From the Programming Guide:
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There are four QuadratureEncoder modules in the FPGA and 8 Counter modules that can operate as quadrature encoders. One of the differences between the encoder and counter hardware is that encoders can give an oversampled 4X count using all 4 edges of the input signal. Counters can either return a 1X or 2X result based on one of the input signals. If 1X or 2X is chosen in the Encoder constructor a Counter module is used with lower oversampling and if 4X (default) is chosen, then one of the four encoders is used.
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The LabVIEW encoder open will return an error if you try to allocate more then 4 encoders (with 4x). In the C++ Encoder::InitEncoder, there is a comment that says
Code:
//TODO: need to check for errors here