The differential voltage measured by the oscilloscope when the RS485 bus is idle is ±3.7V, and the periodic waveform with a frequency of 50hz

王相茗

The differential voltage measured by the oscilloscope when the RS485 bus is idle is ±3.7V, and the periodic waveform with a frequency of 50hz [Copy link]

When the RS485 bus is idle, the oscilloscope measures a differential voltage of ±3.7V between the AB lines, with a periodic waveform of 50 Hz. This does not affect the normal communication of the device. I would like to ask why.

chunyang

This is caused by the spatial coupling between the bus cable and the power grid, and is related to your measurement method.

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chunyang posted on 2022-10-18 16:52 This is caused by the spatial coupling between the bus cable and the power grid, and is related to your measurement method.

Thank you very much for your reply. I use the AD acquisition board to measure the communication voltage. All 485 devices are Siemens electric meter devices that support 485 communication. So how can I do a good job of static bias in this case?

chunyang

It is certain that your bus has no static bias or even terminal resistance. If the measurement method is correct, no grid space induction interference will be observed in the circuit with bias and terminal resistance.

chunyang

Wang Xiangming posted on 2022-10-18 16:57 Thank you very much for your reply. I use the AD acquisition board to measure the communication voltage. All 485 devices are Siemens devices that support 485 communication that I purchased...

The static bias of the bus is to add bias resistors to the power supply and ground respectively, so that the static level of the bus remains in the logic 1 state when no data is sent.

chunyang

In addition, when measuring the 485 bus, the oscilloscope cannot clamp the ground wire on the AB line, but should clamp it on the ground wire end of the interface chip, and then use two inputs to observe the AB line at the same time.

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chunyang posted on 2022-10-18 17:01 In addition, when measuring the 485 bus, the oscilloscope cannot clamp the ground wire on the AB line, but should clamp it on the ground wire end of the interface chip, and then use two inputs to observe A at the same time...

Thank you very much, I will make adjustments based on your reply

chunyang

The electric meters with 485 interface are all 485 terminal devices, which have no terminal resistance and static bias. The bias circuit should be added to the host end, and the terminal resistance should be added to the host end and the other end of the bus.

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chunyang posted on 2022-10-18 17:04 Electric meters with 485 interfaces are all 485 terminal devices. Terminal devices do not have terminal resistance and static bias. The bias circuit should be added to the host side, and at the same time...

Good teacher, thank you

lkh747566933

Is 50HZ industrial frequency interference? Is there AC power around your device? You can try a DC power supply.