Abstract: Diagnostic monitoring apparatus for connection in-line with a cable connecting a first device to a second device and carrying binary information between the first device and the second device over a wire contained in the cable. There is a series connector inserting a connecting wire in series with the wire in the cable. Signal sampling and triggering logic is connected to the connecting wire for sensing any binary signals on the connecting wire and for producing a trigger output when a binary signal is sensed. There is also an illuminatable visible indicator in the form of an LED and a monostable device having the trigger output connected to an input thereof and an output connected to the indicator for producing a signal at the output of sufficient duration to illuminate the indicator for a visibly detectable period of time. The high speed signal sampling and triggering logic is high impedance and connected in parallel to the connecting wire whereby the signal in the wire in the cable is not loaded down.