Encoders have no velocity output

[andypugh]

The LinuxCNC encoder drivers have a velocity output pin. (typically both rpm and rps for convenience)

This is useful for velocity-feedback applications, for example with spindles.

A reason to do this inside the driver rather than with an external ddt is that there is a large step-change in position output on index-reset, which appear to cause problems under some circumstances.
See: https://forum.linuxcnc.org/38-general-linuxcnc-questions/45882-rigid-tapping-and-pid-control#243134

Velocity calculation inside the driver can easily compensate for index-reset, whereas compensating externally is troublesome (and is something that it is easy to overlook the necessity for)