Abstract: A modular data processing unit is adapted to form part of a processing system comprising a plurality of such processing units connected in master-slave relationship. Each processing unit comprises a control section responsive to a control instruction to perform the execution of a program stored therein and to switch the processing unit to an idle mode on completion of the program execution, an internal memory section including an area of read/write memory; a computation section for performing calculations in accordance with the program instructions during which it can access the internal memory; and interface means through which the processor unit can be connected as a master unit to one or more slave units to monitor and control, and to access the internal memories of the slave units while it is executing a program, and through which it can be connected as a slave to a master processing unit by which it can thus be monitored and controlled, and have its internal read/write memory accessed.