Enum used in our yard gate:

TYPE YARDGATESTATE :
(
	YARD_UNINIT := 0,
	YARD_INIT,
	YARD_CLOSED,
	YARD_OPENING,
	YARD_OPENED ,
	YARD_CLOSING,
	YARD_BLOCKED
);
END_TYPE

Each state is described by an enum. Some people describe the states by a number. For example, YARD_CLOSED can be expressed by 50. But this may not be expressive. The above enums will describe the state of the Gate at a particular time. First, the state will be uninitialized, and then the initialization phase. The initialization phase depends on what kind of actuators. The actuators can be compressed air systems, hydraulic power, or direct AC/DC motor. In this gate project, we are using a direct DC motor.

If we express by number, it can be like the following but not so much expressive as compared to the code in the handling state section.

CASE state OF
10:
.........
20 :
.........
50 :
.........
END_CASE

Now, let’s create a skeleton state machine with the above enums. We declare a variable state of type YARDGATESTATE and initialize it to YARD_UNINIT. We use a case  (switch in other programming languages).

state     : YARDGATESTATE := YARD_UNINIT;
stateInit : BOOL := FALSE;

With this initialization, in each PLC cycle, the code will check that the state is YARD_UNINIT and execute the statements under this case. If it finds any logic to jump to another state it will do, but in our skeleton, there is no way to go to other states. We need to change the code for that. If you have noticed, we have another variable called stateInit which is initialized to FALSE. When the state changes then we set this variable to FALSE. This will give a chance to execute state-specific code. In each state, we set the variable to TRUE for avoiding multiple time execution.

CASE state OF
    YARD_UNINIT:
    IF NOT stateInit THEN
        stateInit := TRUE;
    END_IF
    YARD_INIT  :
    IF NOT stateInit THEN
        stateInit := TRUE;
    END_IF
    YARD_CLOSED:
    IF NOT stateInit THEN
        stateInit := TRUE;
    END_IF
    YARD_OPENING :
    IF NOT stateInit THEN
        stateInit := TRUE;
    END_IF
    YARD_OPENED :
    IF NOT stateInit THEN
        stateInit := TRUE;
    END_IF
    YARD_CLOSING:
    IF NOT stateInit THEN
        stateInit := TRUE;
    END_IF
    YARD_BLOCKED :
    IF NOT stateInit THEN
        stateInit:= TRUE;
    END_IF
END_CASE

In the above code, there is no way to jump to other states. It will be all the time in the same state according to initialization. Here will show how we jump to other states.

YARD_UNINIT:
    IF NOT stateInit THEN
	stateInit := TRUE;
    END_IF
	state :=YARD_INIT;	
	stateInit := FALSE;
		
YARD_INIT  :
    IF NOT stateInit THEN
	stateInit := TRUE;
	stateTimer(IN:=FALSE);	
    END_IF	
	stateTimer(IN:=TRUE, PT:= initTime);
    IF stateTimer.Q THEN		
	state :=YARD_CLOSED;	
	stateInit := FALSE;	
    END_IF		
...	

When we declared the state variable, we initialized it to YARD_UNINIT. In this case, we are changing the state to YARD_INIT directly which means, in the next PLC cycle the state will be YARD_INIT.

Now in the YARD_INIT state, we initialize a timer (just to simulate, in a real system we might get some sensor input or wait for another device status). When the timer expires the state takes a new value (in this case YARD_CLOSED) and we set stateInit to FALSE.