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Cascade Basics

Cascades in FOUNDATION ™ Fieldbus follow a strict if simple set of rules. A cascade is a two (2) way

communication. The driving or master block provides an output that is used as the cascade input by alower block commonly referred to as the slave. The slave provides an output that informs the master as towhen its output is being accepted and the limit conditions that exist below it; this output is the backcalculation output. The master reads this input through a back calculation input. (Refer to Advanced Topics- BKCAL Communications for more information.) For each cascade mode in a block, there is at least onecascade input and one back calculation output. For each block that is defined as control block, there is atleast one output and one back calculation input.

When the target mode is changed to a cascade mode in the slave, the back calculation output is set with asubstatus of Initialization Requested (IR). Upon seeing IR at the back calculation input, the master setsInitialization Acknowledge (IA) in its output substatus. The combination of IR in its back calculation outputand IA in its cascade input is the trigger for the slave block to change the actual mode to the cascade. Thislogic is generally applicable to all cascade inputs and outputs and cascade modes (Cas, RCas, ROut).(See below for the exception.) The following figure and table illustrate the initiation and completion of acascade with a master target mode of Auto and a slave initial mode of Auto. In fact, the initial slave modecould be any of the operating modes Man, Auto, RCas, ROut. The master target mode could be Man, Auto,RCas, ROut, Cas and, as long as the actual mode was not being forced to LO, the logic completes, asshown in the following figure.

Cascade Configuration

Example Cascade with Auto as Target Mode

Substatus Mode

Condition Slave BKCALOutput

Master OUT Slave Master

Cascade IsNot Possible

Not Invited Anything Not Cas Target: AutoActual: IMan

InitializationRequested bySlave

InitializationRequested

Anything but IA Target: CasActual: Auto

Target: AutoActual: IMan

Master SeesInitializationRequest

InitializationRequested

InitializationAcknowledge

Target: CasActual: Auto

Target: AutoActual: IMan

Slave SeesInitializationAcknowledge

Drops InitializationRequested andChanges to Normal

InitializationAcknowledge

Target: CasActual: Cas

Target: AutoActual: IMan

Master SeesNormal

Normal Drops InitializationAcknowledge andChanges To Normal

Target: CasActual: Cas

Target: AutoActual: Auto,assuming no otherforcing condition.

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The exception to the cascade handshaking behavior described above occurs when a CAS_IN orCAS_IN_D parameter has a NonCascade substatus. This is the case when there is no master control blockproviding a GoodCascade status to CAS_IN, but there is a parameter or calculation block providing aGoodNonCascade status to CAS_IN. In such cases the receiving block does not need to see an

InitializationAcknowledge (IA) on CAS_IN. If the target mode is Cas, the actual mode will change to Casimmediately if the status on CAS_IN is GoodNonCascade (and nothing else is preventing the actual modefrom climbing