The second level trigger system ( SLT) has the full detector resolution and segmentation available but is a massively parallel system of transputers which operate each on a small region of the detector data and combine their results in a tree-like structure. Algorithms are implemented in software but only have limited access to the data of neighboring regions. This puts considerable constraints on the possible trigger algorithms. Moreover, it has to operate at , so only relatively simple calculations are possible. The SLT uses partly calibrated detector information and a simple tracking algorithm is available.