Group Update Rate
The Execution Schedule controls the rate at which the transaction group executes. On the Action tab of a group you have selected, under Execution Scheduling, there are two options: Timer and Schedule. Timer, executes the group at a certain rate. Schedule, executes the group at specific times. When the Schedule option spans across a period of time, you must specify the rate at which the group executes during that time.
Timer
The Timer acts as the heartbeat of the transaction group. It can run at millisecond, second, minute, hour, or day rates. The Timer specifies the OPC tag subscription rate for the OPC tags. When a Timer is running the transaction group first analyzes the tags inside the Basic OPC/Group Items section of the transaction group. Then it looks at the trigger configuration and evaluates for tag changes. Then it evaluates the specific trigger conditions and decides to execute on a trigger. If the triggered condition is true, the transaction group proceeds to the Triggered Expression Items section of the transaction group. Only after this flow is complete, will the transaction group interacts with the database and for example, insert the tag values into the database.
Schedule
An important difference between the Timer and the Schedule options is that the schedule option will automatically align to the specified start time on the update rate. For example, if the Schedule option is selected and the times are entered as 8,12pm-3pm and the update rate is 15 minutes, the transaction group will execute at 8am and then at 12pm, 12:15pm, 12:30pm and so on.