''A strip chart recorder:'' 

http://upload.wikimedia.org/wikipedia/en/b/b0/Patent_4333084.png

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.

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I've repeated Bill's measurements using a digital oscilloscope to observe the dialog between a server and a BasicStamp microcontroller. For pictures, see...

* http://home.c2.com/~ward/bs2/ (Empty link)
* moved to bs.home.c2.com. (according to readme file, however this is a BrokenLink, Ward will have to correct this if it still exists)