Opc interface to the pi system

Quality Information

The OPC Data Access standard uses Fieldbus quality flags. The interface translates those quality flags to the closest approximation within the PI System State table. The low 8 bits of the Quality flags are currently defined in the form of three bit fields, Quality, Sub-status and Limit status. The 8 Quality bits are arranged as follows:

QQSSSSLL

The tables at the end of this section give the transformation to PI status. The second table gives the status codes that will be used if the PI system is version 3.3 or higher. That version of PI is the first that defines the states needed by the OPC interface so that the interface will not use the Archive Offline and other states which were causing some confusion.

Questionable Qualities -- Store the Status or the Value?

Because the PI archive stores either the quality or the value, the interface will translate the qualities in the “questionable” category to GOODSTAT, and set the “questionable value” flag for the data value. “Bad quality” flags get the corresponding PI status stored for the tag. If the quality is SUBSTITUTED_ST, the interface will currently store the status rather than the value sent. This behavior can be changed with the /SQ parameter in the startup file. Setting /SQ=Y parameter will cause the interface to store the quality (translated to a PI digital state) if the quality is anything but GOOD. Similarly, if /SQ=I is set, the interface will ignore the "questionable" quality, and treat the value as if it had good quality.

Quality	no /SQ parameter	/SQ=Y	/SQ=I
GOOD	value	value	value
Questionable	value, parameter	digital state	value
BAD	digital state	digital state	digital state

The behavior can be enhanced to send only GOOD quality data by setting the /SG parameter. If the /SG is set, only GOOD quality data will be sent to PI. Questionable quality data and BAD quality data will be ignored.

If the /SQ=I or /SQ=Y flag is also set, then Questionable Quality data will be sent to PI as described above. BAD quality data will continue to be ignored.

The quality information will continue to be sent to tags that are configured to store quality instead of values.

Storing Quality Information Directly

Storing the quality in a separate PI tag is also an option, so both the values reported and the qualities that came with those values are recorded, with no loss of granularity. Setting Location2=4 for a tag tells the interface to store the quality for the associated ItemID rather than the value. Since OPC qualities are unsigned, 16-bit integers, the interface expects to have an Int32 tag to receive them. The values are stored in PI without any change, and their status is always GOOD. To understand what those quality values represent, please go to http://www.opcfoundation.org and download the OPC Data Access specifications, which contain a brief discussion of quality data. The OPC quality table can also be found in the PI OPCClient User’s Guide.

Note: The column on the left in the tables below is a bit mask. The first number, for instance, would correspond to a hexadecimal value between 0xC0 (11000000 bit mask) and 0xFF (11111111 bit mask).

Quality States Used with PI Systems Earlier than PI 3.3

Good Quality

Quality	OPC Definition	PI Status
11SSSSLL	Non-specific	Good
Except
110110LL	Local Override	_SUBStituted

Not used by OPC

Quality	OPC Definition	PI Status
10SSSSLL	Invalid	Bad Input

Questionable

Quality	OPC Definition	PI Status
010110LL	Sub-Normal	Bad_Quality
010101LL	Engineering Units Exceeded
QQSSSS01	Low Limited	Under LCL
QQSSSS10	High Limited	Over UCL
Otherwise		Inp OutRange
010100LL	Sensor Not Accurate
QQSSSS01	Low Limited	Under Range
QQSSSS10	High Limited	Over Range
Otherwise	Out of calibration	Invalid Data
010011LL	Invalid	Bad Input
010010LL	Invalid	Bad Input
010001LL	Last Usable Value	No_Sample
010000LL	Non-specific	Doubtful

Bad Quality

Quality	OPC Definition	PI Status
000111LL	Out of Service	DCS failed
000110LL	Comm Failure	Arc Off-line
000101LL	Last Known Value	Scan Timeout
000100LL	Sensor Failure	Equip Fail
000011LL	Device Failure	Unit Down
000010LL	Not Connected	Access Denied
000001LL	Configuration Error	Configure
000000LL	Non-specific	Bad

Quality States Used with PI Systems Version PI 3.3 and Later

Good Quality

Quality	OPC Definition	PI Status
11SSSSLL	Non-specific	Good
Except
110110LL	Local Override	_SUBStituted

Not Used by OPC

Quality	OPC Definition	PI Status
10SSSSLL	Invalid	Bad Input

Questionable

Quality	OPC Definition	PI Status
010110LL	Sub-Normal	Bad_Quality
010101LL	Engineering Units Exceeded
QQSSSS01	Low Limited	Under LCL
QQSSSS10	High Limited	Over UCL
Otherwise		Inp OutRange
010100LL	Sensor Not Accurate
QQSSSS01	Low Limited	Under Range
QQSSSS10	High Limited	Over Range
Otherwise	Out of calibration	Invalid Data
010011LL	Invalid	Bad Input
010010LL	Invalid	Bad Input
010001LL	Last Usable Value	No_Sample
010000LL	Non-specific	Doubtful

Bad Quality

Quality	OPC Definition	PI Status
000111LL	Out of Service	Out of Service
000110LL	Comm Failure	Comm Fail
000101LL	Last Known Value	Scan Timeout
000100LL	Sensor Failure	Equip Fail
000011LL	Device Failure	Unit Down
000010LL	Not Connected	Not Connected
000001LL	Configuration Error	Configure
000000LL	Non-specific	Bad

Through the use of the /AS=system_digital command line option, it is possible to replace the default PI statuses (i.e. digital states) listed in the table above with custom states. To use this command line option it is necessary to create a marker digital state in the system digital state set that corresponds to system_digital. Following this marker state, 18 digital states can be created to override the default statuses listed in the above tables. The order of the digital states along with the PI statuses that are replaced are listed in the table below.

Order after Marker state	Default PI status
1	Bad_Quality
2	Under LCL
3	Over UCL
4	Inp OutRange
5	Under Range
6	Over Range
7	Invalid Data
8	Bad Input
9	No_Sample
10	Doubtful
11	Out of Service
12	Comm Fail
13	Scan Timeout
14	Equip Fail
15	Unit Down
16	Not Connected
17	Configure
18	Bad