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Chapter 7. openETCS Meta Model
gEVCStateMachine
ModeTransition
ModeTransition
CurrentState
1...1
NextState
1...1
CurrentState
1...1
NextState
1...1
oMode
1...*
oMode
0...*
oMode
0...*
oMode
1...*
Figure 7.3.: gEVCStateMachine bindings
It must be observed that the gEVCStateMachine graph itself does not provide any syntax
to define temporal behaviour, but the evaluation of its guard objects is manipulated in
gMainFunctionBlock graphs where causal and time-dependent behaviour can be described, as
shown below.
A certain question arises if the gained abstraction for modes and their transitions in the
openETCS meta model is directly evaluated here. A representation of 8 modes and 30 guard
conditions for transitions would result in a huge graph, which would be difficult to read
and therefore would not provide good abstraction. On the other hand, the transition table
in [88, p. 40] already is somehow in an optimized form. Fortunately, MetaEdit+ not only can
instantiate graph types directly with objects and bindings but also in matrix representation [56].
In contrary to the transition table, the modes are listed on the row and column index of the
matrix while the transitions or rather the ModeTransition relationships are located in the cells.
Any transition between a state i to state j can be found then in the cell (i, j). An example
taken from [88, p. 40] is shown in Figure 7.4. The EVC can be switched from the mode NP to
NP
-p2-
SB · · ·
. .
. ..
Figure 7.4.: Simple example from SRS transition table
SB under the condition / guard “4” and from SB to NP under condition “29”. Both transitions
have the priority 2. The result of converting a gEVCStateMachine to a matrix instance is
presented in Figure 7.5. A more concrete example for a gEVCStateMachine matrix is presented
in Chapter 10.
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