Introduction
The PS EPB represents a two dimensional time/space container. Vertically, the resource rows (queues) are
defined ; whereas in the horizontal direction we represent the passage of time. Earlier, we talked about "white
space". White space, in our context, represents the availability of productive time for a particular resource on the
EPB.
Now, unless a resource is available to produce 24/7 (twenty four hours a day seven days a week) without respite,
we need a way to tell the PS engine when and when not the resource is productive. To do this, we introduce the
concept of down time. In order to block out non-productive time for a particular resource, we add a series of
down bars. The down bar represents a contiguous piece of time on a resource queue when the resource is not
available. In the Finite Capacity model this normally means that the machine that underlies the "resource" is
closed down; typically there is no way of operating the machine during that period. This usually means no crew
availability. So in essence, the down bar can be said to define "no shift".
Let's look at an example. A press department runs two shifts during the week, the last ending at 1am Saturday
mornings and starting again on Monday mornings at 7am. The shifts are nine hours each. This means that during
the week, the presses are non-productive from 1am till 7am (6 hours); and over the weekend from 1am Saturday
until 7am the following Monday (54 hours). For each resource in the press department, we need to introduce 5
down-bars for each week that we are likely to run these shift patterns into the foreseeable future. These downbars
will have the following spans: