AC Inductive Load

  • RC Snubber Board -

Cause analysis

ground bounce

Almost always, RS232 disconnects are from something called ground bounce, which means that some inductive device like a motor or a solenoid puts a lot of power onto the ground. When that happens, the signaling, which is relative to ground, can end up being high or low.

float ground by seperated power supply

The problem is worse when the systems all share a ground. If you powered the new board with a battery, not a power supply, this can help to float the ground. Or, if the PC is a laptop, disconnect from the wall and run it on battery. Try that first if you haven’t. if you haven’t tried, change the cycle time, where maybe the motor is on 5 seconds and then off 5 second. That makes it easier to see when the error happens- at the start or the stop of the motor. If the start, it is going to be a voltage drop on the RS232 chip, and a local capacitor of 5uf or so can make a huge difference. If this happens with the motor turning off, this is almost certainly inductive spike causing a ground bounce.

extra capacitances

I usually put some extra capacitance on CPU, comms chips, and any logic chips and make sure there is a good ground between them- either a ground plane, or at least a 20mil trace. Same with power. The caps will handle all short term power changes-

power trace

but you need the larger power traces to get that power to or from the chip before the cap is no longer able to keep voltage in limits.

oscilloscope check

Do you have a DSO oscilloscope? You will never see this with a power supply meter- you need to check for voltage spikes that are much shorter than 0.1 second. Often, they are more like 0.00001s. If you don’t have that test equipment, I do, and maybe you can send the proto board over with just the bare minimum parts- RS232 chip, motor drive chip, controller. I have motors and all that stuff.

Solution Tried

  • add 100uf capacitors to 5V/GND and 3V3/GND
  • use a seperated 12V/2A battery, USB-RS232 board does not keeping reboot, can still can not be opened COM port to use
  • mosfet control is already optocoupler isolated as the design, and used seperated power supply
  • DPR1084-dat RS232 board only connects with RXD, TXD, GND, no power supply


  • USB_hub is unstable, cause the RS232 or RS584 connection broke

Common Application

  • involve inductive loads- motors
  • fuel injection
  • solenoids- things