Interval and Offset

Introduction

Certain tag types support Interval and Offset fields to define when and how often a timed activity occurs. Examples of timed activities include:

When a calculated tag is evaluated. For example Calculated Analog
When a register block is scanned from a device. For example Modbus Register Block Tag.

Interval and Offset fields are Duration Literals. For example T#1m30s represents 1 minute and 30 seconds.

Timed Activity Frequency

The Interval field defines how often the activity occurs. For example an Interval of T#5s sets the timed activity to occur every 5 seconds.

Fernhill SCADA will attempt to perform the timed activity at the interval. However, Fernhill SCADA does not guarantee the timed activity is performed at this interval. External and internal influences such as computer load, and the response time of external devices, may result in timed activities being delayed or dropped.

When a Timed Activity Occurs

Both the Interval and Offset fields define when a timed activity occurs:

The current time is rounded up to a multiple of the Interval. This is the base time of the timed activity.
The Offset is added to the base time.

Examples

Some examples of timed activity:

Interval	Offset	Comment	Event times
T#500ms	T#0s	Every 500 milliseconds at the start of second and at the start of the second plus 500ms	12:43:00.000 12:43:00.500 12:43:01.000
T#500ms	T#250s	As above, but offset by 250ms	12:43:00.250 12:43:00.750 12:43:01.250
T#10s	T#0s	Every 10 seconds at the 10-second boundary	12:43:00.000 12:43:10.000 12:43:20.000
T#10s	T#5s	As above but offset by 5 seconds	12:43:05.000 12:43:15.000 12:43:25.000

Further Information

Duration Literals

To learn about duration literal values.

Glossary

For the meaning of terms used in Fernhill SCADA.