Watlow PM LEGACY™ Limit Controller
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Chapter 8 Features
Range High and Range Low
With a process input, you must choose a value to represent the low and high ends of the current
or voltage range. Choosing these values allows the controller’s display to be scaled into the actual
working units of measurement. For example, the analog input from a humidity transmitter could rep-
resent 0 to 100 percent relative humidity as a process signal of 4 to 20mA. Low scale would be set
to 0 to represent 4 mA and high scale set to 100 to represent 20 mA. The indication on the display
would then represent percent humidity and range from 0 to 100 percent with an input of 4 to 20mA.
Select the low and high values with Range Low
r.Lo
and Range High
r.hi
(Setup Page, Analog Input
Menu).
Outputs
Retransmitting a Process Value or Set Point
The retransmit feature allows a process output to provide an analog signal that represents the set
point or process value. The signal may serve as a remote set point for another controller or as an in-
put for a chart recorder documenting system performance over time.
In choosing the type of retransmit signal the operator must take into account the input impedance of
the device to be retransmitted to and
the required signal type, either voltage
or milliamps. Typically applications
might use the retransmit option to re-
cord one of the variables with a chart
recorder or to generate a set point for
other controls in a multi-zone applica-
tion.
Output 3 can be ordered as process
output. Select retransmit
rЛЏ t
as
the Function
fn
(Setup Page, Output
Menu). Set the output to volts
v o lt
or milliamps
ЛЏ a
with Type
o.ty
. Select the signal to retransmit
with Retransmit Source
r.sr
. Set the range of the process output with Scale Low
s.Lo
and Scale High
S.hi
. Scale the retransmit source to the process output with Range Low
r.Lo
and Range High
r.hi
.
When the retransmit source is at the Range Low value, the retransmit output will be at its Scale Low
value. When the retransmit source is at the Range High value, the retransmit output will be at its
Scale High value.
Resetting a Tripped Limit
Output 2 will always be a Form A (normally open) Mechanical Relay and it will always be internally
tied to the limit function. When the limit is in a safe state the internal coil for this relay will be ener-
gized, therefore the relay will be closed. When a condition occurs that causes the limit to trip, the in-
ternal coil will deenergize causing the relay to latch open. When the condition that caused the limit to
trip has been resolved, the relay will remain latched open until manually reset.
Retransmit
Retransmit Source
Output Scale
Range High
Range Low
Scale High
Scale Low
