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Reference Manual
00809-0100-4696, Rev AA
September 2004
3-17
Rosemount 848L
Counter Functions
CTU (clock, reset, target)- The parameters clock and reset are functions. The
target is a 16 bit constant. Whenever reset is true, the internal counter is set to
zero and the value of the function is false. The value of clock is ignored while
reset is true. If reset is false, the internal counter will increment once for each
rise of the clock parameter. When the internal counter equals the target value,
the value of the function is true and the counter stops counting in order to
avoid rollover. The value of the function is false if the internal counter does not
equal the target.
The internal counter is not visible from Fieldbus and is not available to any
other function. The value of the internal counter is not retained during a device
restart. This function is not suitable for a totalizer, but can be used as a
prescaler to adjust the external mechanical counter rate. The pulse rate must
be less than five per second.
The following expression increments the counter whenever hardware input 1
turns on. The counter is reset whenever hardware input 2 is on. If input one is
from a mechanical displacement flowmeter that delivers 76.54 pulses per
gallon, then the highest flow rate is 3.5 gallons per minute. The following
equation will deliver one 0.1 second pulse per 100 gallons:
OUT1_EQ contains CTU(IN(1),OUT(1),7654);
Starting at zero, 7653 pulses go by and then pulse 7654 turns on the output.
On the next evaluation cycle, the counter is reset because Output 1 is on.
This is a result of the order of execution of equations. Output 1 becomes true
because the count is reached, but the OUT(1) function has already been
evaluated as false. The counter must reset before the next pulse comes in.
The output pulse may be extended with a TP function.
TON (power, target) - Whenever power is false, the value of the internal timer
is set to zero and the value of the function is false. When power is true then
the value of the function will become true after the target amount f time has
elapsed. This condition persists as long as power is true. The timer resets
when power is false.
The following equation filters the level switch in a stirred tank so that high
level bouncing of the float does not create nuisance alarms for the operator.
Hardware input 1 senses the level switch and hardware output 1 drives the
alarm annunciator with its big horn. The level switch must stay closed for 5
minutes before the alarm is energized and the operator is startled by the horn.
OUT1_EQ contains: TON(IN(1),3000);
TOF (power, target) - Whenever power is true, the value of the internal timer is
set to the target and the value of the function is true. The value of the function
will become false after the target amount of time has elapsed. This condition
persists as long as power is false.
The following equation keeps the outlet valve open for about 5 seconds after
the pump is shut off, so that the pressure across the pump can equalize.
Hardware output 1 runs the pump and hardware output 2 opens the valve
OUT1_EQ contains: <something that controls the pump>;
OUT2_EQ contains: TOF(OUT(1),50);