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.
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.