Chapter 30. Compatibility utilities

The event format of the ring buffer data acquisition system is fundamentally different from that of earlier NSCL data acquisition systems. In order to ease the transition to the ring buffer system, several utilities' are provided that allow user software written for use with spectrodaq to operate unmodified.

The key utitity is compatibilitybuffer. This is a unix filter. compatibilitybuffer accepts a stream of ring items on its standard input and produces a stream of spectrodaq style nscl data buffers on its output. A trivial application of this program allows you to convert a ring event file to a spectrodaq event file as shown below:

Example 30-1. Converting a ring buffer event file with compatibilitybuffer

$DAQROOT/bin/compatibilitybuffer <run-1234-00.evt >run-1234-4096.evt

Other utilities and pipeline creating scripts allow you to directly write spectrodaq style event files when running your experiment, as well providing an SpecTcl pipeline data source that accepts ring items and provides spectrodaq style buffers to SpecTcl.

This section provides a brief tour of the compatibility utilities. See the 1compatibility reference section for detailed descriptions of each utility.

30.1. Format conversion with compatibilitybuffer

Compatibility utilities were build with a toolkit approach in mind. This allows you to build unix pipelines that compose these utilities to easily build actual applications.

The core tool in the compatibility toolbox is compatibilitybuffer which takes a stream of ring buffer items and transforms it into a string of spectrodaq style data buffers. You can specify the size of the output buffer using the --buffersize option. For example if, in the example Converting Event file, we wanted to write data with a 16384 byte buffersize we would instead write:

Example 30-2. Converting ring buffer data to a 8Kword (16Kbyte) old style event file

$DAQROOT/bin/compatibilitybuffer --buffersize=16384 <run-1234-00.evt >run-1234-4096.evt 

There is not a perfecdt match between ring buffer data and spectrodq buffered data. This is because:

  1. Sizes in ring buffer items are always 32 bits wide while buffer sizes and event lengths in the spectrodaq buffered data are only 16 bits wide.

  2. Sampled clients in ring buffer data count event triggers and can determine the sampling fraction by looking at the ratio of triggers seen to triggers emitted as documented by the PHYSICS_EVENT_COUNT ring items it sees. In spectrodaq bufffered data, each buffer has a sequence number and the sampled fraction is approximated by counting event buffers and taking the ratio of the event buffers seen to the last sequence number seen.

Due to these differences; PHYSICS_EVENT items must be shorter than 64Kwords of data. Similarly you cannot choose a --buffersize larger than 131070.

Sequence numbers are computed from the sampling fraction seen by compatibilitybuffer. If not all events are seen, you will see sequence number skips that will allow consumers of the output data to approximate the sampling fraction.