Compression Record (compress)

The data compression record is used to collect and compress data from arrays. When the INP field references a data array field, it immediately compresses the entire array into an element of an array using one of several algorithms, overwriting the previous element. If the INP field obtains its value from a scalar-value field, the compression record will collect a new sample each time the record is processed and add it to the compressed data array as a circular buffer.

The INP link can also specify a constant; however, if this is the case, the compression algorithms are ignored, and the record support routines merely return after checking the FLNK field.

Parameter Fields

The record-specific fields are described below, grouped by functionality.

Scanning Parameters

The compression record has the standard fields for specifying under what circumstances the record will be processed. These fields are listed in "Scan Fields". In addition, "Scanning Specification" explains how these fields are used. Since the compression record supports no direct interfaces to hardware, its SCAN field cannot specify I/O Intr.

The user specifies the algorithm to be used in the ALG field. There are six possible algorithms which can be specified as follows:

IndexIdentifierChoice String
0 compressALG_N_to_1_Low_Value N to 1 Low Value
1 compressALG_N_to_1_High_Value N to 1 High Value
2 compressALG_N_to_1_Average N to 1 Average
3 compressALG_Average Average
4 compressALG_Circular_Buffer Circular Buffer
5 compressALG_N_to_1_Median N to 1 Median

The following fields determine what channel to read and how to compress the data:

FieldSummaryTypeDCT DefaultReadWriteCA PP
ALGCompression AlgorithmMENU (compressALG)Yes YesYesNo
INPInput SpecificationINLINKYes YesYesNo
NSAMNumber of ValuesULONGYes1YesNoNo
NN to 1 CompressionULONGYes1YesYesNo
ILILInit Low Interest LimDOUBLEYes YesYesNo
IHILInit High Interest LimDOUBLEYes YesYesNo

As stated above, the ALG field specifies which algorithm to be performed on the data.

The INP should be a database or channel access link. Though INP can be a constant, the data compression algorithms are supported only when INP is a database link. See "Address Specification" for information on specifying links.

IHIL and ILIL can be set to provide an initial value filter on the input array. If ILIL < IHIL, the input elements will be skipped until a value is found that is in the range of ILIL to IHIL. Note that ILIL and IHIL are used only in N to 1 algorithms.

OFF provides the offset to the current beginning of the array data. Note that OFF is used only in N to 1 algorithms.

The RES field can be accessed at run time to cause the algorithm to reset itself before the maximum number of samples are reached.


Circular Buffer algorithm keeps a circular buffer of length NSAM. Each time the record is processed, it gets the data referenced by INP and puts it into the circular buffer referenced by VAL. The INP can refer to both scalar or array data and VAL is just a time ordered circular buffer of values obtained from INP. Note that N, ILIL, IHIL and OFF are not used in Circular Buffer algorithm.

Average takes an average of every element of the array obtained from INP over time; that is, the entire array referenced by INP is retrieved, and for each element, the new average is calculated and placed in the corresponding element of the value buffer. The retrieved array is truncated to be of length NSAM. N successive arrays are averaged and placed in the buffer. Thus, VAL[0] holds the average of the first element of INP over N samples, VAL[1] holds the average of the next element of INP over N samples, and so on. The following shows the equation:

N to 1 If any of the N to 1 algorithms are chosen, then VAL is a circular buffer of NSAM samples. The actual algorithm depends on whether INP references a scalar or an array.

If INP refers to a scalar, then N successive time ordered samples of INP are taken. After the Nth sample is obtained, a new value determined by the algorithm (Lowest, Highest, or Average), is written to the circular buffer referenced by VAL. If Low Value the lowest value of all the samples is written; if High Value the highest value is written; and if Average, the average of all the samples are written. The Median setting behaves like Average with scalar input data.

If INP refers to an array, then the following applies:

N to 1 Low Value

Compress N to 1 samples, keeping the lowest value.

N to 1 High Value

Compress N to 1 samples, keeping the highest value.

N to 1 Average

Compress N to 1 samples, taking the average value.

N to 1 Median

Compress N to 1 samples, taking the median value.

The compression record keeps NSAM data samples.

The field N determines the number of elements to compress into each result.

Thus, if NSAM was 3, and N was also equal to 3, then the algorithms would work as in the following diagram:

Operator Display Parameters

These parameters are used to present meaningful data to the operator. They display the value and other parameters of the record either textually or graphically.

