  We present the design and commissioning of a new multiplicity veto for the
HERA-B detector, a fixed-target spectrometer originally designed to study the
physics of B mesons in proton-nucleus interactions. The HERA-B trigger is a
message-driven multi-level track trigger. The first level trigger (FLT)
consists of custom-made electronics, and the higher trigger levels are
implemented as PC farms.
  The multiplicity veto has been designed to reject high-multiplicity events
before they enter the trigger chain. A veto signal is generated based on the
comparison of the number of photons in part of the HERA-B ring-imaging
Cherenkov counter (RICH) with a programmable threshold.
  The RICH Multiplicity Veto is a modular system. First the hits in 256
detector channels are summed by Base Sum Cards (BSC), then FED Sum Cards (FSC)
sum the subtotals of up to eight BSCs. Finally the Veto Board (VB) takes the
veto decision based on the sum of up to 14 FSCs.
  The RICH Multiplicity Veto has been successfully installed and commissioned
in HERA-B. The measured veto efficiency is (99.9991+/-0.0001)%, and the system
is used in the routine data-taking of the HERA-B experiment.
