Sequential Function Chart

[adam2392]

Hi,

I am programming a process in SFC.

Does anyone know what the function of the DN bit does? In my manual it reads that it signals when the timer reaches a preset value. Can it also be used to just say that the previous action step is "done"?

As a side note, I am also using phase logic interface (PLI). Does any1 know what the difference between using the DN bit, like STEP_001.dn; and something like COLLECT_EQ_PLI.ST = PH_STATE.COMPLETE;? Thanks!

 

[Lancie1]

As a side note, I am also using phase logic interface (PLI).

That is not a side note, but the most important statement that you made. Without knowing the brand and model of your device, there is no way anyone can help you, because they all are different.

I know nothing about what brand and model that your PLI might really consist of.

 

[adam2392]

Hi I am using the program RSLogix5000 (rockwell automation).

Sorry, I am relatively new to the site.

 

[dmargineau]

The .DN BOOL flag of any RSL5K Data Type(Instruction) does nothing unless used within an application's programmed code; if FALSE (0), it inhibits the execution of the code it preceeds and if TRUE (1) the .DN tag enables the execution.
What the .DN bit represents though, is the state of the instruction it belongs to; for example the Your_Timer.DN flag TRUE (1) denotes that the Your_Timer Accumulator (Your_Timer.ACC) has reached the Preset (Your_Timer.PRE) value.
As for SFC programming Data Types(Instructions), the only one carrying a .DN flag is the ACTION STEP; Your_Step.DN BOOL tag will be TRUE (1) when Your_Step ACTION STEP has been active for a time period equal to the preset value entered at the time of the Your_Step SFC Step instruction configuration(Timer = Preset).
The "previous action step" will have its own .DN bit (Previous_Action_Step.DN if you wish).
Obviously, the two .DN tags (Previous_Action _Step.DN and Your_Step.DN)you are refering to are completely different entities and they will be TRUE (1) at different real time values since Your_Step's execution will not commence unless Previous_Action_Step has been completed (presuming the Transition "connecting" the two Action Steps' "condition" is evaluated as TRUE).

 

[adam2392]

Thanks for the reply!

Since the DN bit does nothing, but interact with the preset timer of the action/transition. Is there a way for me to signal the transition to be true, when the previous action is complete? So that way the SFC moves forward action by action (not determined by a preset time). Thanks.

 

[dmargineau]

...the .DN bit does nothing unless being used into the program...
So, do use the Previous_Action_Step.DN tag as the "condition" of a Transition SFC instruction which "connects" the two consequent steps.
For example, Your_Transition TRANSITION instruction's Condition expression could be defined as "Previous_Action_Step.DN = 1" or just "Previous_Action_Step.DN".
Thus, at the time the Previous_Action_Step is completed, the Previous_Action_Step.DN tag becomes TRUE, hence the Condition of the Your_Transition instruction is satisfied, fact which will commence the execution of the Your_Step instruction.
If you do not want to use the .DN bit to "trigger" the next step, try using the Previous_Action_Step.X BOOL flag as the Condition of the Transition instruction in a "Previous_Action_Step = 0" expression, meaning that Previous_Action_Step is not active anymore.