Applications Guide Tracer Graphical Programming - Trane

®
Using a Constant block
Figure 15: Equip Rm Space Temp input block
Using a Constant block
You want to check if the space temperature is above or below the setpoint
of 85°F. Because you know the setpoint and do not want it to change, use
a Constant block to represent it.
➤ Constants and variables
When is it appropriate to use a constant? Or a variable? The primary
distinction between constants and variables is that constants remain
unchanged; whereas, variables change. You might ask, “How do these
definitions apply to the Tracer MP580/581?”
Constants may be used in programs as unchanging values. They may
be analog, binary, or time/date values. However, they cannot be
known outside of the program in which they reside. In other words, a
constant cannot be seen or changed unless you edit the program itself.
Variables, on the other hand, may be changed using a variety of methods.
Variables can be communicated from the Tracer Summit system
and changed using the Rover service tool. Variables may also be calculatedinaprogram,ortheymaybemadeadjustablethroughthe
operator display. Ask the following questions when you are considering
using a constant or a variable:
• Does the value change during program execution?
• Must the value be displayed on the operator display?
• Must the value be adjustable by the operator, through the operator
display, the Tracer Summit system, or the Comm5 network,
including the Rover service tool?
• Must the value be communicated (to Tracer Summit, Rover, or
another Comm5 device)?
If you answered no to all of the above questions, then it is appropriate
to use a constant. If you answered yes to any of the questions, then
use a variable.
To use a Constant block:
1. From the Blocks menu, choose Input/Output. From the Input/Output
menu, choose Constant.
2. Click in the design space to place the Constant block (Figure 16 on
page 20).
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