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

212 Practical Industrial Data Networks: Design, Installation and TroubleshootingFIELDBUS SEGMENT+24VPOWERSUPPLYFieldPOWERCONDITIONERControlRoomHomerun1900 metresMaximumSPUREXPANSIONTERMINATOR(grounded)TERMINATOR(isolated)Figure 14.10Layout of a systemNotice that the home run in the last drawing connects the control room equipment withthe devices via common terminal blocks (chickenfoot or crowsfoot). The signal cable alsoprovides the power to the devices. There is a terminator at each end of the cable. Powersupplies require power conditioners.Power exampleHere is an example of the power requirements for a system.• The power supply output is 20 volts• The two wires are 1 km long with 22 ohms per wire (44 ohms total)• Each device draws 20 mA• Minimum voltage at each device 9 volts (20–9 = 11 volts)• 11 volts/44 ohms = 250 mATherefore• 250 mA/20mA = 12 devices on the system14.9.3 Communication problemsOnce the power is ruled out as a problem, it can be assumed that the communicationsystem is at fault. Initially, it is important to check the following items:• Are the wires connected correctly?• Is the shield continuous throughout the system?• Is the shield grounded at only one place?