Due to the Diagnostic Manager signal level, noise and jitter can easily be identified.
The signal level is the voltage of the communications signal that a device transmits on the fieldbus cable. It is generated through a current change created by the device. This current change is then “translated” into the voltage change detectable by all fieldbus participants through the terminators located at each end of the fieldbus trunk.
Noise is an unwanted disturbance that is induced in the fieldbus cable from outside sources, e.g., through EMI.
Jitter is the deviation of the ideal zero crossing point of the transmitted signal curve during the nominal bit duration, measured with respect to the previous zero crossing point. This is an extremely sensitive measurement.
Any hardware connected to the fieldbus segment influences the electrical attributes of the segment. Each change in connected hardware causes a change in electrical parameters, which in turn changes the signal form and typically increases jitter:
- Cables with bad parameters
- Faulty components in an instrument
- Over or under termination
In reality, a system is able to run with higher jitter levels but with reduced immunity against EMI such as noise. The amount of jitter is only dependent on the quality of the connected components.