A prototype of the trigger and digital processing electronics for the electromagnetic calorimeter of the CMS experiment, coupled to a prototype of the PbWO 4 crystal calorimeter, was tested during summer 1996 in the H4 beamline at the CERN SPS. A very successful operation was achieved for this system, which runs in synchronous and pipelined mode at the LHC clock frequency, and performs the basic trigger and data acquisition functions needed in the CMS electromagnetic calorimeter. The performance of the trigger front-end electronics is well within the established requirements: a highly efficient bunch-crossing identification (>99.9%), a good trigger-energy resolution ( σ/E∼9%/ E ⊕2% ) and a highly efficient electron cluster shape identification (∼99%) have been achieved. The FERMI digitising system based on a dynamic analog compressor and a sampling ADC showed a very good performance, in particular the energy resolution for 150 GeV electrons was 0.54%, equal to the resolution obtained with a conventional charge integration ADC system.