An assembly of devices programmed to deliver precisely timed electrical impulses to the heart, during spontaneous and paced rhythms, for diagnostic cardiac stimulation. It typically includes electrodes/leads applied to the heart and an external pulse generator (EPG). It may be used in asynchronous or synchronous mode, to deliver stimuli of variable strength, and for pacing in a wide range of cycle lengths (typically from 150 to 1500 ms). It is used for performing physiologic cardiac tests such as to determine the function of various components of the atrioventricular conduction, factors required for induction and termination of tachycardia, and to assess sinus node function.