The input and output is a signal flow through each subsystem.
The input and output represent the causal signal used in the calculation mode.
Block diagram is a computational structure and do not reflect the physical structure of a system
is a bond graph where the vertices represent subsystems or technological components denoted by a word and the inputs and outputs are a power variables
represents the physical structure of the system.
Input and output are the power variables .
The direction of half arrow indicates the orientation of the power.
However, the computational structure can be superimposed on a bond graph by adding a perpendicular stroke to each half arrow. Then the calculation (simulation) structure can be deduced as it is shown in the causal word bond graph