# How do you get eye diagrams?

I understand that eye diagrams give an indication of the quality of a signal. But I don't understand how they are displayed on an oscilloscope. With proper triggering shouldn't you get either positive or negative pulses, but not both?

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I believe it is as simple as triggering on one transition earlier then the eye. Assuming your data is fairly random, you should get a pretty even set of positive and negative transitions. – Connor Wolf Sep 23 '11 at 9:04
Alternatively, there are oscilloscopes which are level triggered, not edge triggered. – Connor Wolf Sep 23 '11 at 9:04
Typically you would trigger on the signal that determines the time when data is sampled. Either an earlier transition, for example the falling edge at a start bit for RS-232. Or on another signal, for example a clock signal. – starblue Sep 23 '11 at 18:19

If you don't have a separate clock signal the data itself is used to trigger the scope. If you look at a Manchester coded signal you'll see that pulses aren't always equally spaced. If you have 01 and trigger on the 0 the second bit will show low and then high, but when you have 00 the second bit will show a high followed by a low, the transition (which is actually the clock) will be at the same place.