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TF and the wire
More and more machines where I work are being wired using Profibus. So that the PLC CPU is connected to remote I/O units by this type of 2 wire shielded cable.
Occasionally a machine will go into fault and a Bus fault (BF) LED will be lit on the CPU. This usually means that we have to physically check the wiring which may run into long distances, removing trunking lids or access covers looking for damage. The last couple of faults were caused by damage from a fork lift truck, and a turntable that had gone round and round twisting the cable until it broke. Both took a lot of finding.
I have been trying to think of an easier way of finding which length of cable is faulty. I know that the cable starts off at the CPU then runs in and out of each Profibus connector with the end one having its "end of line resistor" turned ON and in circuit. The remainder being turned OFF.
If I turn all power off and connect my meter on ohms to the two outgoing cores, in effect looking into the bus. Should I be able to read the end of line resisitor in an intact circuit, or will the I/O units affect the reading? If so, could I disconnect the plug and connect a dummy made up connector to simply connect the ingoing and outgoing cables together so it makes a continuous cable?
Assuming a broken bus, I could then start at nearby connectors switching their resistors ON, seeing if I have a reading and gradually moving out until i have no response. The fault being in between that one and the previous.
Occasionally a machine will go into fault and a Bus fault (BF) LED will be lit on the CPU. This usually means that we have to physically check the wiring which may run into long distances, removing trunking lids or access covers looking for damage. The last couple of faults were caused by damage from a fork lift truck, and a turntable that had gone round and round twisting the cable until it broke. Both took a lot of finding.
I have been trying to think of an easier way of finding which length of cable is faulty. I know that the cable starts off at the CPU then runs in and out of each Profibus connector with the end one having its "end of line resistor" turned ON and in circuit. The remainder being turned OFF.
If I turn all power off and connect my meter on ohms to the two outgoing cores, in effect looking into the bus. Should I be able to read the end of line resisitor in an intact circuit, or will the I/O units affect the reading? If so, could I disconnect the plug and connect a dummy made up connector to simply connect the ingoing and outgoing cables together so it makes a continuous cable?
Assuming a broken bus, I could then start at nearby connectors switching their resistors ON, seeing if I have a reading and gradually moving out until i have no response. The fault being in between that one and the previous.