Serial communication-What is the reason for the decrease of impedance to ground?

sfcsdc

Serial communication-What is the reason for the decrease of impedance to ground? [Copy link]

A device that uses serial port communication has been working normally before.

After unplugging and plugging it back in today, I found that the communication was faulty.

After troubleshooting, it was found that the RXD impedance of the device to ground was only 34Ω.

The communication port is protected by TVS, and the equipment is used normally without human contact.

The port's impedance to ground will become smaller, which may be due to the TVS being broken or the port being broken.

Why does the good serial communication suddenly become bad?

maychang

"Why does the good serial communication suddenly become bad?"

Presumably the OP is hot-swappable.

sfcsdc

maychang posted on 2020-6-29 11:40 "Why does the good serial port communication suddenly become bad?" I guess the OP is hot-plugging while powered on.

How can the voltage spike generated by hot plugging be transmitted to the serial port?

Would having a TVS on the power supply solve this problem?

maychang

sfcsdc posted on 2020-6-30 08:35 How can the voltage spike generated by hot plugging be transmitted to the serial port? Can TVS on the power supply solve this problem?

"How can the voltage spike generated by hot-plugging be transmitted to the serial port?"

Hot-plugging does not "create voltage spikes".

The devices on both sides of the plug and socket may not share a common "ground". There may be a voltage between the "grounds" of the devices on both sides, which may be tens of volts or even hundreds of volts, but this voltage may have a considerable internal resistance. If the serial port line is connected well, the ground wire in the serial port line will short-circuit this voltage source with a large internal resistance, so that the devices on both sides share a common ground and work normally.

However, during hot plugging, the ground wire in the serial port line may be disconnected first, so there is voltage between the "ground" of the devices on both sides, although it is very short. This voltage is applied to the input end of the receiving party, which may damage the input end.

maychang

sfcsdc posted on 2020-6-30 08:35 How can the voltage spike generated by hot plugging be transmitted to the serial port? Can TVS on the power supply solve this problem?

"Would having a TVS on the power supply solve this problem?"

Instead of adding TVS to the power supply, overvoltage protection should be added to the serial port input and output. Of course, it is best not to hot-swap.

qwqwqw2088

The standard RS232 voltage of the serial port can reach ±15 V, and hot-plug is not supported.

轩辕默殇

It is best not to hot-swap. I have fallen into this pit several times.

miaods2002

I would like to ask the experts, can RS485 be hot-swappable?