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PICS has three main data flows: input, output and control. Input covers any of the data scanned from field inputs. Output includes manual substituted values, on/off state, alarm limit changes, etc. Control generally comprises all of the communication between the PICS System Monitor and the Task Monitors.
In general, input data originates at field input hardware located throughout the plant. These field inputs are connected to analog-to-digital (A2D) converter devices that are read by the 8800s (at PGDP the A2D system is either AVCO or CPI hardware). The values read from the A2D are called raw values in PICS. In the PICS database, each field point being scanned (identified by a scanner hardware address) is assigned a conversion (or data) type that determines the algorithm used to convert the raw value into an Engineering Units (EU) value. The algorithms allow inclusion of different constants for different sensors (perhaps for calibration).
Prior to conversion, the 8800's check the raw value against the point's validity limits to determine if an EU conversion is possible (to try to prevent things like division by zero and/or computations outside the domain of a conversion function). If not, then the point is marked as having a high (or low, as appropriate) validity alarm and the last known value is sent to PICS. Otherwise, an EU value is computed and then the 8800's perform alarm checking on the value, returning the appropriate alarm status along with the value.
The 8800s time stamp each value as it is scanned from the A2D device, providing very accurate time readings for each individual data point.
The actual application path may vary, but a typical path for a single field point value would be: 8800 scans, converts, alarms the value and sends it to MUX Control, who immediately forwards it to RTDBA (the master real time database) from whence it is shipped across the network, and also to all local applications registered to receive that field point. The point record may be transported even further, for example via RTServer to RTClient (via TCP/IP) and then to an end-user application, such as Recall Display. The large number of applications involved in transporting the data make things sound complicated, but they also provide PICS with much of its power and flexibility.
Actual output points, along with various database changes and things like on/off scan and on/off monitor (alarm) are handled by the output data stream. For data destined to the 8800s, this means that the PICS static database is updated to contain the desired output value, and the appropriate 8800 automatically requests the information immediately after it is posted to the static database. The MUXSDB application processes data requests from the 8800s and provides a change notification service as part of the network time message (sent once per second).
The PICS Task Monitor (TaskMon) uses UDP network messages to communicate with all other Task Monitors (TaskMon). Applications, such as the PICS System Viewer (SysView) may ask the local TaskMon to send control messages (such as 'reboot now' or 'set your failure action to log off') to any operating TaskMon (including the local one).
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