Practical Industrial Data Networks:Design, Installation and ...

66Practical Industrial Data Networks: Design, Installation and Troubleshootingleads on the multimeter need to be reversed. The procedure for measuring the idle voltageis as follows:13. Verify that the power is on14. Verify that all stations are connected15. Verify that the master is not polling16. Measure the voltage difference between the A– and B+ lines starting at themaster17. Verify and record the idle voltage at each stationIf the voltage is zero, then disconnect the master from the system and check the outputof the master alone. If there is idle voltage at the master, then plug in each station one at atime until the voltage drops to or near zero. The last station probably has a problem.Common mode voltage measurementCommon mode voltage is measured at each station, including the master. It is measuredfrom each of the A and B lines to ground. The purpose of the measurement is to check ifthe common mode voltage is getting close to maximum tolerance. It is importanttherefore to know what the maximum common mode voltage is for the system. In mostcases, it is +12 and –7 volts. A procedure for measuring the common mode voltage is:18. Verify that the system is powered up19. Measure and record the voltage between the A and ground and the B andground at each station20. Verify that voltages are within the specified limits as set by themanufacturerIf the voltages are near or out of tolerance, then either contact the manufacturer orinstall resistors between each line to ground at the station that has the problem. It isusually best to start with a high value such as 200 k ohms ¼ watt and then go lower asneeded. Both resistors should be of the same value.4.2.5.2 OscilloscopeOscilloscopes are used for:21. Noise identification22. Ringing23. Data transferNoise identificationAlthough the oscilloscope is not the best device for noise measurement, it is good fordetection of some types of noise. The reason the oscilloscope is not that good at noisedetection is that it is a two-dimensional voltmeter; whereas the effect of the noise is seenin the ratio of the power of a signal vs the power of the noise. But having said that, theoscilloscope is useful for determining noise that is constant in frequency. This can be asignal such as 50/60 Hz hum, motor induced noise or relays clicking on and off. Theoscilloscope will not show intermittent noise, high frequency radio waves or the powerratio of the noise vs the signal.