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Chapter 7. openETCS Meta Model
can be only used at the beginning of a telegram to define a telegram header [85]. Therefore,
they only can be modelled in the order between the oRootNode object and the first oPacket or
oAnyPacket instance. The VariableOrder relationship is used between the connection between
two oVariableInstance objects. PacketOrder is valid between oAnyPacket and/or oPacket types,
and between the last oVariableInstace and the two packet types.
The oAnyPacket object type is different from the others because it cannot be directly found
in SRS [85]. It can be used to model that at a certain point in the variable-packet order
several oPacket objects are possible. The concrete set of oPacket objects is specified in the
decomposition of the oAnyPacket instance. In that way, ETCS telegrams only differing from
each other in a certain packet can be easier and faster modelled.
7.3.7. gPacket Graph Type
Similar to the gTelegram graph type, the gPacket graph type is used to specify the order of
oVariableInstance objects in a certain ETCS packet [85]. Three object types are provided:
oRootNode defines the start point of a packet order.
oVariableInstance defines a certain ETCS variable of a certain type [85]. Therefore, the
oVariableInstance object type holds an oVariableType object as property. The oVariableType
type is not graphically used (only as property) and specifies the type of an
oVariableInstance object, which is similar to a template-mechanism [79] in object-oriented
programming languages. The oVariableType type holds several properties for defining
the attributes of an oVariableInstance object:
1. size in bits
2. the variable’s resolution [85]
3. the variable’s physical unit
oLeafNode determines the last gPacket object in an order.
The advantage of defining a property of oVariableType object type is that the concrete
oVariableType objects can be reused for several oVariableInstance objects.
Apart from enabling the specification of variable orders according to the ETCS specifications
[85], also a possibility to model the scaling of certain variables is needed. Scaling means
that a value of a certain variable can scale (by multiplication) the value of another. This is
modelled by the additional Scaling relationship while the VariableOrder is used for specifying
the oVariableInstance object order. The full syntax is illustrated in Figure 7.12.
Moreover, the ETCS language specification [85] defines so-called iterating variables. This
concept is similar to arrays in C/C++ [79] or other high-level programming languages. A
certain variable holds the size of an array and the following variables are components of an
array of the specified size. An iterating variable is not modelled by a relationship but by
graphical containment, as introduced in Chapter 4. Therefore, oVariableInstance objects that
are iterated by another oVariableInstance object are drawn within the iterating instance. In
this way, it is also possible to specify nested iterations.
A specialisation of iterating variables are so-called conditional iterator variables. In contrast
to the normal iteration concept, conditional iterators do not iterate the graphically contained
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