Deutsches Elektronen-Synchrotron DESY
A Research Centre of the Helmholtz Association

FE

Fast Control System for HERA-B

The FCS is a central part of the data aquisition system of the HERA-B experiment. It provides

  • the distribution of the Bunch Crossing Pulses (every 96 ns) to more than 250 crates along the detector with high accuracy (jitter and skew < 500 ps)
  • the filtering, sorting and distribution of the information generated by the First Level Trigger
  • the handling and distribution of the Slow Control Commands like starting and stopping of runs, fine tuning of the timing signal delays, etc.
  • the generation of statistical information and histograms about the accepted and rejected events
  • different test features to generate triggers and test pulses to control the functioning of the data aquisition modules

The FCS contains several modules:

FCS Master Module

This VME module receives the Bunch Crossing Signals from the HERA controls and the event trigger information generated by the First Level Trigger (FLT). Operator control is possible via the VME bus.

The incoming event trigger information is checked for correct timing and accepted or rejected according to different flags (which are under operator control).

Since the event triggers from the FLT are not necessarily in the correct order they are delayed as long as possible and sorted via a dual port memory. Each incoming bunch crossing signal (every 96 ns) causes the transfer of a clock signal and a 20 bit data word (parallel) on the backplane via the VME P2 connector. The data lines carry information needed to synchronize the different parts of the DAQ system, encoded commands issued by the operator to control the different parameters of the FCS, or (at the correct time) the trigger information from the FLT. This information is fed into a group of up to eight identical modules for further distribution.

FCS Fiber Transmitter Module

This module receives the 20 bit data words and the clock pulse from the backplane, serializes the information via a HP-HDMP1012 chip, and drives (every 96 ns) the serial telegram via an 8-fold fanout into 8 optical fiber links (the drivers used are SIEMENS FC266).

This scheme allows the setup of up to 64 optical fiber links (which all carry the same information).

FCS Fiber Receiver Module

This module receives every 96 ns the 20 bit long serial telegram via the optical fiber (the receivers used are SIEMENS FC266) , deserializes the information via a HP-HDMP1014 chip, and drives the parallel data and the clock pulse via a 10-fold fanout into 10 twisted pair cables (2x30, differential drivers DS90C031). This scheme allows the setup of up to 640 parallel data path (which all carry the same information, 20 data bits and one clock signal each).

FCS Daughter Module

This module works as a crate controler in a DAQ crate and receives every 96 ns the 20 data bits and the clock signal from the parallel twisted pair cable. The information is decoded, interpreted and used to drive the backplane according to the DAQ crate specifications. A programmable (via the FCS system) delay is used to synchronize all DAQ crates. Since the DAQ crates are not accessible during run time an additional CAN bus link allows the readout of a few bytes of information to verify the functioning of the FCS data transmission system.
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