A strip chart recorder:
It is like a volt meter except the needle sweeps across a moving strip of paper making a line or impression dependent on the magnitude of the signal presented to it. It however can consist of multiple pens with different colors to represent more than one parameter in the same time frame.
BillCroft used a two needle version to monitor duplex link traffic while tuning his network protocols for the PurdueComputingCenter.
A DigitalEquivalent? is a LogicAnalyzer that writes into a CircularBuffer which it formats after detecting a TriggerEvent?.
I've repeated Bill's measurements using a digital oscilloscope to observe the dialog between a server and a BasicStamp microcontroller. For pictures, see...