User`s guide

Using Analog I/O Drivers
4-29
Using Analog I/O Drivers
Control systems have unique requirements for I/O devices that the Real-Time
Windows Target supports.
This section includes the following topics:
• “I/O Driver Characteristics” on page 4-29
• “Normalized Scaling for Analog Inputs” on page 4-30
I/O Driver Characteristics
The Real-Time Windows Target uses off-the-shelf I/O boards provided by many
hardware vendors. These boards are often used for data acquisition
independently of the Real-Time Windows Target. In such environments, board
manufacturers usually provide their own I/O device drivers for data acquisition
purposes. This use differs significantly from the behavior of drivers provided
with the Real-Time Windows Target.
In data acquisition applications, data is often collected in a burst or frame
consisting of many points, perhaps 1,000 or possibly more. The burst of data
becomes available once the final point is available. This approach is not
suitable for use in automatic control applications since it results in latencies
equal to 1000 * Tsample for each point of data.
In contrast, drivers used by the Real-Time Windows Target capture a single
point of data at each sample interval. Considerable effort is made to minimize
the latency between collecting a data point and using the data in the control
system algorithm. This is the reason why a board that specifies a maximum
sample rate (for data acquisition) might be described as achieving sample rates
well in excess of the rates that are achievable in the Real-Time Windows
Target. For data acquisition, such boards are usually acquiring data in bursts
and not in a point-by-point fashion, which is more appropriate for stable control
systems.