PTOLEMY II - CiteSeerX

Using Vergil
these ports are shown in the diagram. The Expression actor is a very flexible actor in the Ptolemy II
actor library. It can have any number of input ports, with arbitrary names, and uses a rich and expressive
expression language to specify the value of the output as a function of the inputs (and parameters
of the containing model, if desired).
Three of the actors in figure 2.1 are composite actors, which means that their implementation is
itself given as a block diagram. Composite actors are indicated visually by the red outline. You can
look inside to reveal the implementation, as shown in figure 2.5, which shows the implementation of
the Signal Source in figure 2.1. It is evident from the block diagram how a sinusoidal signal is generated.
2.2.3 Executing a Pre-Built Model: A Continuous-Time Example
A key principle of the Ptolemy II system is that the model of computation that defines the meaning
of a block diagram is not built-in, but is rather specified by the director component that is included in
the model. The box labeled “SDF Director” in figures 2.1 and 2.5 specifies that these block diagrams
have synchronous dataflow semantics, which is explained further below. The second example in the
quick tour of figure 2.3, by contrast, has continuous-time semantics (the one labeled “Continuous-
Time Modeling”). The example is the well-known Lorenz attractor, a non-linear feedback system that
exhibits chaotic behavior.
FIGURE 2.4. The run window for the model shown in figure 2.1.
48 Ptolemy II