Simultaneity in signal-event systems

Signal-event systems provide a modular modelling technique based on Petri nets. Activities of a module can be made possible or can be prevented by another module through conditions. One sided synchronization of modules is done by signal-events, which cause the execution of actions in steps. But due to use of conditions and signal-events it is possible, that the simultaneous firing of steps leads to markings, which are not reachable by normal interleaving. We give a criterion, in which situations simultaneous firing of steps can be safely omitted, without missing reachable markings.