Timers
Mechanical Timing Relay
Time delay relays are simply control relays with a
time delay built in.
Their purpose is to control an event based on
time.
Timers
• On delay : With an on-delay timer, timing begins when coil is
activated. When the time has expired, the contacts close/open(depends on
the contacts normal condition) — and remain closed/opened until coil is deactivated. If coil is de-activated before time-out, the time delay resets
• Off delay :When using an off-delay timer, when coil is activated, the
contacts close/open (depends on the contacts normal condition). Deactivating the coil causes timing to begin. When the time has expired, the
contact will return to their normal state.
Normally-open, timed-closed Contact
NOTC (normally open, timed close contact)- contact is open, when relay is de
energized; when relay is energized, there is a time delay in closing.
Normally-open, timed-closed Example
normally closed, timed open contact
NCTO (normally closed, timed open contact)-contact is closed when relay
coil is de energized. When relay is energized, there is a time delay in
opening.
normally closed, timed open example
Normally open, timed open contact
NOTO (Normally open, timed open contacts)- contact is normally open, when
relay coil is de energized when relay coil is energized contact closes instantly.
When relay coil is de energized, there is a time delay before the contact opens.
Normally open, timed open contact example
normally closed, timed close contact
NCTC (normally closed, timed close contact)- contact is normally closed when
relay coil is de energized. When the relay coil is energized, contact open
instantly. When the relay coil is de energized, there is a time delay before the
contact closes.
normally closed, timed close contact
example
Timer instructions
PLC timers are output instruction that provides the
same function as mechanical timing relay.
• ON delay provides the delay when the relay is
energized.
• OFF delay provides the delay when the relay is
de energized.
• Retentive Timer is similar to on delay timer
except for the fact that the Accumulated value is
retained even if the relay is de energized.
On delay timer example
On delay timer example
off delay timer example
On delay timer example
Timer instructions
SLC 500
CompacLogix
Timer instruction
• Address: By default, SLC 500 uses file #4 for timers. Compactlogix uses
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tags.
Time Base: SLC 500 uses 1.0s, 0.01s, 0.001s. CompactLogix uses only
.001s
Preset value (pre): is the value up to which the timer will time.
Accumulated value (ACC): is the value that increments as the timer is
timing. This value can range from 0 to +32,767.
Done (DN) bit: Changes state whenever .PRE=.ACC value
Enable (EN) bit: is on when the timer instruction is true.
Timer timing(TT) bit: is on when the timer is timing.
Timer memory structure
SLC 500 timer memory structure
CompactLogix timer memory structure
On delay timer instruction
• When the TON is true, it will write a "1" to the EN (Enable).
• If the TON is true and the ACC (Accumulated) is less than the PRE (Preset),
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the TON will write a "1" to the TT (Timer Timing) bit and a "0" to the DN
(Done) bit.
If the TON is true and the ACC (Accumulated) is greater than the PRE
(Preset), the TON will write a "0" to the TT (Timer Timing) bit and a "1" to
the DN (Done) bit.
The processor resets the accumulated value when the TON is false.
Reset instruction (RES), will reset the timer at any time.
Off-Delay Timer
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When a TOF is true, it will write a "0" to the
accumulated value, a "1" to the EN (Enable), a "0"
to the TT (Timer Timing), and a "1" to the DN
(Done) bit.
When the TOF is false, it will write a "0" to the EN
(Enable).
If the TOF is false and the ACC (Accumulated) is
less than the PRE (Preset), the TON will write a "1"
to the TT (Timer Timing) bit and a "1" to the DN
(Done) bit.
If the TOF is false and the ACC (Accumulated) is
greater than the PRE (Preset), the TOF will write a
"0" to the TT (Timer Timing) bit and a "0" to the
DN (Done) bit.
A reset instruction resets the timer
Off delay timer operation
Retentive timer
The RTO instruction begins timing when it rung goes true.
As long as the rung remains true, the timer updates the
accumulated value each program scan, until it reaches the
preset value. The RTO instruction retains its accumulated
value even if one of the following occurs:
• the rung goes false
• you change to Program mode
• the processor faults of loses power
SLC 500 Timer addressing
Example
T4:0 means that it references an internal Timer file
T4:0 means that it uses the first Timer in the table
T4:0/DN means that it references the Done bit.
T4:0/EN means that it references the Enable bit.
T4:0/TT means that it references the Timer Timing bit.
T4:0.ACC means that it references the 16 bit Accumulated value.
T4:0.PRE means that it references the 16 bit Reset value.