servo runaway prevention

A poster recently commented that he experienced a servo runaway and
some slight machine damage when the encoder cable fell off.

It would seem to me that a proper following error setting should shut
the machine down very quickly in such a case it is connected to an
emergency stop circuit which kills power to the amps. Alternatively,
a circuit could be added to the amps which would shut them down if no
motion [in the correct direction] is detected by the time the output
ramp up reaches a certain limit.

Having fun with the stepper machine & thinking about the servos that
will be on the next one,

Chris