Default
ptolemy.domains.sr.lib.Default

When the <i>preferred</i> input has a token, then the output is equal to that token. If the <i>preferred</i> input is absent, then the output is equal to the <i>alternate</i> input (whether it is absent or not). This actor is non-strict, in that can produce an output even if <i>alternate</i> input is unknown. Thus, it can be used to break causality loops. <p> This actor is inspired by the "default" operator in the synchronous language SIGNAL, and is similar to the "followed by" operator in Lustre. But in the context of the Ptolemy II SR domain, its behavior is deterministic. This is because the Ptolemy II SR domain has a Lustre or Esterel style master clock, and this actor is given the opportunity to fire on each tick of that master clock. In SIGNAL, by contrast, this actor can be used to "upsample" to a higher rate clock; however, without considerable care, its use in SIGNAL results in nondeterminism.

Author(s): Edward A. Lee
Version:$Id: Default.doc.html,v 1.1 2006/02/22 18:40:26 mangal Exp $
Pt.Proposed Rating:Yellow (eal)
Pt.Accepted Rating:Red (cxh)


preferred
The preferred input port. If there is a token here, then that token is produced on the output. Any data type is accepted.
alternate
The alternate input port. If there is no token on the preferred input port, then the output will equal whatever is here (including absent). Any data type is accepted.
output
The output port. The type is greater than or equal to the types of the two input ports.