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Off Delay Timer
The Off-Delay Timer (OFDT) increments while power flow is off, and
the timer's Current Value (CV) resets to zero when power flow is on.
OFDT passes power until the specified interval PV (Preset Value) has
elapsed.
Time may be counted in the following increments:
▪
Seconds
Tenths (0.1) of a second
▪
▪
Hundredths (0.01) of a second
▪
Thousandths (0.001) of a second
The range for PV is 0 to +32,767 time units. If PV is out of range, it has no effect on the timer's word 2.
The state of this timer is retentive on power failure; no automatic initialization occurs at power-up.
When OFDT receives power flow, CV is set to zero and the timer passes power to the right. The
output remains on as long as OFDT receives power flow.
Each time the OFDT is invoked with its power flow input turned OFF, CV is updated to reflect the
elapsed time since the timer was reset. OFDT continues passing power to the right until CV equals or
exceeds PV. When this happens, OFDT stops passing power flow to the right and stops accumulating
time. If PV is 0 or negative, the timer stops passing power flow to the right the first time that it is
invoked with its power flow input OFF.
When the function receives power flow again, CV resets to zero.
Notes:
▪
The best way to use an OFDT function is to invoke it with a particular reference address exactly
one time each scan. Do not invoke an OFDT with the same reference address more than once per
scan (inappropriate accumulation of time would result). When an OFDT appears in a program
block, it accumulates time once per scan. Subsequent calls to that program block within the
same scan will have no effect on its OFDTs.
▪
Do not program an OFDT function with the same reference address in two different blocks. You
should not program a JUMP around a timer function. Also, if you use recursion (where a block
calls itself either directly or indirectly), program the program block so that it invokes the timer
before it makes any recursive calls to itself.
▪
For information on using timers inside parameterized blocks, refer to Using OFDT, ONDTR and
TMR in Parameterized Blocks.
▪
An OFDT expires (turns OFF power flow to the right) the first scan that it does not receive power
flow if the previous scan time was greater than PV.
▪
When OFDT is used in a program block that is not called every scan, the timer accumulates time
between calls to the program block unless it is reset. This means that OFDT functions like a timer
operating in a program with a much slower scan than the timer in the main program block. For
program blocks that are inactive for a long time, OFDT should be programmed to allow for this
catch-up feature. For example, if a timer in a program block is reset and the program block is not
called (is inactive) for four minutes, when the program block is called, four minutes of time will
already have accumulated. If the enable input is OFF, these four minutes are applied to the timer
(that is, CV is set to 4 minutes).
GFK-2950C
Chapter 4. Ladder Diagram (LD) Programming
February 2018
Mnemonics:
OFDT_SEC
OFDT_TENTHS
OFDT_HUNDS
OFDT_THOUS
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