This report presents a formal specification in the Z notation for a safety-critical control system. It describes a particular medical device but is quite generic and should be widely applicable. The specification emphasizes safety interlocking and other discontinous features that are not considered in classical control theory. A method for calculating interlock conditions for particular operations from system safety assertions is proposed; it is similar to ordinary Z precondition calculation, but usually results in stronger preconditions. The specification is presented as a partially complete framework that can be edited and filled in with the specific features of a particular control system. Our system is large but the specification is concise. It is built up from components, subsystems, conditions and modes that are developed separately, but also accounts for behaviors that emerge at the system level. The specification illustrates several useful idioms of the Z notation, and demonstrates that an object-oriented specification style can be expressed in ordinary Z.