State Machine

The state machine is a convenient LabVIEW construct where a case structure is contained with a while loop. Execution of particular cases in the structure is determined by the output from the previous case (or in the instance of the first execution) by the control selector input. The control of the order of case execution is controlled through the use of a shift register. By wiring the output from one case as the determinate to the subsequent case (wiring to the shift register on the right hand side) and wiring the input to the case selector from the left hand side shift register, operations can be ordered according to which state is executed. See attached image.

The state machine has the advantage of allowing several steps to be linked in series so that each individual step can be executed (or debugged) easily. The trap for the inexperienced programmer is the use of the state machine to step in a haphazard fashion through the cases in the loop. This can lead to complicated pathways through the state machine reminiscent of convoluted goto statements in conventional text based languages. However state machines are a valuable tool and can be used very effectively with this in mind.

State machines can be based around integers, strings or enumerated types. The relative merits of each have been discussed in detail elsewhere. In summary an enumerated type state machine has the advantage of being automatically updated when the enum is changed (if the enum is type def'ed) but has the disadvantage of requiring careful checking of all the code if new states (enums) are added or redundant states deleted. A string state machine has the advantage of easily added or removed states, but has the disadvantage of spelling mistakes causing prorgamming errors (Note: this can be reduced by including the "Default" state showing the mispelt case in a popup dialog box).