A downhole explosive detonation comprises a high voltage electro-explosive initiator comprising an input high voltage power supply with a low impedance shunting fuse, a flexible electrical link and a capacitor discharge unit. Explosive is initiated in a direction approximately parallel, or in another version perpendicular to the capacitor discharge unit. A unique configuration and construction of the assembly allows installation through a small service port in the gun housing structure for more efficient gun arming. A real time downhole voltage monitoring is described that transmits voltage readings to the surface during a firing sequence.