Event Structure

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Only two formats are currently supported by the frontend,

Note that a frontend cannot write data directly into ROOT format. A conversion to ROOT may be done (e.g. by the data logger Mlogger) from one of the supported formats.

MIDAS Format Event

The MIDAS event format is a variable length event format. It uses "banks" as subsets of an event. A bank is composed of a bank header followed by the data.

Figure 1: Structure of MIDAS event showing Event and Bank headers with data banks.

Event Header

Each event carries a 16-byte header, generated by the Frontend with the bm_compose_event() routine and is used by consumers to distinguish between different events.

The event header is defined in the EVENT_HEADER structure in the file midas.h in the midas package. It has following structure:

the EventID identifies the event by number. Usually 1 is used for triggered events, 2 for scaler events, 3 for HV events etc.
the trigger mask can be used to describe the sub-type of an event. A trigger event can have different trigger sources like "physics event", "calibration event", "clock event". These trigger sources are usually read in by the front-end in a pattern unit. Consumers can request events with a specific triggering mask.
Serial number 
The serial number starts at 1 and is incremented by the front-end for each event.
Time Stamp 
the time stamp is written by the front-end before an event is read out. It uses the time() function which returns the time in seconds since 1.1.1970 00:00:00 UTC.
Event Data Size 
The event data size contains the size of the event in bytes excluding the header.

Event headers are always kept in the byte ordering of the local machine. If events are sent over the network between computers with different byte ordering, the event header is swapped automatically, but not the event contents.

If the byte ordering of the contents of a complete event has to be swapped, the routine bk_swap() can be used.

Data Area

The data area of the event can contain information in any user format (integer, real etc.), although only certain formats are supported when events are copied to the ODB or written by the logger in ASCII format.

The Data Area of a MIDAS event consists of a global Bank Header followed by one or more MIDAS Data Banks.

A Data Bank is a substructure of an event and can contain only one type of data, either a single value or an array of values. Each Data Bank has an individual bank header containing a name of exactly four characters, which is treated as a bank ID.

Bank Header (global)

There is one global Bank Header per event, defined in the BANK_HEADER structure in the file midas.h. It has the following structure:

All Bank Size 
Size in bytes of the following data plus the size of the bank header
used for endian detection (byte ordering)

The global Bank Header is initialized by the bk_init() call.

MIDAS Data Bank

Each data bank contains a header defined by the BANK (or BANK32) structures in the file midas.h.

It has the following structure:

Bank name 
four characters for the name of each bank. Each bank in an event must have a unique name.
Bank type 
one of the Midas Data Types TID_xxx values to encode the data type
Bank length 
size in bytes of the following data.

Following the header is the data in the format designated by the Bank type parameter (e.g. integer or float). A separate MIDAS bank must be created for each data type needed.

See MIDAS Event Construction.

Example of MIDAS event

Figure 2 shows a MIDAS event containing banks coloured to match the structure in Figure 1. This has been obtained from a MIDAS data file using the mdump application.

Figure 2: Example of MIDAS banks dumped by mdump.

------------------------ Event# 2 --------------------------------
Evid:000d- Mask:0000- Serial:0- Time:0x4c7a6869- Dsize:48/0x30
\#banks:1 - Bank list:-SDAS-

Bank:SDAS Length: 32(I*1)/8(I*4)/8(Type)Type:Real*4 (FMT machine dependent)
1-> 4.000e+00 1.000e+01 1.000e+00 3.400e+00 3.400e+00 3.400e+00 3.400e+00 3.400e+00

------------------------ Event# 3 --------------------------------
Evid:0001- Mask:0000- Serial:0- Time:0x4c7a686b- Dsize:344/0x158
\#banks:2 - Bank list:-MPETMCPP-

Bank:MPET Length: 304(I*1)/76(I*4)/76(Type) Type:Unsigned Integer*4
1-> 0x80010000 0x00000002 0x10010000 0x00004e21 0x80020000 0x00000002 0x20020000 0x000015f4
9-> 0x20020000 0x00001660 0x20020000 0x0000185f 0x20020000 0x0000191e 0x20020000 0x000019d6
17-> 0x40020000 0x00001a37 0x20020000 0x00001a77 0x20020000 0x00001ba2 0x10020000 0x00004e22
25-> 0x80030000 0x00000002 0x20030000 0x00001637 0x20030000 0x000018d1 0x20030000 0x000019bc
33-> 0x20030000 0x00001b35 0x20030000 0x00001bb2 0x10030000 0x00004e21 0x80040000 0x00000002
41-> 0x10040000 0x00004e22 0x80050000 0x00000002 0x20050000 0x000013c5 0x20050000 0x000017f2
49-> 0x20050000 0x0000185f 0x20050000 0x00001976 0x20050000 0x00001aa8 0x10050000 0x00004e21
57-> 0x80060000 0x00000002 0x20060000 0x000015c3 0x20060000 0x000018d8 0x20060000 0x0000198d
65-> 0x20060000 0x00001ac4 0x10060000 0x00004e22 0x80070000 0x00000002 0x20070000 0x00001747
73-> 0x20070000 0x000019ae 0x10070000 0x00004e21

Bank:MCPP Length: 16(I*1)/4(I*4)/4(Type) Type:Unsigned Integer*4
1-> 0x00005e4c 0x0000352d 0x00006453 0x00006d5b

--------------------------------------End of MIDAS Format ---------------------------------------------------------

FIXED Format Event

The "FIXED" format event is the simplest event format. The event length is fixed and is mapped to a C structure that is filled by the readout routine. Since the standard MIDAS analyzer cannot work with this format, it is only recommended for an experiment which uses its own analyzer and wants to avoid the overhead of a bank structure, or for monitoring purposes in the ODB.

