User manual

Chapter 4 NI-DAQ Double Buffering

The double-buffered output operation begins when the output device begins

outputting data from the first half of the circular buffer (Figure 4-4a). After

the device begins retrieving data from the second half of the circular buffer,

NI-DAQ can copy the prepared data from the transfer buffer to the first half

of the circular buffer (Figure 4-4b). Your application can then update the

data in the transfer buffer. After the output device is finished with the

second half of the circular buffer, the device returns to the first half buffer

and begins outputting updated data from the first half. NI-DAQ can

now copy the transfer buffer to the second half of the circular buffer

(Figure 4-4c). The data in the transfer buffer is again available for update

by your application. The process can be repeated endlessly to produce

a continuous stream of output data from your application. Notice that

Figure 4-4d is equivalent to the step in Figure 4-4b and is the start of a

two-step cycle.

Potential Setbacks

Like double-buffered input, double-buffered output has two potential

problems. The first is the possibility of the output device retrieving and

outputting the same data before NI-DAQ has updated the circular buffer

with new data from the transfer buffer. This situation is illustrated by

Figure 4-5.

Notice in Figure 4-5b, NI-DAQ has missed the opportunity to copy data

from the transfer buffer to the first half of the circular buffer while the

output device is retrieving data from the second half. As a result, the

device begins to output the original data in the first half of the circular

buffer before NI-DAQ has updated it with data from the transfer buffer

(Figure 4-5c). To guarantee uncorrupted output data, NI-DAQ is forced

to wait until the device finishes retrieving data from the first half before

copying the data from the transfer buffer. After the device has begun to

output the second half, NI-DAQ copies the data from the transfer buffer

to the first half of the circular buffer (Figure 4-5d).