Emerson E2 operation manual Possible Data Errors, Data Compression, Base Log Group, Clipping

olution (how precisely the analog values are recorded).

11.5.1 Possible Data Errors

For point log groups, regardless of the resolution, the following applies when errors occur in log data:

Displayed	Logged Value	Description
Value
NO DATA	(LOG_DATA_NONE)	Actual data value of the out-
		put is NO_VALUE but out-
		put status is not
		CELL_OUT_FAIL or
		CELL_OUT_INACT
PT FAIL	(LOG_PT_FAIL)	Data is NO_VALUE and sta-
		tus of output is
		CELL_OUT_FAIL -- could
		occur when point cell has
		passed one minute update
PT INACT	(LOG_PT_DISABLED)	Status of output is
		CELL_OUT_INACT and this
		appears regardless of the
		value of the data – this may
		occur at startup
ERROR		Indicates log error type could
		not be determined but the
		LOG_ERROR_FLAG was
		set in the data (this is display
		only and might indicate cor-
		rupted data)

Table 11-8- Data Errors

Adisplayed value of DATA N/A (LOG_DATA_NA) is used in medium and low resolution logs when that particu- lar type of data cannot be logged in that resolution. Pulse, one shot, and system list types cannot be logged in medium and low resolution log groups.

11.5.2 Data Compression

Because the user may need to log more data than the default logging of the E2 can hold in memory, the user can set the resolution type in the log manager to determine how the desired points will be logged. E2 converts these analog sample values to this lower resolution (Low or Medium) by:

•Adding/subtracting an offset

•Multiplying by a scaling factor

•Rounding to the nearest integer value of the size appropriate for the desired resolution

11.5.2.1Clipping

Clipping occurs whenever the conversion process results in a sample value that falls outside the range of the selected log resolution. For example, converting a value of 750 PSI to low resolution would result in a compressed

sample value of 609 PSI (refer to Appendix E: Com- pressed Point Log Sample Limits and Precision Tab). Low resolution only allows values up to 609 PSI; there- fore, the sample is said to be “clipped.” If the value that will be logged is anticipated to exceed the high or low range for a particular log resolution, a higher resolution should be chosen.

High Clipping occurs when a compressed sample value is above the range allowed by the set resolution. In this case the sample value is considered “high-clipped” and the data recorded will be the high-clipped value and logged at the highest possible valid value.

Low Clipping occurs when a compressed sample value is below the range allowed by the set resolution. In this case the sample value is considered “low-clipped” and the data recorded will be the low-clipped value and logged at the lowest possible valid value.

11.5.2.2Incompressible Data Types

Points of certain data types cannot be compressed due to the nature of their data values. These points are called Incompressible Points. Refer to the following table for the data types that are considered to be incompressible at cer- tain resolutions:

Data Type	Incompressible at Resolution
PULSE_TYPE	Low and Medium
ONE_SHOT_TYPE	Low and Medium
SYS_LIST_TYPE	Low and Medium

Table 11-1- Incompressible Data Types and Their Resolutions

11.5.3 Base Log Group

The Base Log group is a permanent logging group and cannot be deleted. When a new application is added to the system, the inputs and outputs that are logged by default are associated with the Base Log group.

Points are pre-configured to be logged for newly added applications. Many of the most important inputs and out- puts within an application are already pre-configured to be logged, and all of these inputs belong to the Base Log Group. Simply define the inputs and outputs to enable log- ging; no special set up is required. Logging is done auto- matically.

When you first create an application in the E2, you can tell which inputs and outputs belong to the Base Log Group: an “L” will be displayed at the right-hand side of their definitions. The “L” is a marker that signifies that the input or output is currently part of a logging group (either the Base Log Group or another group as assigned by the

Logging Groups Software Overview • 11-17