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

178 Practical Industrial Data Networks: Design, Installation and Troubleshooting• In the case of multiple power supplies, connect this ground only at thepower supply closest to the middle of the network• Ensure that all nodes on the network connect to the shield, the signal andpower linesNote: CAN frame errors are a symptom of grounding problems. CAN error messagescan be monitored with a handheld DeviceNet analyzer or CAN bus analyzer.Break the shield at a few points in the network, and insert a DC ammeter in the shield.If there is a current flow, then the shield is connected to DC common or ground in morethan one place, and a ground loop exists.PowerThis can be measured with a voltmeter or handheld DeviceNet analyzer and is measuredbetween V+ (red) and V– (black).Measure the network voltage at various points across the network, especially at the endsand at each device. The measured voltage should ideally be 24 V, but no more than 25 Vand not less than 11 V DC.If devices draw a lot of current, then voltages on the bus can fluctuate hence busvoltages should be monitored over time.If the voltages are not within specification, then:• Check for faulty or loose connectors• Check the power system design calculations by measuring the current flowin each section of cableOn some DeviceNet analyzers, one can set a supply alarm voltage below which awarning should be generated. Plug the analyzer in at locations far from the power supplyand leave it running over time. If the network voltage falls below this level at any time,this low voltage event will be logged by the analyzer.Note that ‘THIN’ cable, which has a higher DC resistance, will have greater voltagedrop across distance.12.12.3.4 Incorrect signal voltage levelsThe following signal levels should be observed with a voltmeter, oscilloscope orDeviceNet analyzer. Readings that differ by more than 0.5 V with the following valuesare most likely indicative of a problem.CAN_H can NEVER be lower than CAN_L and if this is observed, it means that thetwo wires have probably been transposed.(a) If bus communications are OFF (idle) the following values can be observed with anymeasuring device.CAN_H (white) 2.5 V DCCAN_L (blue) 2.5 V DC(b) If bus communications are ON, the following can be observed with a voltmeter:CAN_H (white) 3.0 V DCCAN_L (blue) 2.0 V DCAlternatively, the voltages can be observed with an oscilloscope or DeviceNet analyzer,in which case both minimum and maximum values can be observed.These are:CAN_H (white) 2.5 V min, 4.0 V maxCAN_L (blue) 1.0 V min, 2.5 V max