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23. SEQUENTIAL LOGIC DESIGN

•This is for systems that move between different ‘modes’ of operation.

•These techniques allow different levels of design sophistication.

•In general the techniques and tools can be selected as shown below.

sequential problem

simple/small

complex/large

single process

multiple

processes

no clear steps

buffered (waiting)

STATE DIAGRAM

state triggers

an orderly

SCRIPTS

performance

PETRI NET

process with steps

short

important

no waiting with

development

FLOW CHART

single states

time

BLOCK LOGIC

EQUATIONS

SFC/GRAFSET

23.1 SCRIPTS

•Another method that can be used to explore a state based program is a script. Another way to describe this is “what is happening, what started it and what will stop it?”

•The general hierarchy for a script is below,

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The output/state/memory [name]....

.....is active.....

......is stopped.....

..... stays on after.....

.....turns off after.....

.....is followed by output/state/memory [name] when.....

.... delays turning on/off for [time] after input/state/memory [name] turns on/off

..... while input/state/memory [name] is on/off

..... after input/state/memory [name] turns on/off

• After we get a process description it must be rewritten as a script. Care is needed to check for consistency.

The output [press advance] stays on after activated. This follows output [press idle light], it is activated by input [both hand buttons pushed]. It is stopped by [bottom limit hit], it is followed by output [press retract]. Or, it is stopped by input [E-stop or gate open], it is followed by output [press stop light].

The output [press retract] stays on after activated. This follows output [press advance] or output [press idle light] and [start]. It is stopped by input [top limit], it is followed by output [press advance]. It is also stopped by [E-stop].

ETC......

• This can be put into ladder logic. (clearly this logic is not complete and extra work is needed make it ready for the PLC)

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press		hand			hand
idle light		button A			button B

press bottom advance limit

press E-stop advance

gate closed

• Implemented for an Allen Bradley PLC it might look like.

press

L advance

press idle light

U

press

retract

L

press

advance

U

press stop

light

L

press

advance

U

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first scan S2:1/14

Start		top limit
I:000/01		I:000/02

ETC.............

press idle light L O:000/00

press advance U O:000/01

press retract U O:000/02

press stop U O:000/03

press idle light U O:000/00

press advance L O:000/01

•General rules for scripting are,

1.At least one sentence is needed for at least each output.

2.The list of conditions should refer to other conditions.

3.The script should refer to actual inputs/outputs, memory locations or state names.

•This method is not rigorous and can be inadequate for many designs.

•Use the method to design a parking gate controller.