We present the set-up and performance of a high energy neutron trigger based on a detector system consisting of a scintillating tile hodoscope and a compensating lead/scintillating fibre calorimeter. A first level trigger signal is generated by a minimum hadronic energy detected in the fiducial volume of the calorimeter. This signature is complemented at a second trigger stage by the isolation of a shower whose origin cannot be attributed to a charged particle. We detect such a topology by comparing the hit pattern of the hodoscope with online available calorimeter information. This complex task can be fulfilled in an efficient and flexible way by Digital Signal Processors (DSP). The presented method of online neutron detection was developed in the framework of the hyperon beam experiment WA89 at CERN and yields an average trigger decision time of ≈11 μs. From an analysis based on Σ − decays ( Σ − → n π −), we find a second level trigger efficiency for neutrons of 70% at a refection level of 95% for hadronic showers originating from charged particles.