From: Markus Montigel To: designCPN@daimi.aau.dk Date: Wed, 20 Nov 96 11:52:47 +0100 Subject: Re: Asynchronous i/o, initial state, port assignments Claus Reinke made the a proposal concerning asynchronous I/O by introducing external port places. I agree completely that it should be possible to feed a CPN with external sensor data for the same reasons Claus mentioned already. However, the markings of these external port places are then changed by some "external force" which isn't modelled in the CPN itself. IMO this contradicts the fundamental Petri-Net axiom that the state of the modelled system changes only if a transition occurs. Giving up this axiom can cause problems when analyzing the net properties (reachability, deadlocks etc.): how should all these external changes of the markings be considered? I would propose to leave output as it is, i.e. an occurring transition can send some output to a connected process (like in the 'Process' example). On the other hand it should be possible for an external process to trigger an enabled transition if the process state changes. This approach is in line with the semantics of an enabled transition which is allowed to but not forced to actually occur. It would then be a fatal modelling error if an external process tries to trigger a transition which is not enabled. In this way we could have an automatic run of a net where external processes determine which enabled transitions actually occur. Such a testing facility seems to be quite convenient. (Otherwise transitions must always be selected by hand). Markus Montigel Transport Automation Systems Alcatel Austria AG Scheydgasse 41 A-1211 Vienna Austria email: Markus.Montigel@aut.alcatel.at Fax: +43 1 277 22 173 Tel: +43 1 277 22 3476 --------------------------------------------------------------------------- --- [[ Post messages and summary of replies: designCPN@daimi.aau.dk ]] [[ To (un)subscribe, send "help" to: Majordomo@daimi.aau.dk ]] [[ The moderator's address: designCPN-owner@daimi.aau.dk ]] [[ World Wide Web URL: http://www.daimi.aau.dk/designCPN/email/ ]]