Long time storage of data for a trend graph.

 In every node where trend graphs shall be stored there
 is a need of a DsTrend job, which is searching through
 the local data storage objects of the node.
 Every measurant which has to be plotted demands a data
 storage object. Attributes in the objects specify for
 example the name of the measurant and how often it is
 to be stored, see figure 3-13. A measurant may be used
 by several different data storage objects, for example
 to store at different frequencies.

 DsTrend Job with Data Storage Objects

 Note! Remember that the sampling rate of physical hardware
 is finite. At present Ai-signals are sampled at 5 Hz,
 Co-, Di-, Do-, and Dv-signals at the base frequency
 determined by the fastest running PLC program of the
 node. Highest possible base frequency is 50 Hz.
 A trend graph consists of measurant data collected at
 some frequency.

 Collected values are stored in DataBuffer. Internally
 DataBuffer is divided into two equal sub-buffers. The
 way of working is to update every element in one of the
 sub-buffers and then continue with the first element in
 the other sub-buffer. When also the last element in the
 second sub-buffer has been updated the updating process
 continues with the first element in the first
 sub-buffer. The sub-buffers are updated in turn.
 BufferStatus specifies how the sub-buffers are used in
 a certain moment.
 The measurant has to be a local quantity. So, DsTrend
 objects have to reside at the same node as the
 mesurants in the runtime environment.
 The dsTrend module is included in the EBUILD-file of
 the node by means of program
 PWR_EXE:RS_DS_TREND.EXE_ELN /warm_debug

 The measurant is supposed to be of any of the types:
 - pwr_tBoolean
 - pwr_tFloat32, pwr_tFloat64
 - pwr_tInt8, pwr_tInt16, pwr_tInt32
 - pwr_tUInt8, pwr_tUInt16, pwr_tUInt32

 If a signal is associated with an object by the type
 DsTrend, DsFast, or DsHist this relation may be marked
 by putting the object direct below the current signal
 object.

 Configuration of a DsTrend Object

 Optional text.

 The scanning period, in seconds, of the DsTrend job.
 See also Multiple.

 The attribute, is assigned the same value as
 corresponding attribute in the DsTrendConf object of
 the node. This is done at the initialisation of the
 DsTrend job.

 The individual quantity is stored at the period of
 ScanTime * Multiple.

 Specifies at which scan this data storage object has to
 be updated. It is set equal to Multiple and then
 decreased at every scanning. If NextMultiple = 0
 storage will take place, and NextMultiple is set equal
 to the value of Multiple.

 The datatype of stored measurant. It has to be any of
 the types pwr_tBoolean, pwr_tFloat32, pwr_tFloat64,
 pwr_tInt8, pwr_tInt16, pwr_tInt32, pwr_tUInt8,
 pwr_tUInt16, or pwr_tUInt32.

 The complete name of the analog measurant, for example
 PROC1-TEMP1.ActualValue. The measurant has to be
 represented by a local object.

 Pointer to the analog measurant which is to be stored.

 Identity at subscription of the measurant.

 Length, in seconds, of the testing result. If the time
 exceeds what is possible to store in DataBuffer, the
 storage is stopped when DataBuffer has been filled.
 StorageTime, Multiple , BaseFrequency and
 NoOfBufElement determines NoOfSample.

 Curve window layout.

 Number of samples per sub-buffer.

 Specifies which sub-buffer is used for writing at the
 moment.

 = 0, .... (NoOfBuffers - 1)
 NextWriteIndex[2]

 Specifies the location in the current sub-buffer where
 the next writing will be done.

 Specifies the current status of the sub-buffers.

  -- 0     The sub-buffer is not used for writing.
  -- 1     The sub-buffer is used for writing.

 Specifies the number of sub-buffers in the area
 DataBuffer.

 Specifies the number of elements in every sub-buffer.
 BufferTime[2]

 Time (per sub-buffer) when the first storage in a
 writing session was made.

 Total accessible storage memory. The maximum size is
 determined by NoOfBuffers NoOfBufElement, which is
 equal to the maximum number of samples in the same
 storage cycle.