User manual

Chapter 4 NI-DAQ Double Buffering

For the previously described situation, NI-DAQ returns an overwrite

before copy warning (overWriteError). This warning indicates that the

data in the transfer buffer is valid, but some earlier input data has been lost.

Subsequent transfers will not return the warning as long as they keep pace

with the DAQ device as in Figure 4-1.

The second potential problem occurs when an input device overwrites data

that NI-DAQ is simultaneously copying to the transfer buffer. NI-DAQ

returns an overwrite error (overWriteError) when this occurs. The

situation is presented in Figure 4-3.

Figure 4-3. Double-Buffered Input with an Overwrite

In Figure 4-3b, NI-DAQ has started to copy data from the first half of the

circular buffer into the transfer buffer. However, NI-DAQ is unable to copy

theentirehalfbeforetheDAQdevicebeginsoverwritingdatainthefirst

half buffer (Figure 4-3c). Consequently, data copied into the transfer buffer

might be corrupted; that is, it might contain both old and new data points.

Future transfers will execute normally as long as neither of the problem

conditions re-occur.

Incoming

Device Data

Circular Buffer

Transfer Buffer

Empty

Buffer

Untransferred

Data

Transferred

Data

Corrupted

Data

Overwrite Error