Chapter 4: API and Third-Party Software
33
• In Synchronous Mode (known as Single Update on
the AO subsystem) there is a set of calls available
which gives direct and immediate access to the
DACs (DIO ports and DSP Timers). Timed access can
be made using the operating system based timing
loops.
• In Asynchronous Mode (which has two sub-modes
on PowerDAQ AO boards – event-based and
regenerate mode) , a dedicated acquisition buffer is
allocated in host PC memory. This buffer is divided
by the number of logical blocks called frames,
usually between 4 and 16 frames should be
allocated. The following steps should be considered
when Asynchronous acquisition mode is used:
# User application uses API calls to allocate the
acquisition buffer and fill it with initial data for all
output subsystems.
# Subsystem configuration word should be assembled
using the API constants provided.
# The user application should define a set of events
from the board. A typical set of events include data
availability events and error events
# Start of asynchronous operation
# WaitForSingleObject operating system function call
or an equivalent should be used to check for events
from the board. It is preferable to put this call into
a separate thread. After an event, the application
should re-initialize it and process the data.
# Terminate asynchronous operation
# Auto-regenerate mode user application can allocate
one or more frames in the buffer, fill it with data
and start acquisition. The PowerDAQ driver will
control event handling and supply more data into
the on-board FIFO as requested. Note, that frame
size cannot exceed 64Mbytes. The size of the
buffer itself is limited by the amount of Physical
memory installed on your PC.
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