Thanks Diettrich.
Oscilloscope is a nice thing to have, question is then would it be worth it. Obviously a oscilloscope also influences the signals to a degree. I did have a look at some input circuit diagrams from oscilliscopes now, they tend to use high Ohm resistors (10MOhm) and some capacitors.
Other thought I had was, measuring the potential differential between BUS and +9V to calculate the rise time (Image3)... but the bus side would be ground then and adding an additional 10MOhm resistor would mean I need amplification... and then it becomes complicated...
Image3
I guess your right... get an oscilloscope and rather provide easy accessible connectors to the PCB for the probe. The added costs for 50 PCBs would also cost as much as an oscilloscope but be less flexible.