Digital Input Timing Signals

The CompactRIO controller features the following digital input timing signals:

• DI Sample Clock Signal*
• DI Sample Clock Timebase Signal
• DI Start Trigger Signal*
• DI Reference Trigger Signal*
• DI Pause Trigger Signal*

Signals with an * support digital filtering. Refer to PFI Filters for more information.

DI Sample Clock Signal

Use the DI Sample Clock signal to sample digital I/O on any slot using parallel digital modules, and store the result in the DI waveform acquisition FIFO. If the CompactRIO controller receives a DI Sample Clock signal when the FIFO is full, it reports an overflow error to the host software.

A sample consists of one reading from each channel in the DI task. DI Sample Clock signals the start of a sample of all digital input channels in the task. DI Sample Clock can be generated from external or internal sources as shown in the following figure.

Routing DI Sample Clock to an Output Terminal

You can route DI Sample Clock to any output PFI terminal.

DI Sample Clock Timebase Signal

The DI Sample Clock Timebase signal is divided down to provide a source for DI Sample Clock. DI Sample Clock Timebase can be generated from external or internal sources. DI Sample Clock Timebase is not available as an output from the controller.

Using an Internal Source

To use DI Sample Clock with an internal source, specify the signal source and the polarity of the signal. Use the following signals as the source:

• it Sample Clock
• ot Sample Clock
• Counter n Internal Output
• Frequency Output
• DI Change Detection Output

Several other internal signals can be routed to DI Sample Clock. Refer to the "Device Routing in MAX" topic in the NI-DAQmx Help or the LabVIEW Help for more information.

Using an External Source

You can route the following signals as DI Sample Clock:

• Any PFI terminal
• Analog Comparison Event (an analog trigger)

You can sample data on the rising or falling edge of DI Sample Clock.

Routing DI Sample Clock to an Output Terminal

You can route DI Sample Clock to any output PFI terminal. The PFI circuitry inverts the polarity of DI Sample Clock before driving the PFI terminal.

DI Start Trigger Signal

Use the DI 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 delay from the start trigger to the first sample.

Using a Time Source

To use the Start Trigger signal with a time source, configure a specific time in NI-DAQmx. Refer to the "Timestamps" and "Time Triggering" topics in the NI-DAQmx Help for more information on accessing time-based features in the NI-DAQmx API.

Using a Digital Source

To use DI Start Trigger with a digital source, specify a source and a rising or falling edge. Use the following signals as the source:

• Any PFI terminal
• Counter n Internal Output

The source also can be one of several other internal signals on the CompactRIO controller. Refer to the "Device Routing in MAX" topic in the NI-DAQmx Help or the LabVIEW Help for more information.

Routing DI Start Trigger to an Output Terminal

You can route DI Start Trigger to any output PFI terminal. The output is an active high pulse.

DI Reference Trigger Signal

Use a reference trigger signal 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.

Once the acquisition begins, the CompactRIO controller writes samples to the buffer. After the CompactRIO controller captures the specified number of pretrigger samples, the controller begins to look for the reference trigger condition. If the reference trigger condition occurs before the CompactRIO controller captures the specified number of pretrigger samples, the controller ignores the condition.

If the buffer becomes full, the CompactRIO controller 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 CompactRIO controller discards it. Refer to the Can a Pretriggered Acquisition be Continuous? document for more information. To access this document, go to ni.com/info and enter the Info Code rdcanq.

When the reference trigger occurs, the CompactRIO controller continues to write samples to the buffer until the buffer contains the number of posttrigger samples desired. The following figure shows the final buffer.

Using a Digital Source

To use DI Reference Trigger with a digital source, specify a source and a rising or falling edge. Either PFI or one of several internal signals on the CompactRIO controller can provide the source. Refer to the "Device Routing in MAX" topic in the NI-DAQmx Help or the LabVIEW Help for more information.

Routing DI Reference Trigger Signal to an Output Terminal

You can route DI Reference Trigger to any output PFI terminal. Reference Trigger is active high by default.

DI Pause Trigger Signal

You can use the DI Pause Trigger signal 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.

Using a Digital Source

To use DI Pause Trigger, specify a source and a polarity. The source can be either from PFI or one of several other internal signals on your CompactRIO controller. Refer to the "Device Routing in MAX" topic in the NI-DAQmx Help or the LabVIEW Help for more information.