How to measure PROFIBUS faults with an oscilloscope
Using oscilloscope measurement can be very effective in finding PROFIBUS faults. Relevant fault information is obtained by analyzing the measured signal waveforms of different qualities.
1) Technical prerequisites
An oscilloscope used for PROFIBUS measurements needs to have the following characteristics:
Design: Digital Storage Oscilloscope
Bandwidth: 100MHZ
Channels: 2, potentials separated from each other and from device ground (network connection)
Trigger: internal + external
Coupling type: DC
When making measurements with an oscilloscope, it is necessary to ensure voltage separation between the two input channels and separation from the device ground. The separation of the two channels ensures that they do not affect each other. The two channels are also electrically isolated from the device ground. If this is not the case, an inadvertent connection of a channel to a live cable core may cause a short circuit.
For PROFIBUS measurements, the channel ground is usually connected to one of the two data lines. Measurements cannot be made without using a potential-separated oscilloscope, since the signal is tapped to ground on the data lines. Normally it is not allowed to make these measurements while the system is running, as the connection of ground to data lines will cause communication failures. However, this can be avoided if you measure the two signals separately and then separate them from each other. In this case, potential separation of the two channels is not required.
2) Measurement auxiliary equipment
When measuring with an oscilloscope, a 9-pin D-type plug is usually used as an auxiliary device for measurement. Figure 6-4 indicates which signal can be measured on which pin. The PROFIBUS plug connected to the programming device is the most suitable measuring aid. If the PROFIBUS structure does not have a programming device connection, it is very convenient to use auxiliary devices for testing. This measuring plug can be inserted between the PROFIBUS station and the PROFIBUS cable.
For some PROFIBUS stations, like programming devices or operator panels, the CNTR signal is not sent beyond standard pin 4. Use pin 9 on these PROFIBUS stations. For some operator panels, the CNTR signal must also be activated, e.g. via DIP switches.
3) Measure PROFIBUS RS 485
Measurements on PROFIBUS RS 485 must be performed at the interface of the respective PROFIBUS station. If only individual PROFIBUS stations are faulty, it is best to carry out measurements on these (faulty) stations.
