Mitsubishi Digital Electronics FX0 Manual

A SERVICE OF

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FX Series Programmable Controllers Points Of Technique 10

10-4

10.4 Constant Scan Mode

Some times the timin

g

of operations can be a problem, especiall

y

if some co-ordination is

bein

g

attempted with a second control s

y

stem. In cases like this it is ver

y

useful to fix the

PLC’s scan time. Under normal conditions the PLC’s scan time will var

y

from one scan to the

next. This is simpl

y

because the natural PLC scan time is dependent on the number of and

t

y

pe of the active instructions. As these are continuall

y

chan

g

in

g

between pro

g

ram scans the

actual scan time is also a var

y

in

g

. Hence, b

y

usin

g

the additional pro

g

ram function identified

below, the PLC’s scan time can be fixed so that it will be the same duration on ever

y

pro

g

ram

scan. The actual scan duration is set b

y

writin

g

a scan time in excess of the current lon

g

est

scan duration to special data re

g

ister D8039 (in the example the value K150 is used). If the

PLC scans the pro

g

ram quicker than the set scan time, a 'pause' will occur until the set scan

duration is reached.

This pro

g

ram example should be placed at the be

g

innin

g

of a users pro

g

ram.

10.5 Alternating ON/OFF States

It is often useful to have a sin

g

le input control or to

gg

le a situation. A basic,

y

et t

y

pical example

is the switchin

g

ON/OFF of a Li

g

ht. This can be easil

y

achieved b

y

usin

g

standard ladder

pro

g

ram to load an input and switch an output. However, this s

y

stem requires an input which is

latchable. If basic ladder steps are used to latch the pro

g

ram then it soon becomes complex

and prone to mis-pro

g

rammin

g

b

y

the user. Usin

g

the ALT instruction to to

gg

le the ON/OFF

(SET/RESET, START/STOP, SLOW/FAST) state is much simpler, quicker and more efficient.

Explanation:

Pressin

g

the momentar

y

push button X1 once will switch the lamp ON. Pressin

g

the push

button for a second time will cause the lamp to turn OFF. And if the push button is a

g

ain

pressed for a third time, the lamp is turned ON a

g

ain and so the to

gg

led status continues.

The second pro

g

ram shown identifies a possible motor interlock/control, possibl

y

a start/stop

situation.

M8000

M8039

MOV

K150

D8039

Newly set

constant scan

time = 150 msec

X 0

ALT Y 1

X 0 Y 1

X 0

Y 0

Y 1

M 0

M 0

ALT M 0

Program

example 1

Program example 2