I hope I am knocking on the right door here. I have a small RS485 network with a master and two slaves that has been running for more than a year now. I recently replaced a seemingly unrelated power supply and started to experience communication failures. I checked the bus using a differential probe at slave 1, as seen on the images.
The bus is running at 9600 bps, using one pair of wires in a Cat5 unshielded cable (the other pairs of wires are unused). The terminations and fail-safe bias locations are shown. The master is continuously polling the slaves for statuses. While the master signals are looking very good to me, the slave responses are awful. A very low voltage difference starts opening up with the same extent in both positive and negative voltages. All units are using the same RS485 transceivers with the same circuits (except the additional fail-safe resistors at the master).
I have never checked the bus before, probably it was (almost) the same bad from the beginning, but it worked so I have not checked. I do not think the replaced power supply has anything to do with it. (It is not the 5V power supply that is powering the units, maybe a little extra noise?)
I do not have too much experience, but when the bus is released (seen when the master fails to receive the message and timeouts) the line looks fine, (only the failsafe bias pulls it to positive), so I do not think the problem is related to noise.
Any idea what could cause this? Any clue is appreciated that gives me a hint where to start...
The bus is running at 9600 bps, using one pair of wires in a Cat5 unshielded cable (the other pairs of wires are unused). The terminations and fail-safe bias locations are shown. The master is continuously polling the slaves for statuses. While the master signals are looking very good to me, the slave responses are awful. A very low voltage difference starts opening up with the same extent in both positive and negative voltages. All units are using the same RS485 transceivers with the same circuits (except the additional fail-safe resistors at the master).
I have never checked the bus before, probably it was (almost) the same bad from the beginning, but it worked so I have not checked. I do not think the replaced power supply has anything to do with it. (It is not the 5V power supply that is powering the units, maybe a little extra noise?)
I do not have too much experience, but when the bus is released (seen when the master fails to receive the message and timeouts) the line looks fine, (only the failsafe bias pulls it to positive), so I do not think the problem is related to noise.
Any idea what could cause this? Any clue is appreciated that gives me a hint where to start...
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