Function Block Modes

Function block modes are described below. Supported modes vary with each function block. Some blocks support modes that are not supported in other blocks.

Out of Service (OOS): The block algorithm is not active. The output is maintained at the last value or at a specified failure action value.

Initialization Manual (IMan): The upstream block of a cascade pair is put into this mode when its downstream partner is in a non-cascade mode. This prevents the upstream block from closing the cascade. When the downstream block returns to a cascade mode (Cas or RCas), the upstream block leaves IMan and returns to its target mode.

Local Override (LO): The block is put into this mode when tracking is activated; the output is driven to a value other than that generated by normal block execution. When tracking is deactivated, the block returns to its target mode.

Manual (Man): The block output is set directly by the operator.

Automatic (Auto): In this mode, the control algorithm of the block is active. An operator-entered setpoint is used in the control algorithm to determine the block output.

Cascade (Cas): This mode is similar to Auto except that the setpoint is supplied by another function block through the CAS_IN parameter. The block maintains a back calculation output value (BKCAL_OUT) to provide bumpless mode transfer when the mode is changed.

Remote Cascade (RCas): This mode is similar to Cas except that the setpoint is supplied by an external control program through the RCAS_IN parameter. The block maintains a back calculation output value (RCAS_OUT) to provide bumpless transfer when the mode is changed.

Remote Out (ROut): This mode is similar to Man except that the OUT value is supplied by an external control program rather than directly by the operator. OUT is supplied through the ROUT_IN parameter. The block maintains a back calculation output value (ROUT_OUT) to provide bumpless transfer when the mode is changed.

The normal mode is the desired operating mode of the block that is set during configuration. Normal is meant to indicate that this is the mode in which the block should be operating during normal operation.

The target mode is the mode that is set manually or by another function during operation. These target modes must be listed during configuration as permitted modes for the block. IMan and LO are not selectable target modes. Unless you specify otherwise during configuration, the system assumes that you will write mode changes to the target mode field and read them from the actual mode field.

The following table lists the supported modes in specific function blocks. In addition, the Books Online information on individual function blocks lists the supported modes for that block.

Supported Modes for Function Blocks

* These modes are only visible when the function block is extended to a Fieldbus device.

Function Block Mode Basics

Function blocks support modes with the relevant fields:

• Target Mode - The mode that the block is trying to attain. When an operator sets the mode, it is the target mode that is being set.
• Actual Mode - The current operating mode of the block.
• Permitted - Those modes allowed as target modes. Write service prevents setting target mode to a non-permitted mode. Permitted mode is configurable by the user for each instance of each block, as defined by the supported mode.
• Normal - The correct mode for most cases of plant operation.

Each block supports a subset of the following modes through the named set parameter, SHED_OPT:

• RCas - Can be a permitted target mode. Remote cascade connection to setpoint. Setpoint is driven through RCAS_IN (the RCas input). The block algorithm determines block outputs.
• ROut - Can be a permitted target mode. Remote cascade connection to output. The block output is driven through ROUT_IN (the ROut input).
• Cas - Can be a permitted target mode. Cascade connection to setpoint. Setpoint is being driven through CAS_IN (the Cas input). The block algorithm determines block outputs.
• Auto - Can be a permitted target mode. Setpoint is local to the block, (not being driven externally). The block algorithm determines block outputs.
• Man - Can be a permitted target mode. Operator or logic external to the function block determines output.
• IMan - Is never a permitted target mode. Original definition was Initialization Manipulation, which means that the block tracks downstream operation (typically another block that has an open cascade).
• LO - Is never a permitted mode. Local Override or locked output. For a control block, the output tracks a specific input (the action triggered by a discrete track switch input). For an output block, failure action initiates.
• OOS - Is always a permitted mode. The block is out of service.

Modes have priority. The above is in inverse priority order. Higher modes have lower priority. For complete descriptions of all of the function block modes, refer to the Function Block Modes topic.

Mode Shedding Logic

Shed from or climb to a remote mode is determined by the parameter SHED_OPT. A block climbs and sheds through the same path. For example, if SHED_OPT specifies that a block should shed to Auto, then, if the block target mode is set to RCas, the block goes through Auto on the way to RCas.

