> I wonder how a 'lost step' would look and if the voltage
> measurement of the phases while not driven might provide
> hints if and before it happens?

Possibly. Voltage developed is going to be relevant to speed.

The driver knows the speed that the motor *should* be stepping
at, because it supplies the pulses. It can also tell if the rotor has
slipped (lost step) by monitoring the undriven coil, although the
ease of doing that will depend a lot on the type of motor and how
much the coils interact

Here's something I devised for a learning project

http://www.piclist.com/techref/io/sensor/pos/enc/stepper_as_encoder.htm

Moving the motor(s) manually and recording the coil outputs
allows you to replicate the movement under power later

Joe


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