Command SetExternalStepBkwInput :

Configures external step trigger input for selected axis. A trigger on this input results in a backward single step.
Trigger input: 0 -> disabled, 1 ...6 input trigger.

SetExternalStepBkwInput Definition
Input Argument	Tango::DEVVAR_LONGARRAY	axisNo, trigger input
Output Argument	Tango::DEV_VOID
DisplayLevel	OPERATOR	..
Inherited	false	..
Abstract	false	..
Polling Period	Not polled	..
Command allowed for	All states	..