Event data format

Event data format

Event data created by Readout is submitted to the spectrodaq buffer server. Any suitably written application can attach to spectrodaq to obtain this data. The eventlog program runs when data recording is enabled to write runs to files on disk.

This section describes the structure of data in a run as it is seen either live or offline. This section describes the global structure of a run. Subsequent subsections describe the structure of individual data buffers.

Runs consist of a stream of fixed length buffers of data. As of November 7, 2002, these buffers are 8192 (8K) bytes long. Typically buffers are considered to contain 16 bit words of data (4096 of them).

Each buffer has a fixed length 16 word header that describes the buffer. Each buffer contains a homogoneous type of data that may have 0 or more "entities". Entities are never split across buffer boundaries.

The buffer header has the form shown in the table below:

Contents of buffer headers

Offset	Name	Words	Description
0	nwds	1	Number of used words in the buffer.
1	type	1	Type of data in the buffer.
2	checksum	1	Buffer checksum.
3	runnum	1	Run number of active run.
4	sequence	2	Buffer sequence number.
6	nEntities	1	Number of entities in the buffer.
7	unused	3	Unused, obsolete words.
10	format	1	Buffer format version.
11	ssignature	1	word of 0x0102 in source system byte order.
12	lsignature	2	longword of 0x01020304 in source system byte order.
14	Unused	2	Unused words.

Note

Some comments about some of these fields:

Values for the type field will be described below.
The checksum is calculated so that the sum over all used words in the buffer is 0.
ssignature and lsignature allow you to determine the set of transformations required to replay data on systems with different byte orderings than the readout system.

The type field deserves further explanation. Each data contains data of a homongeneous type. The type field describes what type of data each buffer contains.

Values for the type field include:

Values of the buffer type field.

Value	Type of data
1	Event data buffers
2	Scaler data buffers
3	Snapshot scaler data buffers
4	Documentation buffer: State variables
5	Documentation buffer: Run variables
6	Documentation buffer: Packet types
11	State transition: Begin run
12	State transition: End run
13	State transition: Pause run
14	State transition: Resume run

Report documentation errors to Ron Fox (fox@nscl.msu.edu)or NSCL's Bugzilla page

Event data format