Digital Output Triggering Signals - National Instruments NI cDAQTM-9138 User Manual

Eight-slot stand-alone chassis with integrated controller

Table of Contents

Buffered Digital 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 cDAQ chassis onboard FIFO before it is

written to the C Series I/O module(s).

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.

•

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.

–

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.

Install parallel DO modules in slots 1 through 4 to maximize accessible FIFO

Note

size because using a module in slots 5 through 8 will reduce the accessible FIFO size.

–

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.