SE701:FB:Vyatkin BasicFB cr
Sequential Axiomatic Model of Execution of Basic Function Blocks in IEC61499
No single execution model of the ECC (state diagram), Harel's statechart used as a basis but still doesnt have a definite definition
Still much confusion around the event passing and handling in the current standard, questions that remain consist of:
- Time interval between events
- If there are two events which one arrives first
- event arrives on a busy block
Multiple Applicable Transitions
If multiple transitions apply to an ECC then the one that is defined first in the xml is used. Unfortunatly there is no graphical representation of this on the diagrams, this could lead to confusion.
Lifetime of event
Life of an event is unspecified. Using hte postulates defined within this paper an event can only be used as condition for the first state transistion. This postulate is an addition to the standard as it does not define the lifetime of an event when determine state transisions
draft standards orignally defined boolean latchs on when to clear the event these were removed for hte final standard.
no eventful transitions
State transitions with no event conditions are not completely defined. DOos the state change as soon as the non-event condition is true? does it wait for another event before evaluting? if another event arises and multiple ocnditions are true? FBDK (using NPMTR exectuion model) says non-event conditions are evaluted only when it first moves to that state if all transitions are false it remains there. If there are no event transitions for the current state the ECC will freeze and recommends you can gaurentee that all states have an eventful transitions or at least one (prefferably one) condition is true whenever it reaches that state.
Paper claims NPTMR is counterintuite as eventful and noneventful transitions are treated in seperate ways
Issueing of output events
Postulates described within state that the output event is issued immediatly after an action and in the order they were issued.
Other output options could have been after all actions in a state, or after a single run of the FB is completed
The outputs are 'scheduled' as soon as outputted as per the standard, what scheduled means is still unclear.
Sequential Hypothesis
6th postulate states if multiple events are issued they are done sequentially from this we can state
In a resource there is no more than one non-SIFB function block in the run state at any moment of time
This is also depended on algorithms are short otherwise long delys would ensue and throw off hte schedulder