66BTrip Points 6BChapter Three: Overview
42
Trip Points
The DMA transducer has two software trip points, which are maintained in the S-Analog Sensor
Object:
• Trip Point-Alarm, Low and High
• Trip Point-Warning, Low and High
The software trip point values are adjustable using digital communication commands such that
when the pressure rises above one or below the other, the corresponding trip point changes state.
Refer to S-Analog Sensor Object, page 65, for more information.
Trip Point Hysteresis
Hy
steresis is built into the operation of the trip points to help comp
ensate for the noise inherent in
all systems. Without hysteresis, the noise may cause the trip points to repeatedly switch states, a
condition known as “chatter.”
The default hysteresis value of 0 can be adjusted separately for each of the trip points through
communication commands.
Setting the hysteresis too high creates a deadband around the trip point. The deadband prevents
the trip point from responding to changes in the pressure signal around the trip point. Ideally, the
hysteresis should be close to, but not less than, the peak-to-peak noise. This setting will provide
maximum immunity from chatter while providing the best possible accuracy. It may take some
trial and error efforts to determine the smallest hysteresis setting appropriate for your system.
Trip Point Settling Time
The trip point settling tim
e defines the amount of tim
e a trip point alarm condition must exist
before it is reported to the exception status attribute of the S-Device Supervisor Object. (Refer to
Exception Status, page 75, and Averaging Time, page 69, for more information.) The trip point
delay can be set from 0 to 10,000 m
sec—the initial value is 0.
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