Chapter 38. Format of Event Data In Ring Buffers

The NSCL Ring buffer based data distribution scheme ia a very flexible data distribution scheme that makes no restrictions on the format of the data it transmits. This section describes the format of data that is placed in the ring buffer by NSCL DAQ readout software. These data are formatted in a consistent way so that consumer programs know what to expect and so that library functions can (and have been) be written that can select specific data record types in specific ways from the data stream.

This chapter is divided into these sections:

Reference material is provided in in the 3daq section that describes the class library.

38.1. The basic data formats

Data placed in the ring buffer consists of 'items'. Items have a header, which has the same shape from item type to item type and an item body whose format varies from type to type.

The header DataFormat.h contains, among other things, the struct definitions that define the shapes of each item. These structs are given typenames via the typedef statement. The remaining discussion will use these typenames, rather than the names of the underlying structs.

The header of each item type is of type RingItemHeader for convenience, the type pRingItemHeader is defined to be a pointer to a RingItemHeader. This is a common pattern in the DataFormat.h header.

The header has the following fields:

uint32_t s_size

Contains the size of the item in bytes. s_size should include the size of the header as well

uint32_t s_type

Contains a value that uniquely defines the type of datain the item. While this is a 32 bit field, the actual type values a 16 bits wide, with the remaining 16 bit set to zero. This allows consumer software to detect byte order differences between systems that generate the data and consumer systems.

The NSCL DAQ reserves types 1 through 32767 for itself. Type 0 is illegal, as it's byte order is indeterminate. Types 37678 through 65535 are available for user applications. The constant FIRST_USER_ITEM_CODE provides the symbolic value for the first item code available for user applications.

The NSCL DAQ is currently using the following item codes, defined in the DataFormat.h header:

BEGIN_RUN

The item describes the beginning of a data taking run.

END_RUN

The item descdribes the end of a data taking run.

PAUSE_RUN

The item describes a temporary pause in data taking. This item must be followed immediately by either a RESUME_RUN item, or an END_RUN item.

RESUME_RUN

This item describes the resumption of data taking after a temporary pause. Barring user defined types that may follow a PAUSE_RUN, this item will always follow a PAUSE_RUN item.

PACKET_TYPES

NSCL DAQ readout frameworks can package chunks of a physics event into packets. Packets have essentially the same format as ring buffer items. The frameworks also support documenting the packet types that can occur in a data taking run. This item supplies that documentation to interested consumers.

MONITORED_VARIABLES

Some readout frameworks support the creation of variables that can monitor external conditions such as EPICS channels. This item contains information about the latest state of a monitored variable.

INCREMENTAL_SCALERS

NSCL DAQ readout frameworks support the periodic readout of counters. These counters are called scalers by the experimental community and are used to monitor the rates of trigger components, detector systems, or system live-time. Data from these are put in INCREMENTAL_SCALERS items.

PHYSICS_EVENT

The purpose of NSCL DAQ readout frameworks, is to respond to event triggers and read out digitizer hardware that has captured the data from a nucleus-nucleus collision. These data are placed in PHYSICS_EVENT items.

PHYSICS_EVENT_COUNT

Periodically emitted to tell clients how many PHYSICS_EVENT items have been inserted in the ring. This is can be used to determine sampling efficiency for analysis consumers, as well as to compute event rates.

EVB_FRAGMENT

An event fragment from the ordering phase of the event builder.

These item types break down in to four distinct categories of item which will be described in the remaining subsections of this section.

38.1.1. State Change Items

State change items are those with types BEGIN_RUN, END_RUN, PAUSE_RUN, and RESUME_RUN. As the type names imply, these signal state transitions in data taking.

State change items have the type StateChangeItem. This item has the following fields:

uint32_t s_runNumber

Is the number of the run for which this state transition is being documented. Typically, run numbers are unique, for recorded runs, as the run number is encoded into the name of the run's event file.

uint32_t s_timeOffset

Is the number of seconds the run has been active prior to this state transition. Clearly if the type of the item is BEGIN_RUN this will be zero. For the NSCLDAQ frameworks, the time offset only counts seconds during which the run was active (time during which the run was paused are not counted).

uint32_t s_Timestamp

Is the absolute time at which the transition occured. This is represented in seconds since the Unix epoch of 00:00:00 UTC, January 1, 1970. Once translated into the host's byte order, it can be passed to any of the time formatting functions (e.g. asctime).

char s_title[TITLE_MAXSIZE+1]

Holds the run title. Run titles are restricted in size to TITLE_MAXSIZE characters, with the +1 to accomodate the trailing '\0'. TITLE_MAXSIZE is also defined in DataFormat.h.

All item types have a field s_header of type RingItemHeader that holds the item header.

38.1.2. Text List Items

Text list items contain a list of null terminated strings. They are usually used to provide metadata for the run. At present, two types of text list items are defined. PACKET_TYPES and MONITORED_VARIABLES.

PACKET_TYPES document the packets you might expect to find in a run's PHYSICS_EVENT items. Creating instances of the CDocumentedPacket object automatically generates these. Each packet is documented with a single string. The string consists of five colon separated fields. These fields contain, in order:

  1. The Name the packet.

  2. The id of the packet given as a hex string e.g. "0x1234"

  3. A desription of the packet.

  4. A version string for the packet. Presumably this will change if the packet with this type ever changes 'shape'>

  5. The date and time at which the CDocumentedPacket object creating this entry was created.

MONITORED_VARIABLES items contains a snapshot of the values of process variables that have been declared by the readout software. Each variable takes up one string and is formatted like a Tcl set command that, if executed, would define that variable to the value it had when the item was created.

String list items have type TextItem. This item has the following fields:

uint32_t s_timeOffset

The number of seconds of data taking that have gone on in this run prior to the generation of this item. This time offset does not count time in the paused state.

uint32_ts_timestamp

Is the absolute time at which the item was created. This is represented in seconds since the Unix epoch of 00:00:00 UTC, January 1, 1970. Once translated into the host's byte order, it can be passed to any of the time formatting functions (e.g. asctime).

uint32_t s_stringCount

Number of strings in the item.

char s_strings[]

An array of characters large enough to hold all the strings. Each string is a null terminated set of characters immediately followed by the next string.

38.1.3. Scaler Items

NSCLDAQ readout frameworks support periodically reading scaler data. These data are represented as ScalerItem items in ring buffers. These items have the following fields:

uint32_t s_intervalStartTime

The number of seconds of active data taking prior the start of the time period represented by the counts in this scaler item.

uint32_t s_intervalEndTime

The number of seconds of active data taking prior to the end of the time period represented by the counts in this scaler item.

uint32_t s_timestamp

Is the absolute time of the end of the scaler counting period. This is represented in seconds since the Unix epoch of 00:00:00 UTC, January 1, 1970. Once translated into the host's byte order, it can be passed to any of the time formatting functions (e.g. asctime).

uint32_t s_scalerCount

The number of scalers in the item.

uint32_t s_scalers[]

The array of scaler counts. This contains s_scalerCount elements.

38.1.4. Event Data Items

These items are of type PHYSICS_EVENT. The contain the data read from the hardware. Depending on the readout framework, this can be the response to one trigger or to a block of triggers. It is up to the analysis software to know which is which.

This item is of type PhysicsEventItem and contains the field uint16_t s_body that is the data from the event.

38.1.5. Event count items

These items are of type PhysicsEventCountItem. uint32_t s_timeOffset is the number of seconds into the active run the event occured. uint32_t s_timesamp is the absolute timestamp indicating when this item was created. uint64_t s_eventCount is the total number of events that have been contributed to this ring for this run.