OPC90 Server
1.) OPC90 can control the STN block in both local and computer level. The STN_LEVEL tag
indicates the current STN block level (0 = local, 1 = computer). The STN block level can be
commanded to local or computer by writing the respective values 0 or 1 to STN_LEVEL.
When a client commands the STN block to the computer level it must satisfy the Bailey STN
block computer ok timer. This is accomplished by periodically issuing supervisory control
writes of the mode, set point or ratio index values. The client can also periodically write a
value of 1 to STN_OK or STN_LEVEL to satisfy the Bailey STN block computer ok timer (see
Device block AUTOMATIC SEND STN CPU_OK setting to delegate management of
computer ok to OPC90). The STN_OK tag reports the status of the Bailey STN block
computer ok timer. The client can monitor the STN_TIMER tag to determine the elapsed
time that has transpired since the last computer level supervisory control (mode, set point,
ratio index), STN_OK or STN_LEVEL write occurred. This timer is active when STN_LEVEL
indicates computer level and STN_OK indicates ok. When writing mode, OPC90 first checks
the current level of the STN block and sends out the appropriate new mode command based
on the current level. So for example if you command the STN block mode to auto by writing
a value of 1 and the STN level is currently computer manual, OPC90 will send out the
appropriate mode command for computer auto. The important thing to remember is the
values used to indicate and control Bailey STN block mode are always 0 = manual, 1 = auto,
2 = cascade regardless of STN block level (local or computer).
2.) The default value of zero instructs the driver to automatically determine the PID/SMITH block
number associated with the Bailey control station block. It follows the block input stream
starting with the STN S3 input looking for the existence of a PID of type FC 18, 19, 156 or
SMITH Predictor (FC 160). If one is found, it’s block number is written to this tag. Otherwise,
the value is set to -1 to indicate PID/SMITH block not found. This will be the case if a
PID/SMITH is not configured or two or more PID/SMITHs are found in the STN input stream.
In this circumstance the user must write the associated PID/SMITH block number to this tag
to make the other STN.K* tuning items functional. It is important to note that if the other
STN.K* tags are not functioning, check the value of this tag, making sure it has a valid
PID/SMITH block number. Until an actual PID/SMITH block is determined, the OUT_HI_LIM,
OUT_LO_LIM, K, KFC, KTYPE, KDIR, KIONLY, KP, KI, KD and KDLAG remain unknown.
3.) OUT_HI_LIM and OUT_LO_LIM are the high and low limit specifications read from the Bailey
PID/SMITH block associated with the Bailey Control Station block. Writing these values from
OPC90 causes the driver to automatically tune them in the appropriate Bailey PID/SMITH
block specification.
4.) K, KDIR, KIONLY, KP, KI, KD and KDLAG are specifications read from the Bailey PID/SMITH
block associated with the Bailey Control Station block. Writing these values from OPC90
causes the driver to automatically tune them in the appropriate Bailey PID/SMITH block
specification.
5.) When any of the K, KDIR, KIONLY, KP, KI, KD and KDLAG tags are written (even writing the
same value), OPC90 Server automatically puts the STN block into the “fast update” state.
This state causes OPC90 Server to acquire the STN.PV and STN.CO values with a polling
command instead of exception reporting. This operation is very useful for loop tuning
software. The duration for the fast update state and polling interval are device settings (see
DEVICE block). The KFAST attribute indicates the fast update state is in effect. The fast
update interval can be refreshed by writing a one to the KFAST attribute or writing a new
value to any of the other K tags (see note 4). The fast update interval is canceled
automatically when the fast update interval for the STN block has transpired or a value of
zero is written to the KFAST attribute. Some applications may want to have direct control of
the KFAST feature and not have writes to the other K* values automatically turn it on. This
can be accomplished by turning off the DEVICE block “Set .KFAST on .K* writes” property.
6.) When the analog output associated with a digital control station is bad, the STN block is
locked into manual mode. Attempted mode changes when the DS_BAD value indicates bad
will be rejected.
The RoviSys Company Version 7.0 Page 117
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