Technical Specifications

217
Multiple Modes and States
Foundation Fieldbus
Automatic (Auto) A local Set Point value is used by the normal block algorithm in
determining the primary output value. The local Set Point value
can be written to by an operator through an interface device.
Manual (Man) The block output is not being calculated, although it can be lim-
ited. It is directly set by the operator through an interface
device. The algorithm must initialize so that no bump is experi-
enced when the mode switches. The Set Point can be main-
tained or initialized to the process variable parameter value or
to the Set Point value associated with the previous retained tar-
get mode.
Local Override (LO) Applies to control and output blocks that support a track input
parameter. In the local override mode the block output is being
set to track the value of the track input parameter. The algo-
rithm must initialize so that no bump is experienced when the
mode switches from LO back to the target mode. The Set Point
can be maintained or initialized to the process variable parame-
ter value.
Initialization Manual
(IMan)
The block output is set in response to the back-calculation input
parameter status. When the status indicates there is no path to
the final output element the control blocks must initialize to pro-
vide for bumpless transfer when the condition clears. A
back-calculation out parameter is supported by all output and
control class function blocks. The Set Point can be maintained
or initialized to the process variable parameter value.
Out of Service (OOS) The block is not being evaluated. The output and Set Point are
maintained at last value.
Multiple Modes and States
The Analog Output Block (AO) must go to CAS mode to connect to a PID function block
in cascade. To prevent the Set Point signal out of the Analog Output Block (AO) from
setting the valve position into the transducer block, the Analog Output Block (AO) must
go into OOS mode. Table 19 shows several examples of mode operation.
When a block is in OOS mode its output status is bad. Any connected blocks know and
automatically change their modes. For example, when a PID block sees bad status in
BKCAL IN it goes to IMAN mode for initialization to the downstream block while
awaiting the status to return to good cascade.