Data Translation DT9837, User Manual

Page: 75 / 150

Principles of Operation
75
Data Format and Transfer
DT9837 Series modules use offset binary data encoding, where 000000 represents negative
full-scale, and FFFFFFh represents positive full-scale. Use software to specify the data
encoding as binary. The ADC outputs FFFFFFh for above-range signals, and 000000 for
below-range signals.
Before you begin acquiring data, you must allocate buffers to hold the data. A Buffer Done
event is returned whenever a buffer is filled. This allows you to move and/or process the data
as needed.
We recommend that you allocate a minimum of two buffers for continuous analog input
operations. Data is written to multiple allocated input buffers continuously; when no more
empty buffers are available, the operation stops. The data is gap-free.
Note: The DT9837A and DT9837 C modules have an input FIFO of 2 kBytes; the DT9837B
has an input FIFO of 4 kBtyes; and the DT9837 does not use an input FIFO.
Error Conditions
DT9837 Series modules report any overrun errors by sending an overrun event to the
application program. This event indicates that data buffers are not being sent from the module
to the host fast enough, and the A/D converter ran out of buffers. To avoid this error, try one
or more of the following:
• Reduce the clock rate of the A/D
• Increase the size of the buffers
• Increase the number of buffers
• Close any other applications that are running
• Run the program on a faster computer
If one of these error conditions occurs, the module stops acquiring and transferring data to the
host computer.