ABB AC 800M Library Object Style Manual page 38

Connections
Point-to-Point Connection using ControlConnection Data Type
A natural way to represent the main signal flow of Control Modules is by structured
connections. In this way a single connection, which preferably is done graphically,
handles the main signal flow between two adjacent objects.
Bidirectional connections should have components for each direction. The
substructures are preferably named Forward and Backward, but other names may be
used if they better fit the context they are used in, for example Previous and Next,
Upstream and Downstream, etc. This makes it easier for the programmer to handle
communication between objects. It also supports data communication over task and
system borders, since it is clear what components are sent in each direction.
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Figure 4. Point-to-point connection between two objects. The node names reflect the
main direction of the signal. The blow-up shows in principle data flow.
In each of the directional substructures, for example Forward and Backward, a
component Connected should exist. All objects shall set the Connected component
true in the direction-component they are allowed writing to. The Connected
component can then be checked by the object on the other side and determine if
there is a connected object or not. This is useful when an object connection is
optional, thus may not be connected.
Point-to-Point Connection Using Reverse Attribute
In bi-directional data types, the components used to transfer data in backward
direction must have the reverse attribute. Otherwise the data type cannot be used for
parameters with direction defined.
The reverse components must be grouped in consecutive order at the end of the
data type. They can also be put in a sub structure at the end of the data type.
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Forward
Backward
Direction
Section 3 Parameter Interface
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