Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Digital systems, linear algebra, introduction to formal proof resit / 3 / b

The lower network pulls the output low exactly when either series branch conducts. Therefore the Boolean function isThe inversion is important: $ab\vee cd$ describes connection to the low rail, not a high output. The pull-up network found in part a implements the complementary high condition.