PowerPoint® Presentation
Chapter 6
Programming PLC Timers
and Counters
Timer and Counter Instructions • Timer Instructions •
Timer Instruction Words • Timer On-Delay (TON)
Instructions • Timer Off-Delay (TOF) Instructions •
Retentive Timer (RTO) Instructions • Reset (RES)
Instructions • Special Applications • Counter
Instructions • Counter Instruction Words • Count Up
(CTU) Instructions • Count Down (CTD) Instructions
Chapter 6 — Programming PLC Timers and Counters
PLCs provide a cost-effective alternative to using
individual hardwired timers and counters because the
smallest PLCs (nanos and micros) already contain 10
or more timers and counters.
Chapter 6 — Programming PLC Timers and Counters
Timer instructions
contain three
words, with word
“0” containing
three bits that can
be used in the
program of a PLC.
Chapter 6 — Programming PLC Timers and Counters
Timers following on-delay (TON)
instructions begin timing when
the logic preceding the
instruction on a rung changes
from false to true.
Chapter 6 — Programming PLC Timers and Counters
A PLC with a programmed timer using timer on-delay
(TON) instructions can be used to replace hardwired
on-delay timers.
Chapter 6 — Programming PLC Timers and Counters
Timers following off-delay
(TOF) instructions start timing
when the logic preceding the
instruction on a rung changes
from true to false.
Chapter 6 — Programming PLC Timers and Counters
Off-delay timers are used
in applications that
require a load to remain
energized after the input
has been removed.
Chapter 6 — Programming PLC Timers and Counters
Retentive timers begin timing
when the logic preceding the
instruction on a rung changes
from false to true. Retentive
timers retain their accumulated
value until reset.
Chapter 6 — Programming PLC Timers and Counters
Free running timers are
continuously timing because
the XIO instruction preceding
the timer is always true.
Chapter 6 — Programming PLC Timers and Counters
Timers are cascaded by having
the DN bit of timer #0 activate
timer #1. The only limit to the
number of cascaded timers is the
size of the memory in the PLC.
Chapter 6 — Programming PLC Timers and Counters
Cascaded timers are used to
start equipment sequentially,
such as when starting three
conveyors with a 10 sec
delay between the start of
each conveyor.
Chapter 6 — Programming PLC Timers and Counters
Counter instructions
contain three words.
The bits and words of
counter instructions
have addresses that
can be used in the
program of a PLC.
Chapter 6 — Programming PLC Timers and Counters
Count up (CTU) instructions
increase the accumulated value
incrementally on each false to
true transition of the logic
preceding the instruction.
Chapter 6 — Programming PLC Timers and Counters
Counters are used
in applications that
require items or
products to be
counted.
Chapter 6 — Programming PLC Timers and Counters
A CTD instruction counts down on
every false to true transition of the
logic preceding the instruction and
shares the same status bits, preset
values, and accumulated values as
the corresponding CTU instruction.
Chapter 6 — Programming PLC Timers and Counters
CTU and CTD instructions can be used to keep track
of the cars entering and exiting a parking lot.