[en] This paper reports on a Fastbus module, the Cosmic Logic Unit (CLU), designed and built to form a muon trigger using the digital data coming from the limited streamer tubes of the Warm Iron Calorimeter (WIC) of the SLD detector at SLAC. The trigger configuration can be changed simply by down-loading new trigger tables to two levels of fast access time RAMs, without interfering with the SLD Data Acquisition System. The module can handle two different types of trigger configurations: one for muons produced in the e⁺e^- collisions, and one for cosmic muons, with the purpose of monitoring the calibrating the whole apparatus on line