DO Function Block
March 2006
5-147
Retained Target—The retained target mode is the
target mode for the block before changing the target
mode to a remote mode. On failure of a remote
cascade connection, the block attempts to attain the
retained target mode.
Auto—On failure of a remote cascade connection, the
block attempts to attain Auto, if permitted, until remote
cascade is restored.
Man—On failure of a remote cascade connection, the
block sheds to Man until a remote cascade connection
is restored.
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. Cas is the highest permitted
non-remote mode and Auto is is the next highest
permitted non-remote mode. If Cas or Auto are not
available, the block will shed by default to Man.
Retained Target—The retained target mode is the
target mode for the block before changing the target
mode to a remote mode. On failure of a remote
cascade connection, the block sets the target mode to
the retained target mode.
Auto—On failure of a remote cascade connection, the
block sets the target mode to Auto, if permitted.
Man—On failure of remote cascade connection, the
block sets the target mode to Man, if permitted.
The user may configure SHED_OPT [23] so that it
calls for a target mode that is not permitted. When
doing this, the mode logic uses 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.
Block Initialization
The Fieldbus Foundation specification requires that
certain parameters have initial values of uninitialized in
function blocks. In addition to setting the Resource
block mode to AUTO, the control system or the user
must change those parameters from their uninitialized
value to a valid value in order for the function block to
move from the Out of Service mode. For the DO
function block, the parameters that must be initialized
are:
SHED_OPT [23] (see page 5-146 for valid values)
CHANNEL [18]
Status Handling
Under normal operating conditions, the status of
OUT_D [9] is Good Non-Cascade, and the status of
BKCAL_OUT_D [21] is Good:Cascade. If the output
hardware fails, the status of BKCAL_OUT_D [21] is
set to Bad:Device Fail, and the BLOCK_ERR [6]
shows Output Failure. If the hardware used for output
feedback fails, the status of READBACK_D [16] and
PV_D [7] is set to Bad:DeviceFail, and the
BLOCK_ERR [6] shows Process Variable has Bad
Status. If the transducer block mode is Out of Service,
the status of READBACK_D [16] and PV_D [7] is set
to Bad:Out of Service.
I/O Selection
To select the I/O associated with the discrete output,
configure the value of the CHANNEL [18] parameter.
Table 5-68 lists the valid Channel selections for the
DO block.
Table 5-68. Channel Selections for the Discrete Output
Function Block
Selection
Transducer Block
Parameter
Transducer
block Index
Description
22 SETPOINT_D 32
Discrete Valve
Control
0 − − Uninitialized
Setting the Output
To set the output for the DO block, you must first set
the mode to define the manner in which the block
determines its set point and output. In Cascade mode,
the set point equals the input value at the CAS_IN_D
[17] parameter. In Automatic or Manual mode, the set
point must be entered manually by the user. For
Automatic, the value must be written to the SP_D [8]
parameter and for Manual, the value must be written
to OUT_D [9]. In Remote Cascade mode, the set point
is determined by a host computer that is writing to the
RCAS_IN_D [22] parameter. Table 5-69 lists discrete
5
To Next Page
Loading...
Need help?
General
