  We review experiments colliding electrons(/positrons) on protons at high
energies at HERA, focussing on the ZEUS experiment. We describe the ZEUS
detector, including its data acquisition system, and look at a specific part,
namely the electronic system for its calorimeter readout control, in more
detail. Data analysis is described, and results pertaining the proton structure
is given. It is found that the naive quark model cannot explain the results
obtained at small fractional momentum, and this requires quantum
chromodynamics. Also it is found that diffractive reactions, with large
rapidity gaps, contribute substantially; efforts are being carried out to
understand them.
