USING SFC STEPS AND TRANSITIONS

Steps and transitions can be used to represent and control consecutive operating phases.

Each operating phase is represented by a symbol called step. When this operating phase is executed, the step is said to be active. In this case, by definition, the step contains a status token. Note that the step is activated by setting an observation digital of this step to ON.

To bring the operating phase to a close, the end of the phase must be authorised or controlled. To do this, set a digital transition control input to ON. The transition is then said to be closed and the status token bypasses it. The token then disappears from the step and makes its way towards the status token circulation output. Consequently, the observation digital changes to the OFF state.

When the operating phase has finished, the step becomes inactive and the observation digital changes to OFF.

The end of one operating phase (B01) is followed immediately by the start of the next operating phase (B02). The next operating phase is also indicated by a new step and its end is also controlled by a transition.

To illustrate the fact that the end of operating phase B01 is followed (in sequence) by operating phase B02, the status token circulation output is wired to one of the status token circulation inputs B02. In this case, when the setting to ON of the transition command of B01 causes its closure, the token present in the step of B01 "falls" and crosses the closed transition towards the step of B02 where it remains as long as the digital transition command input of B02 remains OFF (transition blocked). The digital observation output of the activity of the step of B02 changes to ON. As soon as the transition of B02 closes, the token now present in step B02 moves on in the status token circulation output, the operating phase associated with the step of block B02 is stopped and the digital observation output of step 2 changes to OFF.

This mode of operation is illustrated in the four diagrams below.

If step 1 is not active, the associated operating phase (B01) is not in progress, so, by definition, the status token is not present in step 1. Therefore, setting the digital control input of transition 1 to ON to close the transition will have no effect, because there is no status token in step 1. It cannot fall.

The digital control inputs of each transition and the digital observation outputs of each step can be connected to other FBD blocks using the digital inputs or outputs. For example, a Boolean input combination can control transition 1, a button can control transition 2, the observation Boolean of step 1 switches a relay and the observation Boolean of step 2 activates a message display.

Back to Contents