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DIRC Numerology


Dirc Sectors

As seen from the exterior of the Dirc Standoff Box, the twelve sectors [0-11] are numbered clockwise, starting from the upper vertical (noon/midnight position).
Sector 0 covers noon to one o'clock, Sector 11 covers 11 o'clock to midnight, etc.

With this convention, Sectors 0-5 are on one Sob door (the one on the right as seen from behind), and Sectors 11-6 on the other one.

In the BaBar system, the z-axis goes along the high energy beam. Each Sector is assigned a local frame, which Z-axis is parallel BaBar's z-axis and which Y-axis is radial. The origin of this frame is defined at the middle of the bar assembly at the bar/wedge interface.

DFBs and Harnesses

There are 14 DFB cards per sector.

The hardware number of a DFB card is in the range [0-0xBF]. As much as possible, the former hexadecimal digit represents the sector, the latter the DFB card number in the sector. For instance, in sector 6, the hardware DFB number are in the range [0x60-0x6D].
There may be exceptions when cards are replaced by spares for instance.

Each DFB services 64 PMTs. The group of 64 signal cables is called a Harness. The Harness is plugged into the DFB card through a single connector.
Inside a sector, the Harnesses and the DFBs are numbered from 0 to 13. This convention follows as much as possible the hardware DFB number. For instance, the eleventh DFB in Sector 10 is very likely to have a hardware number of 0xAB.

The PMTs [0-63] serviced by a given DFB card, arranged more or less radially in a Sector, are numbered from the innermost to the outermost row, and in a row from left to right , as seen from behind (Y-axis vertical, i.e. with Row 0 at bottom and Row 40 at top).
There are known exceptions. For instance in row 29 the last 4 Holes (belonging to Harness # 13) should have PMT numbers 30-31-32-33, and have instead, by a circular permutation, numbers 32-33-30-31.
There are also possible miscablings.

The radial stripes of PMTs corresponding to Harnesses [0-13] are numbered from left to right in a sector as seen from behind (Y-axis vertical).

Each sector supports an Electronic Crate.
The DFB cards are numbered 0 to 13 starting from the right. In this convention the DCC is plugged in slot 16 ( which corresponds to the slot 5 in the WIENER crate convention).
Note that the backplane uses yet a different convention to identify a DFB

DFBNumber = 15 - BackplaneSlotNumber

but this is hidden to the user since the official convention is restored inside the ROM.

The PMTs in a sector are numbered from 0 to 895. PMTs 0 to 63 belong to DFB number 0, PMTs 832 to 895 to DFB number 13, etc.

The 168 Harnesses are labelled (on the DFB side : sticker on the connector) from 0 to 167, with the following convention :

HarnessLabel = ( 11 - SectorNumber )*14 + DFBNumber

Data Flow

In Data Flow world, a channel is referenced by an odfTag identifier.
In addition to the unique identifier for the DIRC detector, a Dirc Data Flow Channel is identified by four numbers : the Module Id [0-5], the Section Id [0-27], the Chip Id [0-3], the Channel Id [0-15].

There are 6 Readout Modules ROM for the DIRC identified by a Module Id [0-5].
Each ROM is connected via an optical link to two Dirc Crate Controllers (DCC) for two different Dirc Sectors (A and B). Each of the 28 DFB cards has a Dataflow Section Id [0-27], given by :

For Sector A :

 SectionId  =  DFBNumber

For Sector B :
 SectionId  =  14 + DFBNumber

The PMT number in a Harness [0-63] is related to the Data Flow Chip Id [0-3] and Channel Id [0-15] by :
 PMTNumber = ChipId*16 + ChannelId

High Voltage

A HV Card supplies 16 PMTs channels with High Voltage.
The design ( view from behind, same view with labels on the HV group) is such that each HV Card services two Harnesses with 8 Channels per Harness.


For allowing the translation between the online identifier (odfTag) and the reconstruction (geometrical) identifier (DrcTubeId), one builds a hash table where the channels are labelled by a 'compact index' compactId [0-10751] defined as follows :

compactId = ModuleId*1792 + SectionId*64 + ChipId*16 + ChannelId

The position in the sector is given by the Row number [0-40] and the Hole number [0-29], Row 0 being the innermost row (with 12 Holes), and Line 40 the outermost (with 30 Holes). Holes are numbered from right to left, as seen from the bars (Y-axis vertical).

In a sector, tubes (==Holes) are numbered form 0 to 895 :

tube(Line i, Hole j) = sum_k=0_k=i-1 { n_k } + j

where i in [0,40] and j in [0,n_i]

Some Numbers to Remember
  • Hardware
    Number of Sectors  12
    Number of Dirc Frontent Boards (DFB) per Sector  14
    Number of DFBs  168
    Number of Analog Chips per DFB  8
    Number of Digital TDC Chips per DFB  4
    Number of TDC Channel per Digital Chip  16
    Number of PMTs per DFB  64
    Number of PMTs per Sector  896
    Number of PMTs  10752
    Number of PMT Rows  41
  • Data Flow
    Number of Readout Modules (ROM)  6 0-5
    Number of Sections per Readout Module  28 0-27
    Number of Chip per Section  4 0-3
    Number of Channel per Chip  16 0-15

This page created by Gautier Hamel de Monchenault(