In Control

Rab,

The problem here is that you have allowed 4 different tasks to "own" the siren, so control of the siren resource has become diffuse and hard to manage.

Try making a separate task to control the siren. It receives requests from it's "clients" (the 4 existing tasks) when they want an alert. The requests can be in the form of semaphores. The siren task then looks at the requests plus the inhibit input and decides what to do.

By doing this you will have separated the two aspects of the problem:

1. Deciding that an alarm condition exists, and
2. Sounding or muting the siren

from each other. This should make it easier to make the program do exactly what you want it to do. It will also make maintenance easier, so if you want to change the "rules" for sound or silence there will be no impact on how and when alarm conditions are generated.