How sequential function chart (SFC) blocks work

How sequential function chart (SFC) works

SFC are used to make sequencies within programs. Most PLC programs used them.

An example of an sequential flow is one barrier on an train crossing.

Steps last some time and transitions define when to leave the current step for a new one.

  • Initiale Step 1; waiting until train is coming
    • Transition; Train Request to cross the crossing
  • Step 2; traffic light turns red.
    • Transition; Traffic is no longer on the crossing
  • Step 3; barriers go down to stop Traffic
    • Transition; barriers down
  • Step 4; train gets green light to cross
    • Transition; train is clear
  • Step 5; Barriers go up
    • Transition; barriers are up
  • Step 6; Traffic light off
    • Transition; traffic light are off
  • Return to Step 1
SFC 1

STEP

B019 is a Step block.

Steps are made out of two functions.

  1. Action what will be done when the step active (on)
  2. The condition what needs te be true before it can transition to the next step.

When B019 (Step) is active it will make every thing which is connected to it active.  In this case it will put output Q.3. on.

The condition before it can transition from on step to the other. In this case I.3. needs to be on.

The step will only become active when its connected to a step above and the transition is true.

SFC 2
SFC 2b

INIT STEP

(Initial SFC step)

This is a initial step what will be activated (active) when the PLC starts for the first time. You can recognise the step by the second square in the normal square. The rest of the function is the same as a normal step.

SFC 3

Reset INIT STEP

(Resettable initial SFC step)

The resettable initial SFC step is the same as a initial step but with an extra function. If the program is in a different step it will jump back to the initial step when the reset input is on.

SFC 10

DIF-OR 2 Step

Divergence to OR with 2 SFC branches is not more then a step with 2 different transition conditions. Its mostly used to give the programmer a option. The first transition to be true, that will be the road the program will follow and the other road will be skipped. This block is capable of splitting the program in to two valid flows. If this is not your intention then always ensure that only one input can be high (usually by inverting one input and using an AND function).

SFC 4

Nog maken!!!!! (usually by inverting one input and using an AND function).

SFC 4

CON-OR 2

(Convergence to OR with 2 SFC branches)

Convergence to OR with 2 SFC branches. When you use an DIF-OR 2 Step you have split the program up in to multiple roads within you program. When you would to combine the different roads of the program back in to one. You can use this block to combined the roads back in to one.

SFC 6
SFC 5

DIV-AND 2

(Divergence to AND with 2 SFC branches)

Divergence to AND with 2 SFC branches. This makes the program split up into two parallel programs. After this function block there can be more then one step active.

SFC 8
SFC 7

CON-AND 2

(Convergence to AND with 2 SFC branches)

Convergence to AND with 2 SFC branches. When you use an CON-AND 2 Step you have split the program up in to multiple roads within you program. When you would to combine the different roads of the program back in to one. You can use this block to combined the roads back in to one. Both "steps" will be active until the transition condition is true.

CON-AND 2
SFC 5

Simulation zeliosoft

Try the logic yourself. You can use the Simulation in Zeliosoft or the PLCtrainingKIT.

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