DLMUK
Member
Hi,
How do people terminate cable screens on RS485?
I've been reading around and people seem to do various things. A few people have said you shouldn't terminate the screen of more than one device on the network as it would cause ground loops. Is this the case?
I thought grounding multiple devices on a ring would cause ground loops rather than screens?
We have a situation at the moment which has recently arisen where we have two RS485 RTUs situated on a crane, every time the vertical hoist motor is operated it kicks the RTUs off for about 1s and then they come back on but the motor can carry on running. I assume this is because when the motor starts it has its highest current draw to initially get moving.
At present the RTUs are individually wired from the bucket on the crane to a big JB and into terminal blocks. Out of those terminal blocks along another cable for about 10ft and then into some more terminal blocks. Then out of those terminal blocks for about 20cm and into a NI RS485/USB box. Each RTU has its own cable so effectively 1 node on the network.
The screens for both cables go to the same earthing point in both junction boxes, would this be likely to cause problems? I expect there are also screens from other items, like the hoist motor in the same blocks, but I haven't had chance to check yet.
At the moment there are no termination resistors. If I cannot find out the characteristic impedance of the cable from the supplier, would you put two 120ohm resistors in or could this potentially make the problems worse?
Thanks,
How do people terminate cable screens on RS485?
I've been reading around and people seem to do various things. A few people have said you shouldn't terminate the screen of more than one device on the network as it would cause ground loops. Is this the case?
I thought grounding multiple devices on a ring would cause ground loops rather than screens?
We have a situation at the moment which has recently arisen where we have two RS485 RTUs situated on a crane, every time the vertical hoist motor is operated it kicks the RTUs off for about 1s and then they come back on but the motor can carry on running. I assume this is because when the motor starts it has its highest current draw to initially get moving.
At present the RTUs are individually wired from the bucket on the crane to a big JB and into terminal blocks. Out of those terminal blocks along another cable for about 10ft and then into some more terminal blocks. Then out of those terminal blocks for about 20cm and into a NI RS485/USB box. Each RTU has its own cable so effectively 1 node on the network.
The screens for both cables go to the same earthing point in both junction boxes, would this be likely to cause problems? I expect there are also screens from other items, like the hoist motor in the same blocks, but I haven't had chance to check yet.
At the moment there are no termination resistors. If I cannot find out the characteristic impedance of the cable from the supplier, would you put two 120ohm resistors in or could this potentially make the problems worse?
Thanks,
