LabVIEW Encoder not reliably returning Rate

[scooperman]

A look inside any industrial control encoder interface shows that typically there is quite a lot of digital filtering done on the A,B,I signals before introduction to the count tracking logic. Take a look at the old HP HCTL-20xx data sheet as an example. They employed multiple stages of demetastabilizing flip-flops, these are necessary because any input signal is asynchronous to the processor clock, meaning that sooner or later you will get a clock edge which is coincident with a signal edge, causing flip-flops to make wrong decisions. After the inputs are synchronized, then more flip-flops are needed to verify that the A,B signals are not in illegal states, because real encoders do not produce signals which are in strict quadrature. After the signals are verified good, then the up/down logic may increment or decrement the encoder tracking count. I suspect there is none of this within the NI device and we can't do much to fix it. On the electrical side, we can filter the power connections to the encoder, shield the wires, shorten the wires, and provide the correct pullup load (US Digital calls for 2.7K ohm pullups) that's about it.