Technical Specifications

221
RCas Mode
Foundation Fieldbus
RCas Mode
When the positioner is receiving it's position Set Point from a remote computer system,
such as a DCS, then the normal state for the positioner is RCas. In this example, the
level control valve, LV201, is receiving it's signal from a DCS. The Analog Output Block
(AO) is shown in RCas mode.
RCas mode cannot be manually switched to, it is automatically switched to after being
configured. For example:
Configure the set point of the positioner in DCS to receive remote data, when the
positioner turns to Normal mode, the mode of AO block shows RCas in the Block Actual
Mode field on the positioner Main page.
PID in RCas Mode
If an internal PID is used in a cascade where the PID receives a Set Point from a
remote computer system, then the normal state has the PID in RCas mode and the
Analog Output Block (AO) is in Cas mode.
The Foundation Fieldbus Application Process specifies the mode handling for
interoperable function blocks. It also specifies which mode a block must be in when
each parameter is written. ValVueFF provides an intelligent, optimized, process for
managing configuration and calibration changes. For efficiency, it does not place blocks
into Out Of Service mode unless the parameter requires OOS. It analyzes the
requested changes and then sets the correct mode for each affected function block.
The actual block modes appear the IVI.
The Foundation Fieldbus specifies a mechanism for transferring between function block
modes. The user declares a target for the block mode but the actual mode is set by the
block in accordance with rules. The rules allow modes based on the quality of input data
and quality of the connection to the process. FVP function blocks comply with the
Fieldbus Foundation rules. ValVueFF sets target modes for individual function blocks as
required by configuration, calibration, diagnostics and other services. It sets target
modes for a group of function blocks to achieve three operational states of the
positioner: Out of Service, Manual, and Normal.
CAUTION The PID block remains in Out of Service mode after
configuration changes are made. After returning to Normal
Operating State open the PID dialog box to change the PID
block mode. If the AO block is left in Auto mode the valve
cannot be moved.