I have two different usb to serial converters. Both have 5 pins with 3v3, txd, rxd, gnd, 5v. In the past I have used them to connect to raspberry pis and arduinos successfully.

I have a board developed by my work that I can connect to using a pci card in my computer at work. All I do is connect the ground and tx from the board to the gnd and rx going to the pci card on the computer. I can then see the board output.

At home when I plug the usb-to-serial adapter into my computer the device manager recognizes it fine. I can connect the ground pin of the adapter to the ground pin of the board but when I connect the tx of the board to the rx of the adapter windows give the usb disconnect sound and the device manager refreshes and the com port is gone.

Any one have any idea on what is going on? I have never had to connect the 5v or 3v3 pins before. Anyone run into this and have a solution?

  • 1
    \$\begingroup\$ What is the output logic level voltage of the device? What exactly is the device, what does it do? Some things which generate a lot of RFI can confuse a USB host. What happens in you insert a 4.7 K (or whatever is handy) resistor in the signal line? What if you use it with a hub, or don't use it with a hub if you were? \$\endgroup\$ Commented Jun 3, 2019 at 0:41

1 Answer 1


Arduino and Raspberry Pi serial ports are, as far as I know, "TTL" level serial (3.3V or 5V) and it sounds like the USB-serial cable is design for this.

Since you are connecting your work device (does it use a DB9 or DB25 connector?) directly to a PC's PCI card, I'm going to assume it uses RS232 signalling. RS232 is a different voltage level and the signals are inverted vs. "TTL" signalling.

You probably need a USB-RS232 cable.

  • \$\begingroup\$ I think this may be it. I'm going to scope data coming out of the adapter and the board and compare. I'll pick up a usb to db9 adapter that I have at work. \$\endgroup\$
    – NDEthos
    Commented Jun 3, 2019 at 4:07
  • \$\begingroup\$ After scoping both I can see the signals are inverted. I picked up a usb to rs232 DB9 connector and it worked out of the box. Thanks for the insight. \$\endgroup\$
    – NDEthos
    Commented Jun 4, 2019 at 19:08

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