I just ordered my first scope (a Rigol DS1054Z) and have been thinking about some interesting experiments/learning projects to perform with it. Thus far most of my experience has been in building circuits/MCU + FPGA programming.
One thing I'd like to try to play around with is signal reverse engineering on a bus between two unknown MCU's. While tapping the signal either with a scope, or with a logic analyzer which I also have is fairly straight forward, the question occurred to me, how might one determine signal 'direction'– I.e. Assuming it is communication between two MCU's, not just out to a passive peripheral, for any given signal, how might one determine, at any given point in time which chip is actually the one 'doing the talking' ?
My first thought in this regard was to design a simple off board circuit that I could use to tap into the bus that would have scope touch points and two LED's in an opposing parallel configuration with the minimum required resistance to operate them. While of course this may not preserve the complete signal integrity for the duration, at least I might be able to obtain a matching 'time/direction' map on the scope I could use to compare with an unfettered bus signal.
However, reading around a little bit I've seen suggestions that imply in this case 'current flow', is not always equivalent to direction, and so this approach might not work.
Even still, it seems to me that the signal must have some temporal propagation delay, implying direction, however small.
My question then is what might be the most reasonable/logical approach to address such an exploration ?
Best Answer
As "helloworld922" said, put a low value in-line resistor (you can probably get away with something as high as 100R, but I'd go for something like 10R), and also pull both sides of that resistor to GND with equal, high-value resistors (e.g. 10k). You should be able to see the voltage drop on a "high" signal.