Oscilloscope measurement of PROFIBUS faults
Measurements with an oscilloscope are very effective in detecting PROFIBUS faults. By analysing the measured signal waveforms of different qualities, the relevant fault information is obtained.
1) Technical prerequisites
The oscilloscope used for PROFIBUS measurements needs to have the following characteristics:
Design: digital storage oscilloscope
Bandwidth: 100 MHZ
Channels: 2, potentially separated from each other and from the device ground (network connection)
Triggering: internal + external
Coupling type: DC
Measurements with oscilloscopes must be carried out in such a way that the two input channels are separated from each other in terms of voltage and from the device ground. The separation of the two channels ensures that they do not affect each other. The two channels are also potentially separated from the device ground. If this is not the case, an inadvertent connection of one channel to a live cable core can cause a short circuit.
For PROFIBUS measurements, the channel ground is normally connected to one of the two data lines. Measurements cannot be carried out without the use of a potential-separated oscilloscope, because the signals are tapped to ground on the data lines. Normally, it is not permissible to make these measurements while the system is in operation; the connection of the ground to the data lines will cause a communication failure. However, this can be avoided by measuring the two signals separately and then keeping them separate from each other. In this case, no potential separation of the two channels is required.
2) Measurement Aids
Measurements with an oscilloscope are usually performed using a 9-pin D-type plug as a measurement aid. Which signal can be measured on which pin is indicated in Figure 6-4. A PROFIBUS plug connected to a programming device is the most suitable measurement aid. If the PROFIBUS structure does not have a programming device connection, it is convenient to use the auxiliary device for testing. This measuring plug can be inserted between the PROFIBUS station and the PROFIBUS cable.
For some PROFIBUS stations, like programming devices or operating panels, the CNTR signal is not sent beyond the standard pin 4. On these PROFIBUS stations use pin 9. For some operating panels, the CNTR signal must also be activated, e.g. via DIP switches.
