A Capture object manages a single high-speed position capture logic block. It represents the physical hardware capture logic and data. When configured and armed, the capture logic block can latch an axis' position based on one or more source input triggers.

There are two capture types: Position-based and Time-based capture. For the Position type, the position counters are latched in the FPGA and are read directly by the controller. This methodology works well for incremental quadrature encoders. For the Time type, the FPGA latches the clock and the controller reads the clock value and position value for that sample period. The controller interpolates the position value from the previous sample's position, the present sample's position, and the clock data. This methodology works well for cyclic feedback data that is digitally transmitted from the drive to the FPGA. Many drives have a proprietary serial encoder that decodes the encoder position and sends the position information to the FPGA once per sample. In these cases, time-based capture is more accurate than position-based capture.

NOTE: Time-based capture will only work correctly if the speed of an axis is less than 344 million counts per second.

For an overview of the Capture Engine, see diagram below.

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Overview of Capture