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Scheduler Dates Explained 

Tue March 17, 2020 03:29 PM

Start and End Dates

The Gantt bars in a schedule are rendered from a work order or task’s scheduled start date to its finish date.  So Scheduler will either use, or derive and use, these dates on the records that enter the schedule.  In some cases – for example, in a new schedule – dates may not exist yet. 

 

Rules to derive and use dates on the work records being displayed:

First, if both schedule dates already exist (schedstart and schedfinish), they are used no matter what the duration is..

Second, if one schedule date exists (schedstart or schedfinish) the other date is calculated by adding or subtracting the duration.

Third, if target dates exist (targstartdate and targcompdate), and duration value exists the target start date is used + duration, if no duration then the target start and end dates are used.

Forth, if one target date exists (targstartdate and/or targcompdate) the other date is calculated by adding or subtracting the duration.

Fifth, if no dates exist, the records are scheduled by their duration beginning at the current system date.  

 

Parent/child

If a child work order or task has no dates that can be used, it will initially use its parent's dates if they exist. If a child work order or a task does have dates that can be used they will override the parent's dates. The parent's dates will be a summary of the start of the earliest child/task through the finish of the latest finish of any of the children/tasks.

Completed, closed and cancelled

Completed records come into the Scheduler as gray read-only bars using their actual start and actual finish dates.

Closed and cancelled work orders are not brought into the Scheduler at all, even if the work order query on the Scheduler's main tab includes them.

Effect of Precedence Constraints:

Precedence constraints, or logical links, determine the order of scheduling by establishing dependencies between work orders or between tasks. Precedence constraints can be inserted, edited and deleted directly in the Gantt View.

They may have originated from a job plan or from work orders that were linked to their predecessors beforehand. Inserting or changing a link does not immediately change the schedule dates. They are processed as one of the inputs to the Critical Path Method (CPM) scheduling algorithm when it is invoked.

Precedence constraints can be finish to start (FS), finish to finish (FF), start to start (SS) or start to finish (SF). They can have lead time (overlaps or head starts) or lag time (gaps or delays).

Note about links: Maximo only allows links to be made between work orders at the same level under the same parent, or between tasks of the same work order. If you want to graphically manage links between your work orders, you may first want to organize them into a work pack under a parent or “header” work order in Work Order Tracking. In such a case, your work query could simply fetch that one work order since all children and tasks automatically come along with parents into the Scheduler.

The introduction of Scheduler Plus 764 on Maximo 7606 allowed any record at any level to be linked to any other, as long as circular references are avoided.

Effect of CPM, Precedence Constraints, Calendars and the Interruptible Flag:

The Critical Path Method (CPM) of calculating schedule dates can be invoked by either of the two CPM tools on the Gantt View’s toolbar, CPM and CPM Selected.

CPM provides very effective scheduling automation when precedence constraints exist between work orders or tasks, and you want the schedule dates to honor those relationships. It is also a very effective way to make sure work is scheduled only during calendar work time.

The initial view of work records (for instance, when a query is first brought into the Gantt View) will use existing dates if they exist. Schedule dates are used first, then targets, then the overall schedule start date, then the system date to establish a start.

If In Progress with an actual start and no actual finish, the actual start is used. If completed, actual start and finish dates are used.

Pressing the CPM button starts a process that follows a set of rules to determine the schedule dates for the work records in the set. It is primarily oriented toward durations, precedence constraints, and working time, and not toward resource availability. The schedule begins at the overall schedule start date, or if there is no schedule start date, it begins at the system date. First, all “source” records are evaluated – meaning those records without predecessors. If the Preserve Schedule Start Dates flag is set, those records will be unchanged from the last saved session. If the Preserve Schedule Start Dates flag is not set, their starts will all move to the schedule start (or today) if not already there.

If a task has a duration greater than 0, and no schedule dates, add the duration to targstartdate if it exists, or subtract it from targcompdate if it exists.

Once a start date is determined, the CPM routine considers each record’s interruptible state. If a record is interruptible, the record’s duration will be scheduled to occur only during working time according to the schedule’s calendar and shift. This may result in the record splitting around a non-work period, which is shown by shading that portion of its bar in the Gantt View. It can also result in a record’s start date moving forward in time to the next working period, if its original date happened to fall in a non-work period. When the interruptible flag is not set, the record is scheduled as soon as possible regardless of calendar/shift.

Next, successor records are evaluated. The precedence type and any lead or lag time is applied to the predecessor’s start or finish date to derive the successor’s start or finish date. Duration is added (to the start) or subtracted (from the finish) accordingly to derive the other date. Then that record’s successors are scheduled, to the end of that branch of the network which is terminated by a “sink” record – meaning one without successors. The total span of that branch of the network is rolled up to its parent. Parent schedules are always derived from the children and are not editable on the Gantt View.

 

Note on lead and lag time: when lead and lag times are applied to a precedence constraint, they are considered by CPM to occur as simple elapsed time – in other words, the calculation of a successor’s date does not have a calendar aspect to the precedence


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