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	T5	T4
STA			STC
	B
T1		D
FS
A			C
T2			T3
		STB
		first scan
T1 = FS		STA = ( STA + T2 + T1)
								T5
T2 = STB A
		STB = ( STB + T5 + T4)			T2 T3
T3 = STB C
		STC = ( STC + T3 T2)			T4

T4 = STC D

T5 = STA B

• For practice,

implement the previous state diagram in ladder logic.

23.4 PARALLEL PROCESS FLOWCHARTS

•Parallel Processes can happen separately but must eventually work together. This happens when PLC’s control multistage machines, etc.

•PLC’s are well suited to this problem.

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•There is a method (based on Petri nets) that goes by a number of names,

-GRAFCET - developed in france

-IEC848 - the standard developed from GRAFCET

-SFC (Sequential Function Charts) - Allen Bradley version

•The basic elements are,

flowlines - connects steps and transitions (these basically indicate sequence) transition - causes a shift between steps, acts as a point of coordination

Allows control to move to the next step when conditions met (basically an if or wait instruction)

initial step - the first step

step - basically a state of operation. A state often has an associated action

step

action

macrostep - a collection of steps (basically a subroutine)

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selection branch - an or - only one path is followed

simultaneous branch - an and - both (or more) paths are followed

• The example below shows a SFQ for a a controller that is watching two separate doors. One door requires a two digit entry code, the second door requires a three digit entry code.

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		Start				lock doors

1

1st digit

1

1st digit

1st digit

wrong

1st digit

wrong

OK

OK

2

2

2st digit

2nd digit

2st digit

2nd digit

OK

OK

wrong

wrong

3

4

unlock#2

3rd digit

3rd digit

5 sec.

OK

wrong

delay

4

unlock#1

5

relock#2

5 sec.

delay

5

relock#1

Parallel because things happen separately, but at same time (this can also be done with state transition diagrams)

•SFC editors are available from many companies.

•SFCs can also be converted to ladder logic the same way state diagrams were.

•Consider the example below,

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4

5

1

reset

7

6

part not

button

detected

1

automatic

button

2

power on

light on

part detect

2

3

advance on

part hold on

bottom

stop

3

limit

button

4

advance off

light off

retract on

advance off

power off

top

5

limit

6

retract off

part hold off

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INITIALIZE STEPS AND TRANSITIONS

first scan

L

U

U

U

U

U

U

U

U

step 1

step 2

step 3

step 4

step 5

step 6

transition 1

transition 2

transition 3

U	transition 4
U	transition 5
U	transition 6

U	transition 7

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CHECK TRANSITIONS

transition 1 automatic on

step 2

L

transition 1

U

transition 7

reset button

transition 1

L

transition 7

U

transition 2

part detect

step 3

L

transition 2

U

transition 3

bottom limit

step 4

L

transition 3

U

transition 4

U

transition 4

stop button

step 5

L

transition 3

U

transition 4

U

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transition 5		top limit

transition 6		part detected

PERFORM ACTIVITIES FOR STEPS step 2

step 3

step 4

step 5

step 6

L

transition 5

U

step 2

L

transition 6

U

power

L

light

L

advance

L

part hold

L

retract

L

advance

U

light

U

advance

U

power

U

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step 6

retract

U

part hold

U

ENABLE TRANSITIONS step 1

step 1

U

transition 1

L

step 2

step 2

U

transition 2

L

step 3

step 3

U

transition 3

L

transition 4

L

step 4

step 4

U

transition 5

L

step 5

step 5

U

transition 7

L