Logic Analyzer with ground probe missing

Question

I'm playing around with a USB logic analyzer (DSLogic Plus) to look at some RS232 signals. Everything seems to be working fine, but I just noticed that the probe to ground wasn't connected properly. So only the RX and TX lines were connected to the logic analyzer.

How was I getting any kind of good data without the ground being connected?

Just to set the expectation level: I'm a software guy who knows something about programming low-level devices, but my electronics knowledge is extremely basic.

A description of the connections between the devices:

The devices communicating over the RS232 link are an iMX6-based board and a device that is based on a Nordic nRF chip. The RS232 link is not TTL level, it's level shifted to RS232 levels:

     +-------+                        +--------+
     | iMX6  | <====== rs232 ========>| Nordic |
     +-------+           ^            +--------+
                         |
                         |
                    +---------+
                    | DS Logic|
                    +---------+
                         |
                        usb
                         |
                    +---------+
                    | PC      |
                    +---------+

The devices are powered by:

• iMX6: a 12V DC brick
• Nordic: a bench power supply at 9V
• PC: household main
• DS Logic: a powered hub connected to the PC

Answer 1

The DS logic input is simply a 7kΩ resistor in series before the FPGA input (there is also a few zeners for input protection). I'm willing to bet that the xilinx FPGA's input is in the uA or greater for input impedance so it would be like having a 1MegΩ resistor as a load on the RS232, regardless not much current is flowing.

The logic analyzer then triggers a 1 or 0 whenever the voltage reaches a threshold of it's Vil or Vih specs*. The Vil and Vih are triggered when referenced to the ground of the FPGA, which is in turn referenced from the ground of the PC. The ground of the PC is connected to AC mains ground.

Ideally the DS probe would also have a grounding clip as most logic analyzers do to avoid having the thing they are measuring and the logic analyzers ground at different potentials, but if that wasn't connected, then the ground would have gone through the 9V supply or the nordic. (the nordic probably isn't connected to AC mains ground because it is most likely isolated and or double insulated as most brick/wall supplies are, the bench supply is probably not isolated unless it is advertised as such, so I'm willing to bet that your ground is going through the bench supply, you can find out by ohming from the ground to the negative terminal of either supply)

The grounding system isn't the greatest because there are loops and could be susceptible to AC mains ground noise. This would only be a problem if the ground noise got into ~50mV range which is where Vih would start to trigger on the FPGA.

Answer 2

guess you have a connection from the safety pins of your electric plugs [wall socket]... which is normaly almost equal to the neutral conductor of your electric plugs - so even better connected.

I also guess you have luck and good designed power supplys in all devices so GND of your DC voltages is not floating to much.

Also the DS Logic is tolerant to some little DC offsets. But to ensure not to damage the equipment by chance: add the gnd connection!