When two components in an Iconic Diagram are connected, one component will always compute the across variable, while the opposite component will always determine the through variable.
We can therefore also interpret a connection as a bilateral signal connection (across-signal and through-signal), of which the directions are opposite to each other.
With "direction" we mean the computational direction, just like signals in a block diagram. In a block diagram the across and through variables, which together form the flow of power, are not shown as a couple. This breaks up the symmetry between the physical system and the the structure of the model.
A connection as a bilateral signal flow.
The interpretation of a connection as a bilateral signal flow, does not fix the individual direction of the across variable and the through variable. It only means the direction of the across variable and through variable are opposite.
The specific computational direction of across and and through variables is called causality. In 20-sim an advanced algorithm detects the possible causalities of each submodel and tries to combine these in such a way, that optimal simulation code will be generated. The result can be inspected using the Causality Info command from the View menu. The result of this command is shown in the example below. The computational direction of forces and velocities is clearly shown for all the connections.