EDOC329

 

EUROBALL DATA ACQUISITION SYSTEM

Raw Data Format (extended) - (edoc329)

Edition 1.0
Oct 27 1999

 

Nuclear Physics Group
Central Laboratory of the Research Councils
Daresbury Laboratory

Euroball Raw Data Event Format

This document defines an extension to the existing format of Raw Event Data within Euroball as defined in EDOC300. This document does not replace EDOC300 and hence EDOC300 should be read before reading this document.
The extension has been defined in such a way that software written to handle the extended format will by default also handle the existing format.

The 32 bit raw data format as defined in EDOC300 is:- 

        Format of a data word

                      I xx I xxxxxx I xxxxxxxx I xxxxxxxxxxxxxxxx I
                        qq    item     group          data

        groups 248 => 255 are reserved for use by the VREs
        the qualifier bits qq are reserved for use by the instrumentation cards

This extension to the format as defined in EDOC300 only considers the 14 bit address (group + item) field of actual data items. All token words and other data items generated by the VREs are unchanged.

Why is an extended format required?

The format as defined in EDOC300 allocates 6 bits to the item field and 8 bits to the group field. A group is taken to define a detector and the items label the data sources within that detector. This was originally defined for Eurogam I and has not been changed since. While this bit allocation is good for the complex segmented germanium detectors which have a large number of data sources for each detector it is not efficient for the simpler detectors commonly used for ancillary detector systems. These normally have only a small number (typically just energy and time) of data sources for each detector.

Euroball currently allocates 90 groups to the 3 phases of Germanium detector (of which 71 are normally in use). The proposed full instrumentation of the Euroball IV BGO Inner Ball will require 110 groups and an ancillary detector such as the Neutron Wall will require up to 80. This is a total of 280 which exceeds the 240 groups available. Hence a method to extend the number of available groups with as little effect on the existing system software as possible is required.

The extended use of the group and item fields

The extended format increases the number of groups available from 240 to 1088 and is totally compatible with existing use and software. 

        Format of a data word address


                      I xx I       xxxxxx     I    0xxxxxxx    I
                        qq          item            group 
        OR

                      I xx I    yyy    xxx    I    1xxxxxxx    I
                        qq     group  item          group 
                             extension
For addresses in which the most significant bit of the 8 bit group field is a zero (i.e. groups in the range 0 to 127) the item field is a 6 bit field as in the basic format. This provides total compatibility with existing use and software.

For addresses in which the most significant bit of the 8 bit group field is a one (i.e. groups in the range 128 to 247) the item field is restricted to the low 3 bits of the 6 bit field as used in the basic format and the high order 3 bits are used as an extension to the group number.

In this extended format the group is a 14 bit number constructed as 


                      I 00 I    yyy    000    I    1xxxxxxx    I
Thus the group can be extracted from the 16 bit address field using the mask 0x38ff and the item extracted using the mask 0x0700 and shifted down 8 bits as at present.

Advantages and limitations of the extended format

This extended format has the major advantage that for practical use it is totally compatible with existing use of the basic format by Euroball.
Also since the NEO++ converts the raw data addresses into "families" the users of Euroball will not be aware of the extension.

There are however some limitations:
1. Only 128 groups can have more than 8 items per group (up to 63).
2. All other groups are restricted to 1-7 items per group.
3. The extended groups are available only in "clusters" of 128 groups. That is the number space is not contiguous. The first "cluster" of extended group numbers is 2176 (0x0880) - 2295 (0x08f8) and the second "cluster" is 4224 (0x1080) - 4344 (0x10f8).