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

DeviceNet overview 17912.12.3.5 Common mode voltage problemsThis test assumes that the shield had already been checked for continuity and current flowand can be done with a voltmeter or an oscilloscope:• Turn all network power supplies on• Configure all nodes to draw the maximum amount of power from thenetwork• Turn on all outputs that draw currentNow measure the DC voltage between V– and the shield. The difference should,technically speaking, be less than 4.65 V. For a reasonable safety margin, this valueshould be kept below 3 V.These measurements should be taken at the two furthest ends (terminator position), atthe DeviceNet master(s) and at each power supply. Should a problem be observed here, asolution could be to relocate the power supply to the middle of the network or to addadditional power supplies.In general, one can design a network using any number of power supplies, providingthat:• The voltage drop in the cable between a power supply and each station itsupplies does not exceed 5 V DC• The current does not exceed the cable/connector ratings• The power supply common ground voltage level does not vary by morethan 5 V between any two points in the network.12.12.3.6 Incorrect termination12.12.3.7 NoiseThese tests can be performed with a MultiMate. They must be done with all buscommunications off (bus off) and the meter set to measure resistance.Check the resistance from CAN_H to CAN_L at each device. If the values are largerthan 60 ohms (120 ohms in parallel with 120 ohms) there could be a break in one of thesignal wires or there could be a missing terminator or terminators somewhere. If, on theother hand, the measured values are less than 50 ohms, this could indicate a short betweenthe network wires, (an) extra terminating resistor(s), one or more faulty transceivers orunpowered nodes.Noise can be observed with a loudspeaker or with an oscilloscope. However, moreimportant than the noise itself, is the way in which the noise affects the actualtransmissions taking place on the medium. The most common effect of EMI/RFIproblems are CAN frame errors, which can be monitored with a CAN analyzer orDeviceNet analyzer.The occurrence of frame errors must be related to specific nodes and to the occurrenceof specific events e.g. a state change on a nearby variable frequency drive.12.12.3.8 Node communication problemsNode presenceOne method to isolate defective nodes is to use the master configuration software or aDeviceNet analyzer to create a ‘live list’ to see which nodes are active on the network,and to compare this with the list of nodes that are supposed to be on the network.