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23.3.1 State Diagrams Example

• A Complex Example - In this case we have a set of traffic lights. The lights will remain green in one direction until a pedestrian cross button is pushed. They will then turn yellow for four seconds and then turn red.

	Red	L1
	Yellow
		L2
	Green
		L3
	North/South
		Walk Button - S1

*This could be done with simple if-then logic (conditional)

	Red	L4
	Yellow
		L5
	Green
		L6
	East/West
		Walk Button - S2

• First we will describe the system variables. These will vary as the system moves from state to state. Please note that some of these together can define a state (alone they are not the states).

We have eight items that are ON or OFF
L1
L2	OUTPUTS	Note that each state will lead
L3		to a different set of out-
L4		puts. The inputs are often
L5		part, or all of the transi-
L6		tions.
S1	INPUTS
S2
A simple diagram can be drawn to show sequences for the

• We can then use outputs, or controlled variables, to indicate system states (you can also think of

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these as modes).

Step 1 : Define the System States and put them (roughly) in sequence

System State
	L1 L2 L3 L4 L5 L6	A binary number
		0	= light off
		1	= light on
State Table

	State Description		#		L1	L2	L3	L4	L5	L6
	Green East/West		1		1	0	0	0	0	1
	Yellow East/West		2		1	0	0	0	1	0
	Green North/South		3		0	0	1	1	0	0
Yellow North/South			4		0	1	0	1	0	0

Here the four states determine how the 6 outputs are switched on/off.

• State transitions are determined based on inputs, and how the system changes. These events cause a change between states. Care must be taken to avoid confusing them with states.

Step 2 : Define State Transition Triggers, and add them to the list of states

	Description		#		L1	L2	L3	L4	L5	L6		transition
	Green East/West		1		1	0	0	0	0	1		S1
	Yellow East/West		2		1	0	0	0	1	0
	Green North/South		3		0	0	1	1	0	0
Yellow North/South			4		0	1	0	1	0	0

• The state transition diagram allows us to visually check for continuous loops. In a simple state diagram multiple transitions may lead to alternate paths.

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Step 3 : Draw the State Transition Diagram

grn. EW	pushbutton NS (i.e., 10)

delay 4sec

first scan

yel. EW

yel. NS

delay 4sec

pushbutton EW (i.e. 01)

grn. NS

• For the previous traffic light example, add a speed up signal for an emergency vehicle.

A strobe light mounted on fire trucks will cause the lights to change so that the truck doesn’t need to stop. Modify the state diagram to include this option.

23.3.1.1 - Block Logic Conversion

• The state diagram can be implemented in ladder logic, in this case for a SLC-150. This example uses states to turn lights on or off. An initial block uses the first scan input ‘868’ to ensure the first state is state 1. Each state runs a section of ladder logic code in ‘MCR’ blocks to wait for transitions.

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STATES			OUTPUTS	INPUTS
701	- state 1 - green E/W		012 - L1	101	- S1
702	- state 2	- yellow E/W	013 - L2	102	- S2
703	- state 3	- green N/S	014 - L3	868	- first scan
704	- state 4	- yellow N/S	015 - L4
			016 - L5
			112 - L6

RESET THE STATES

MCR

868

701

L

702 U

703 U

704

U MCR

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TURN ON LIGHTS AS REQUIRED 701

702

704

703

703

704

702

701

FIRST STATE WAIT FOR TRANSITIONS

701

101

101

012

013

014

015

016

112

MCR

701

U

702

L

MCR

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SECOND STATE WAIT FOR TRANSITIONS 702

901

901

901

THIRD STATE WAIT FOR TRANSITIONS

703

102

102

MCR

901

RTO PR 0040

702

U

703

L

901

RST

MCR

MCR

703

U

704

L

MCR

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FOURTH STATE WAIT FOR TRANSITIONS
704			MCR
			902
				RTO
			PR 0040
902			704
				U
902			701
				L
902			902
				RST
			MCR

• Consider the state diagram below and implement it in ladder logic. You should anticipate what will happen if both A and C are pushed at the same time.

STA	STC
B	D
A	C
	STB

first scan

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first scan

STA

B

STB

C

Note: if A and C are true at the same time then C will have priority. PRIORITIZATION is important when simultaneous branches are possible.

A C

L	STB
U	STA
U	STC
MCR
U	STA
L	STB
MCR
MCR
U	STB
L	STC
U	STB
L	STA
MCR