Page 60 - Programmable Logic Controllers, Fifth Edition - Mobile version

• Create the tag database. Most HMI packages pro-
vide a way to import tags from the PLC program-
ming software.
• Insert the graphical objects on the screen. Graphics
are drawn or imported from a library of common
objects.
• Animate the objects. There are two basic types
of animation: user input and display. User input
types allow an operator to change tag values. A
display animation allows a value to be displayed
and also allows an object to change shape, position,
and color.
Many different types of HMI hardware and software
features are available. These include: Figure 2-53 HMI installed in an industrial environment.
Source: Photo Courtesy PC Enclosures, http://www.pcenclosures.net.
HMI MONITOR AND ENCLOSURE
HMI operator panels typically contain monochrome or ALARM STATUS
256 color display screens. These systems often commu-
nicate directly with the PLC to read or write memory System pump-1 flow System pump-2 flow
locations.
Recirc-1 flow Tank low level
• A monochrome monitor uses one color for the back-
ground and another to display text or images on the Fluid high temp Fluid low temp
screen.
• Color displays enable clearer process representation Figure 2-54 Typical alarm status screen.
and in general brighten up their systems. The color
convention for status and alarms should follow the • Inactive—The condition being monitored does not
same convention as their hardwired equivalents, have any faults present, and there is no associated
namely: alarm message waiting to be acknowledged.
- Red—for alarm, danger, and stop • Active—A fault condition is present, and the alarm
- Yellow—for caution and risk of danger message has not been acknowledged by the operator.
- Green—for ready, running, and safe condition • Acknowledged—The fault condition is present, and
• Screen resolution is expressed as width × height, the operator has acknowledged the alarm message.
with the units in pixels. • OK - The fault condition is no longer present,
• Screen memory is expressed in Megabytes (MB). but the operator has not acknowledged the alarm
• The environmental certification refers to the type message yet.
of electrical enclosure used to protect their contents
from troublesome operating conditions such as EVENT HISTORY
dust, liquids, and extreme variations in temperature An event history presents a time-stamped list of all sig-
(Figure 2-53). nificant events that have occurred in the process. Many
• The screen may or may not be touch-sensitive. The problems within the plant or equipment may occur when
touch-sensitive screen allows for more devices and no one is monitoring the system, and intermittent prob-
data to be displayed in a smaller area. Detailed in- lems may be difficult to diagnose without a history of pre-
formation about an object can be accessed by touch- vious issues.
ing the object.
TREND
ALARMS Values of important process variables, such as flow, tem-
Alarms are messages which indicate that a fault condition perature, and production rate, over a period of time are
is present (Figure 2-54). An alarm summary can present a shown by this type of display. This type of display pro-
complete list of timestamped active alarms. Typically an vides the ability to chart the progress of the process in
alarm can exist in the following states: real time, providing the same function as a strip chart

PLC Hardware Components Chapter 2 41

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