The fast control system for HERA-B

The HERA-B experiment is dedicated to measure CP-violation in decays of neu tral B-mesons. The B-mesons are produced in a nearly overwhelming backgroun d of inelastic proton-nucleon-interactions. To reconstruct a sufficient number of B-decays the experiment has to be ope rated with multiple interactions at a 10 MHz event race. A granularity of 5 40,000 channels is required. The challenge for the DAQ system is deadtimele ss readout. This requires unprecedented speed of storing and labeling the data. A 50 kH z first level trigger rate needs a fast distribution of accepted-event iden tifiers. Ambitious plans for drift time measurements require a high precisi on time reference for the front end electronics. A fast control system has been developed to fulfil these requirements. The system consists of a VME-accessible mother unit with trigger and command pr ocessing logic, a two-stage signal distribution system and one individually configurable daughter module with timing adjustment capabilities in each o f more than 200 front end electronics crates.