Example 4

Example 4.

Using System Variables

Each controller uses several variables while performing its tasks. For example, the controller uses the time kept by its clock and calendar to decide when to log data. The clock keeps the seconds, minutes, and hours, while the calendar keeps the day of the month, the month and the year. Refer to the section “Script Syntax” for more information about the system variables. Many system variables may also be used in the program. These system variables have names that start with the underscore character (‘_’). For instance, “_second” and “_minute” give the current seconds and minutes as the clock reports it. Just like input variables, in general system variables should also be considered as “volatile variables.” That is, their values may change by actions outside the scope of the program.

The following program illustrates how the output, “out1” is switched on for 20 seconds each minute.

title "example 4a. system variables "

output out1;

out1=(_second<20);

This program looks very similar to the simple thermostat control example. As in that example, the output is switched on when a certain condition holds true. The condition, in this case is different. The condition is true when the current seconds is less than 20. So while the current seconds ticks from 0 to 59, the condition is true for the first 20 seconds, and false afterwards.

Unlike the output variable “out1”, note that the system variable “_second” is not included in the declarations segment of the script. A very simple change to the above program is shown below. Here, the system variable “_beep” is used as the output. The system variable “_beep” is the beeper on the controller. This way, you can beep the output for 20 seconds at the beginning of each minute. Note that there are no inputs or outputs in the declaration segment.

title "example 4b. system variables "

_beep=(_second<20);

You may consider using a parameter to change the behavior of the program. Parameters were introduced in Example 2.

title "example 4c. system variables "

parameter pBeep;

_beep= pBeep && (_second<20);

In the above program, the condition is made up of two components. For the condition to be true, both the parameter “pBeep” must be true (nonzero) and the seconds must be less than 20. As in Example 3, the two components of the condition are AND-ed using the Boolean operator “&&.” Refer to the section “Script Syntax” for more information about the Boolean operators. Now the feature of beeping 20 seconds at the beginning of each minute may be turned on or off by modifying the parameter value “pBeep.” As discussed in Example 3, this can be done from iNetGrow, by clicking the “Peek” button, or, if you have a web server on the LAN, from a web page through your browser.

Also see : Programming, Program Examples, Script Syntax