Event Builder

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Introduction to Event building in MIDAS

MIDAS supports event building through a dedicated mevb application. Similar to the Frontend, the mevb application requires the definition of an equipment structure which describes its mode of operation.

The set of parameters for this equipment is limited to:

  • Equipment name (appears in the Equipment list).
  • Equipment type (should be 0).
  • Destination buffer name (SYSTEM if destination event goes to logger).
  • Event ID and Trigger mask for the build event (destination event ID).
  • Data format (should match the source data format).

Based on the given buffer name provided at the start-up time through the " -b <buffer_name> " argument supplied, the mevb utility will scan all the equipments and handle the building of an event based on the identical buffer name found in the equipment list if the frontend equipment type includes the "EQ_EB" flag.

Example

An example eventbuilder can be found in $MIDASSYS/examples/eventbuilder/.

Event Builder and related frontend fragment

It is possible that in the case of multiple Frontends, the same "fragment" code may run in the different hardware frontends. In order to prevent having to build nFragment different Frontend tasks, the Frontend task argument -i <index> will replicate the same application image with the necessary dynamic modification required for the proper Event Building operation.

The index can be passed to the frontend application either by this argument, or by environment variable MIDAS_FRONTEND_INDEX

The -i <index> argument will provide the index to be appended to the minimal set of parameters to distinguish the different frontends. These parameters are:

frontend_name 
Name of the frontend application.
equipment name 
Name of the equipment (from the Equipment structure).
event buffer
Name of the destination buffer (from the Equipment structure).

Asynchronized and synchronized data collection can be envisaged (see Mevb). - For asynchronized fragment collection, the composition of the "equipment name" in the Equipment definition requires the index format to be present (see example 1). - For synchronized gathering of the fragments, the composition of the "equipment name" and "event buffer" both require the index format to be present (see example 2).


Examples of Frontend Code for fragment collection in the event builder

1.Asynchronized

Event buffer remains the same for all the fragments:


 char *frontend_name = "ebfe";    /* The frontend name (client name) as seen by other MIDAS clients */ 
 ...  
  EQUIPMENT equipment[] = {
     {"Trigger%02d",              /* equipment name */
      1, TRIGGER_ALL,         /* event ID, trigger mask */
      "BUFFER",                  /* event buffer */
      EQ_POLLED | EQ_EB,       /* equipment type + EQ_EB flag <<<<<< */
      LAM_SOURCE(0, 0xFFFFFF), /* event source crate 0, all stations */
      "MIDAS",                 /* format */


2.Synchronized

 char *frontend_name = "ebfe";  /* The frontend name (client name) as seen by other MIDAS clients */
 ...
  EQUIPMENT equipment[] = {
     {"Trigger%02d",              /* equipment name */
      1, TRIGGER_ALL,         /* event ID, trigger mask */
      "BUF%02d",                  /* event buffer */
      EQ_POLLED | EQ_EB,       /* equipment type + EQ_EB flag <<<<<< */
      LAM_SOURCE(0, 0xFFFFFF), /* event source crate 0, all stations */
      "MIDAS",                 /* format */


Once the frontend is started with e.g. -i 1 , the MIDAS client name, equipment name and, in the case of the synchronized event builder, buffer name will be modified by adding the index number (1 in this case).


> ebfe -i 1 -D
...
odbedit
[local:midas:S]/Equipment>ls
Trigger01
[local:midas:S]Trigger01>ls -lr
Key name                        Type    #Val  Size  Last Opn Mode Value
---------------------------------------------------------------------------
Trigger01                       DIR
   Common                      DIR
       Event ID                WORD    1     2     18h  0   RWD  1
       Trigger mask            WORD    1     2     18h  0   RWD  65535
       Buffer                  STRING  1     32    18h  0   RWD  BUF01 
       Type                    INT     1     4     18h  0   RWD  66
       Source                  INT     1     4     18h  0   RWD  16777215
       Format                  STRING  1     8     18h  0   RWD  MIDAS
       Enabled                 BOOL    1     4     18h  0   RWD  y
       Read on                 INT     1     4     18h  0   RWD  257
       Period                  INT     1     4     18h  0   RWD  500
       Event limit             DOUBLE  1     8     18h  0   RWD  0
       Num subevents           DWORD   1     4     18h  0   RWD  0
       Log history             INT     1     4     18h  0   RWD  0
       Frontend host           STRING  1     32    18h  0   RWD  hostname
       Frontend name           STRING  1     32    18h  0   RWD  ebfe01
       Frontend file name      STRING  1     256   18h  0   RWD  .../eventbuilder/ebfe.c
...


Independently of the event ID, each fragment frontend will send its data to the composed event buffer (BUFxx). The event builder task will make up a list of all the equipment belonging to the same event buffer name (BUFxx). If multiple equipments exists in the same frontend, the equipment type (EQ_EB) and the event buffer name will distinguish them.


Event Builder Flowchart

The Event Builder flowchart below shows a general picture of the event process cycle of the task. The Event Builder runs in polling mode over all the source buffers collected at the begin of run procedure. Once a fragment has been received from all enabled sources (i.e. those with /Equipment/<equipment-name>/Settings/Fragment Required set to y"), an internal event serial number check is performed prior to passing all the fragments to the user code. A check on the content of each fragment can be done within the user code for further consistency check.

Event Builder Flowchart