Data acquisition system

A data acquisition system, e.g. for an intrusion alarm system, has a plurality of distributed sensors connected in sets to nodal units or concentrators. The concentrators are interrogated by clock pulses broadcast from a central control unit and when each concentrator's address is reached it transmits back to the central unit a set of pulse signals the widths of which represent the voltage levels of the respective sensors connected to it. Analogue sensor outputs may thus be transmitted. The sensor outputs are also preferably integrated at the concentrators, prior to pulse-conversion, over a period corresponding to a complete cycle of the local mains supply, thus to eliminate any mains interference.