FIELDBUSES
PROFIBUS DP
A classic serial fieldbus for cyclic data exchange between a controller and field devices.
Who communicates with whom?
A PROFIBUS DP master exchanges cyclic data with configured uniquely addressed slaves. It sends output data to one device and receives its input/status data in that exchange, then proceeds to the next. A valid reply confirms communication, not physical execution of a command; read an appropriate feedback signal. With multiple masters a token coordinates their bus access.
Voltage and waveform
Typical copper PROFIBUS DP uses RS-485. Measure differential B−A; supplier naming can vary. A loaded driver provides at least about 1.5 V differential. A 5 V supply pin for active termination is not the data amplitude. Cable, profile and loading affect the shape.
Measuring with an oscilloscope
The PI commissioning guide describes measuring differential B−A with an appropriate scope. Check signal shape along the segment for reflections, overshoot, interference and correct termination. Match the timebase to baud rate.
The principle
PROFIBUS DP often runs over an RS-485 segment. A master exchanges data with configured devices. With multiple active masters, a token controls access to the bus; one master polls its connected devices.
An example
Select participants to see a simplified polling cycle. Real telegrams and timing are not reproduced.
Important
Termination, addressing, cable and configuration must match the real network. PROFIBUS and PROFINET are separate technologies.