeneration, the entire buffer is downloaded to the FIFO and regenerated from
there. After the data is downloaded, new data cannot be written to the FIFO. To use onboard
regeneration, the entire buffer must fit within the FIFO size. The advantage of using onboard
regeneration is that it does not require communication with the main host memory once the
operation is started, which prevents problems that may occur due to excessive bus traffic or
operating system latency. There is a limit of 16 waveform channels for onboard regeneration.
– With non-regeneration, old data is not repeated. New data must continually be written to the
buffer. If the program does not write new data to the buffer at a fast enough rate to keep up
with the generation, the buffer underflows and causes an error. There is no limitation on the
number of waveform channels supported by non-regeneration.
Analog Output Triggering
Analog output supports two different triggering actions:
• Start Trigger
• Pause Trigger
• AO Start Trigger
• AO Pause Trigger
An analog or digital trigger can initiate these actions. Up to two C Series hardware-timed digital input
modules can be used in any chassis slot to supply a digital trigger. An analog trigger can be supplied by
some C Series analog modules.
AO Start Trigger Signal
Use the AO Start Trigger (ao/StartTrigger) signal to initiate a waveform generation. If you do not use
triggers, you can begin a generation with a software command. If you are using an internal sample clock,
you can specify a delay from the start trigger to the first sample. For more information, refer to the
NI-DAQmx Help. The NI-DAQmx Help is available after installation from Start»All Programs»
National Instruments»NI-DAQ»NI-DAQmx Help.
Using a Digital Source
To use ao/StartTrigger, specify a source and a rising or falling edge. The source can be one of the
following signals:
• A pulse initiated by host software
• Any PFI terminal
• AI Reference Trigger
• AI Start Trigger
The source also can be one of several internal signals on the NI cDAQ-9178/9174 chassis. Refer to the
Device Routing in MAX topic in the NI-DAQmx Help or the LabVIEW Help for more information.

NI cDAQ-9178/9174 User Guide and Specifications 20 ni.com
The NI-DAQmx Help is available after installation from Start»All Programs»National Instruments»
NI-DAQ»NI-DAQmx Help. To view the LabVIEW Help, select Help»Search the LabVIEW Help in
LabVIEW. Alternately, to download the LabVIEW Help, go to ni.com/manuals.
You also can specify whether the waveform generation begins on the rising edge or falling edge of
ao/StartTrigger.
Using an Analog Source
Some C Series I/O modules can generate a trigger based on an analog signal. In NI-DAQmx, this is
called the Analog Comparison Event, depending on the trigger properties.
When you use an analog trigger source, the waveform generation begins on the first rising or falling edge
of the Analog Comparison Event signal, depending on the trigger properties. The analog trigger circuit
must be configured by a simultaneously running analog input task.
Routing AO Start Trigger Signal to an Output Terminal
You can route ao/StartTrigger to any output PFI terminal. The output is an active high pulse.
AO Pause Trigger Signal
Use the AO Pause Trigger signal (ao/PauseTrigger) to mask off samples in a DAQ sequence. When
ao/PauseTrigger is active, no samples occur, but ao/PauseTrigger does not stop a sample that is in
progress. The pause does not take effect until the beginning of the next sample.
When you generate analog output signals, the generation pauses as soon as the pause trigger is asserted.
If the source of the sample clock is the onboard clock, the generation resumes as soon as the pause
trigger is deasserted, as shown in Figure 12.
Figure 12. ao/PauseTrigger with the Onboard Clock Source
If you are using any signal other than the onboard clock as the source of the sample clock, the generation
resumes as soon as the pause trigger is deasserted and another edge of the sample clock is received, as
shown in Figure 13.
Figure 13. ao/PauseTrigger with Other Signal Source
Pause Trigger
Sample Clock
Pause Trigger
Sample Clock