The structure has to be defined twice: once for the compiler in the form of a C structure, and once for the ODB in form of an ASCII representation. There are several ways of doing this. The ASCII string may be supplied to the system as the init string in the equipment list as follows:

Example of definition of FIXED format using Equipment field "init string"

Alternatively, the structure may be defined first in the ODB, under /Equipment/<eqp_name>/Variables, and an experim.h file generated (see experim.h. The structure is then supplied to the readout routine by the frontend program including "experim.h", as follows:

Example of structure from experim.h for a fixed event

To select "FIXED" format, the Equipment Definition must have the format parameter set to "FIXED". The Equipment definition for this fixed event might be:

{ "Info ODB",     /* equipment name */
   10, 0,         /* event ID, trigger mask */
   "",            /* no banks sent */
   EQ_PERIODIC,   /* equipment type */
   0,             /* interrupt source */
   "FIXED",       /* format */
   TRUE,          /* enabled */
         RO_EOR,  /* read when running; 
                     send to odb */
   500,           /* polling period */
   0,             /* event limit */
   0,             /* number of sub-events */
   0,             /* log history */
   "", "", "",
   info_odb,      /* readout routine */

It is a good idea to check the record size and/or create the record in the ODB when using C structures from experim.h, to make sure that the structures in the ODB and in experim.h are identical. The frontend code might look like this:

/* frontend.c */
#include experim.h
// INFO_ODB_EVENT and  INFO_ODB_EVENT_STR are defined in experim.h
HNDLE hInfo;
INT status, size;
char   str_set[256];
sprintf(str_set,"/Equipment/INFO ODB/Variables");
/* create record /Equipment/INFO ODB/Variables to make sure it exists  */
/* find the key for info odb */
 status = db_find_key(hDB, 0, str_set, &hInfo);
 if (status != DB_SUCCESS)
     printf( "Key %s not found; creating record for info odb\n",str_set);
     status = db_create_record(hDB, 0, str_set, strcomb(info_odb_event_str));
 /* check the record size */
 status = db_get_record_size(hDB, hInfo, 0, &size);
 if (sizeof(INFO_ODB_EVENT) != size)
       cm_msg(MERROR, "bnmr_init", "error; record sizes do not match");
       return DB_TYPE_MISMATCH;

A readout routine for this fixed event is as follows:

INT info_odb(char * pevent, INT off)
/* - periodic equipment updating the ODB ONLY
  - no event generation for the data stream.
 /* fill various values */
 cyinfo.helicity = gbl_ppg_hel;
 cyinfo.current_cycle = gbl_CYCLE_N;
 cyinfo.current_scan = gbl_SCAN_N;
 cyinfo.epicsdev_set_v_ = epics_params.Epics_val;
 cyinfo.epicsdev_read_v_ = epics_params.Epics_read;
 cyinfo.campdev_set = 0;   
 cyinfo.campdev_read = 0; 
 memcpy(pevent, (char *)&(cyinfo.helicity), sizeof(cyinfo));
 pevent += sizeof(cyinfo);
 logMsg ("info_odb %d size:%d\n",gbl_CYCLE_N,sizeof(cyinfo),0,0,0,0);
 return sizeof(cyinfo);

More examples of FIXED events can be found in the slow controls device drivers, for example ../examples/slowcont/frontend.c and ../drivers/class/hv.c

Tape Format

Writing to tape as described below is not maintained. Files are usually written to disk, and may later be archived onto tape or other storage devices using normal file transfer methods.

Events are written to disk files without any reformatting. For tapes, however, a fixed block size is used. The block size TAPE_BUFFER_SIZE is defined in midas.h and usually 32kB.

Three special events are produced by the system.

  • A begin-of-run (BOR) and end-of-run (EOR) event is produced which contains an ASCII dump of the ODB in its data area. Their IDs is 0x8000 (BOR) and 0x8001 (EOR).
  • A message event (ID 0x8002) is created if Log messages is enabled in the logger channel setting. The message is contained in the data area as an ASCII string.

The BOR event has the number MIDAS_MAGIC (0x494d or 'MI') as the trigger mask and the current run number as the serial number. A tape can therefore be identified as a MIDAS formatted tape.

The routine tape_copy() in the utility mtape.c is an example of how to read a tape in MIDAS format.