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The Bus Monitor has:
The BUSY test point is high between an I2C Start, and the next I2C Stop.
Triggering off BUSY should give you a stable synchonous view of the message.
Of course the I2C bus allows repeated starts, so start and stop aren't always paired. There is a 2us pulse at the START and STOP test points.
The LEDS's have pulse stretchers so you can see single pulses.
SDA, SCL, and IRQ leds are lit when those lines go low.
A basic start bit detector is quite simple (below) but the Bus Monitor has a bit more.
The software completely takes over your PC while it monitors the bus. When you have finished monitoring, it displays the results. It decodes the message into ascii, and shows you the time of each message. It also montiors the IRQ line.
The software runs using a linux boot CD so you don't need to setup linux on a pc. The best idea is to use a laptop and boot it off the CD.
Important: Your PC cannot do anything while it is monitoring. So you will need a second PC to be the bus monitor if you need to use your main pc while monitoring.
The protocol analyser galvanically isolates the bus from the PC's printer port. Power is supplied by the printer port for the PC side. The logic signals are inverted.
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