Note Consider the effect of the fault state when selecting the mode shed option. Keep in mind that the fault state can cause the mode to go to Local Override instead of the expected mode from mode shedding.

Configuring Shed

The parameter SHED_OPT determines shed and climb for a remote mode. It can be configured as follows:

Shed With Return Options

Remote cascade connection failure shifts actual mode but keeps trying to restore remote cascade (in other words, the remote cascade target mode stays in effect).

• Normal - On failure of a remote cascade connection, the block attempts to attain the highest permitted non-remote mode until remote cascade is restored. On change to remote cascade target mode from any other mode, the block attempts to attain the highest permitted non-remote mode until a remote cascade connection is established.
• Retained Target - On failure of a remote cascade connection, the block attempts to attain the mode retained in the target mode. On change to the remote cascade target mode from any other mode, the block attempts to attain the mode retained by target mode until a remote cascade connection is established. The DeltaV system does not support the retained target shed. Selecting the retained target results in shed to Auto.
• Auto - On failure of a remote cascade connection, the block attempts to attain Auto, if permitted, until remote cascade is restored. On change to remote target mode from any other mode, the block attempts to attain Auto, if permitted, until a remote cascade connection is established.
• Man - On failure of a remote cascade connection, the block sheds to Man until a remote cascade connection is restored. On change to remote target mode, from any other mode, the block goes to Man until a cascade connection is established.

Shed With No Return Options

For any shed with no return option, the target mode changes as determined by the option. Therefore, there is no attempt to restore the connection following failure. The behavior on change to the remote cascade target mode is identical to that for Shed With Return Options.

• Normal - On failure of a remote cascade connection, the block sets the target mode to the highest permitted non-remote mode. On change to a remote cascade target mode from any other mode, the block attempts to attain the highest permitted non-remote mode until a remote cascade connection is established.
• Retained Target - On failure of a remote cascade connection, the block sets the target mode to mode retained in target mode. On change to the remote cascade target mode from any other mode, the block attempts to attain the mode retained by target mode until a remote cascade connection is established. The DeltaV system does not support the retained target shed. Selecting the retained target results in shed to Auto. This applies to any block, regardless of whether it is in a field device or a DeltaV controller. The writing device sets or clears the retained target bits in the target mode. Therefore, any workstation or other device (DeltaV controller applications, for example) that does not support retained target operation ignores the retained target bits. In a DeltaV system, regardless of where the block is located, the retained target bits are turned off, and the block sheds to Auto.
• Auto - On failure of a remote cascade connection, the block sets the target mode to Auto, if permitted. On change to the remote target mode from any other mode, the block attempts to attain Auto, if permitted, until a remote cascade connection is established.
• Man - On failure of remote cascade connection, the block sets the target mode to Man, if permitted. On change to the remote target mode from any other mode, the block goes to Man until a cascade connection is established.

It is possible for the user to configure SHED_OPT so that it calls for a target mode that is not permitted. When doing this, the mode logic does the following rules as applied by the remote logic:

• Shed logic never results in a non-permitted target mode.
• Shed logic never attempts to attain an actual mode of Auto or Cas if that mode is not permitted.

Cascade Basics

Cascades in FOUNDATION ä 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 a lower block commonly referred to as the slave. The slave provides an output that informs the master as to when its output is being accepted and the limit conditions that exist below it; this output is the back calculation 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 one cascade input and one back calculation output. For each block that is defined as control block, there is at least 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 a substatus of Initialization Requested (IR). Upon seeing IR at the back calculation input, the master sets Initialization Acknowledge (IA) in its output substatus. The combination of IR in its back calculation output and IA in its cascade input is the trigger for the slave block to change the actual mode to the cascade. This logic 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 a cascade with a master target mode of Auto and a slave initial mode of Auto. In fact, the initial slave mode could 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, as shown in the following figure.

Cascade Configuration

Example Cascade with Auto as Target Mode

The exception to the cascade handshaking behavior described above occurs when a CAS_IN or CAS_IN_D parameter has a NonCascade substatus. This is the case when there is no master control block providing a GoodCascade status to CAS_IN, but there is a parameter or calculation block providing a GoodNonCascade 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 Cas immediately if the status on CAS_IN is GoodNonCascade (and nothing else is preventing the actual mode from climbing).