Interfacing procedural elements
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There are several areas where it is necessary for Procedural Elements to communicate with each other.

One is where a higher level needs to invoke a lower level, for example when Unit Procedure needs to start and monitor an Operation (but not in a controller).

When you have a standard Recipe Manager this is generally handled automatically by the products intrinsic capabilities.

Another more important one is where the Procedure needs to hand control to Equipment, such as when an Operation needs to run an Equipment Phase.

This interface and linking between the procedural control (control recipe entities) and the equipment control (equipment entities) is generally done via a so called Phase Logic Interface. (In fact it could be at a higher level than phase such as operation)

The Standard provides an  'Example' State transition diagram for procedural elements and this can provide a basis for the interface.

Whilst being only an example, the version in the standard has become almost an industry standard. Note that in the 2nd edition of S88 it has been further developed

The version below is based on the original version of the standard but with the transitions made clearer

The Phase Logic Interface or PLI as it is known comprises a standard collection of flags that are connected between the Recipe Manager and the Equipment Controller. One for each equipment phase.

For programmers of equipment phases it is necessary to respond to some of the bits in the interface and to set others.

For example Start is set by the Recipe Manager (specifically by a Control Recipe) and Complete by the Equipment controller. When Start is Set then typically the Equipment phase SFC is initiated, and it sets the Complete as it finishes.

If you are using a standard Recipe Manager you should understand the Phase Logic Interface - normally the suppliers will provide a set of examples for PLC's

If you have a DCS with an integrated Recipe Manager then the process of creating the interface should be easier, but you must still program at least the running state.

In many implementations controller sequences are provided for the each of the Running, Holding, Aborting and Stopping states. This is one way, but there are alternatives, not least by embedding all the interface functionality in the one SFC, this is often easy and can save programming effort. Some DCS's may appear to force you to program each 'ing state, but there is always a workaround.

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