FieldSummaryTypeDCT DefaultReadWriteCA PP
EGUEngineering UnitsSTRING [16]Yes YesYesNo
HOPRHigh Operating RangeDOUBLEYes YesYesNo
LOPRLow Operating RangeDOUBLEYes YesYesNo
PRECDisplay PrecisionSHORTYes YesYesNo
NAMERecord NameSTRING [61]No YesNoNo
DESCDescriptorSTRING [41]Yes YesYesNo

The EGU field should be given a string that describes the value of VAL, but is used whenever the get_units record support routine is called.

The HOPR and LOPR fields only specify the upper and lower display limits for VAL, HIHI, HIGH, LOLO and LOW fields.

PREC controls the floating-point precision whenever get_precision is called, and the field being referenced is the VAL field (i.e., one of the values contained in the circular buffer).

See "Fields Common to All Record Types" for more on the record name (NAME) and description (DESC) fields.

Alarm Parameters

The compression record has the alarm parameters common to all record types described in "Alarm Fields".

Run-time Parameters

These parameters are used by the run-time code for processing the data compression algorithm. They are not configurable by the user, though some are accessible at run-time. They can represent the current state of the waveform or of the record whose field is referenced by the INP field.

FieldSummaryTypeDCT DefaultReadWriteCA PP
NUSENumber UsedULONGNo YesNoNo
OUSEOld Number UsedULONGNo YesNoNo
SPTRSumming Buffer PtrNOACCESSNo NoNoNo
WPTRWorking Buffer PtrNOACCESSNo NoNoNo
CVBCompress Value BufferDOUBLENo YesNoNo
INPNNumber of elements in Working BufferLONGNo YesNoNo
INXCompressed Array InxULONGNo YesNoNo

NUSE and OUSE hold the current and previous number of elements stored in VAL.

BPTR is a pointer that refers to the buffer referenced by VAL.

SPTR points to an array that is used for array averages.

WPTR is used by the dbGetlinks routines.

Record Support

Record Support Routines

  long init_record(struct dbCommon *precord, int pass)

Space for all necessary arrays is allocated. The addresses are stored in the appropriate fields in the record.

  long process(struct dbCommon *precord)

See "Record Processing" below.

  long special(struct dbAddr *paddr, int after)

This routine is called when RSET, ALG, or N are set. It performs a reset.

  long cvt_dbaddr(struct dbAddr *paddr)

This is called by dbNameToAddr. It makes the dbAddr structure refer to the actual buffer holding the result.

  long get_array_info(struct dbAddr *paddr, long *no_elements, long *offset)

Obtains values from the circular buffer referenced by VAL.

  long put_array_info(struct dbAddr *paddr, long nNew);

Writes values into the circular buffer referenced by VAL.

  long get_units(struct dbAddr *paddr, char *units);

Retrieves EGU.

  long get_precision(const struct dbAddr *paddr, long *precision);

Retrieves PREC.

  long get_graphic_double(struct dbAddr *paddr, struct dbr_grDouble *p);

Sets the upper display and lower display limits for a field. If the field is VAL, the limits are set to HOPR and LOPR, else if the field has upper and lower limits defined they will be used, else the upper and lower maximum values for the field type will be used.

  long get_control_double(struct dbAddr *paddr, struct dbr_ctrlDouble *p);

Sets the upper control and the lower control limits for a field. If the field is VAL, the limits are set to HOPR and LOPR, else if the field has upper and lower limits defined they will be used, else the upper and lower maximum values for the field type will be used.

Record Processing

Routine process implements the following algorithm:

  1. If INP is not a database link, check monitors and the forward link and return.

  2. Get the current data referenced by INP.

  3. Perform the appropriate algorithm:

    • Average: Read N successive instances of INP and perform an element by element average. Until N instances have been obtained it just return without checking monitors or the forward link. When N instances have been obtained complete the algorithm, store the result in the VAL array, check monitors and the forward link, and return.

    • Circular Buffer: Write the values obtained from INP into the VAL array as a circular buffer, check monitors and the forward link, and return.

    • N to 1 xxx when INP refers to a scalar: Obtain N successive values from INP and apply the N to 1 xxx algorithm to these values. Until N values are obtained monitors and forward links are not triggered. When N successive values have been obtained, complete the algorithm, check monitors and trigger the forward link, and return.

    • N to 1 xxx when INP refers to an array: The ILIL and IHIL are honored if ILIL < IHIL. The input array is divided into subarrays of length N. The specified N to 1 xxx compression algorithm is applied to each sub-array and the result stored in the array referenced by VAL. The monitors and forward link are checked.

  4. If success, set UDF to FALSE.

  5. Check to see if monitors should be invoked:

    • Alarm monitors are invoked if the alarm status or severity has changed.

    • NSEV and NSTA are reset to 0.

  6. Scan forward link if necessary, set PACT FALSE, and return.