OPC90 Server
number of auxiliary values associated with the typical values that are also
included in the OSTN implementation. The RED_TAG tag allows the client to
indicate the device is currently under a red tag condition. The client reports the
mode of the control loop via the MODE tag. Valid modes are manual (0), auto
(1) and cascade (2). OPC90 will receive mode write request from the Bailey
system and transfer it to the client by writing the new requested mode value to
the MODE tag. Bailey logic can lock a control loop into any given mode. The
client uses the MODE_LOCK tag to indicate this condition. The PV tag is used
by the client to report the process variable of the control loop. This value is in
engineering units and will be clamped between the zero and span defined for the
process variable. The OUT tag is used by the client to report the current value of
the control output. By definition for Bailey systems, OUT has a range of –5.0 to
105.0 that expresses a percentage. OPC90 will receive control output write
request from the Bailey system and when the current mode is manual, transfer
the control output to the client by writing it to the OUT tag. Bailey logic can
enable control output tracking while in manual mode. The client uses the
OUT_TRACKING tag to indicate this condition. The SP tag is used by the client
to report the current value of the set point. This value is in engineering units and
will be clamped between the zero and span defined for the set point. OPC90 will
receive set point write request from the Bailey system and when the current
mode is manual or auto, transfer the set point to the client by writing it to the SP
tag. Bailey logic can enable set point tracking while in manual or auto mode.
The client uses the SP_TRACKING tag to indicate this condition. A Bailey STN
block can be configured to be a ratio station. Ratio stations are similar to a
cascaded loop but differs in its method of set point generation when in cascade
mode. The cascaded set point input (wild variable) is multiplied by a ratio
adjustment factor (called the ratio index) to determine the actual set point value.
The RI tag is used by the client to report the current value of the ratio index. By
definition for Bailey systems, the ratio index has a valid range of 0.05 to 10.0.
OPC90 will receive ratio index write request from the Bailey system and when
the current mode is cascade, transfer the ratio index to the client by writing it to
the RI tag. Some older Bailey systems using the station block have also
included a “hand held” station called a Digital Control Station that allows plant
personnel to take direct control of the loop output. The client uses the
AO_BYPASS tag to indicate this condition has occurred. Likewise, the client
uses the DS_BAD tag to also indicate that the controller has lost communication
with the Digital Control Station hand held station.
The RoviSys Company Version 7.0 Page 102
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