Ground Loops and other possible issues when using an FTDI cable

Projects General Guidance
1

On a previous post it was suggested to use an FTDI cable for Serial communication with a PC , if the Arduino Uno is to be powered through the +5V pin (please see Printing to a serial console and powering through the 5V pin. - Project Guidance - Arduino Forum).
I'm a little concerned about grounding issues though. I want to be able to use a laptop safely to send/receive serial data both when the laptop is connected to mains ground via the AC adapter and when it is consuming power from its battery.

Power to the +5V pin of the Uno originates from a separate source that connects to Vin and ground of the shield board I described in the post referenced above. That source could either be an AC/DC adapter or a separate +12V battery. The grounds of the Shield board and the Uno are connected.

Let say the Arduino is connected to the Tx,Rx, and gnd wires of an FTDI cable, but not the VCC, and the USB end of the cable is connected to a Laptop.

Would a ground loop or other signal issues be encountered under the following scenarios?

Scenario 1: Laptop powered through its AC adapter (I'm not sure the USB port ground is isolated from mains ground, someone can please clarify)

  • Shield board powered through a separate AC/DC adapter.

Scenario 2: Laptop powered through its AC adapter +
Shield board powered through external +12V battery.

Scenario 3: Laptop powered throuhg an external +12V battery (via a DC adapter)

  • Shield board powered through an AC/DC adapter.

Scenario 4: Laptop and Shield Board powered through same +12V battery.

Scenario 5: Laptop not connected to an external power source (i.e. consuming power from its battery) +
Shield board powered through an AC/DC adapter.

Scenario 6: Laptop not connected to an external power source (i.e. consuming power from its battery) +
Shield board powered through external +12V battery.

2

I have never particularly heard of "ground loop" problems with USB/Serial cables. Although there are enough badly designed "5V power supply modules" that it might be a legitimate concern. There are "USB Isolators" and "Isolated USB Serial Ports" (sticker shock!)

I would think it would be easier and cheaper to put some optical isolation AFTER the serial conversion (in the RX, TX, and DTR signals) - the signal levels and rates are much more compatible with "easy" isolation than the USB signals.