Arduino negative voltage question

[mavu]

Hi

I'm currently replacing the display in my Fluke 8050A.
I found some nice references online of the conector of the fontpanel and someone even wrote a nice software to display on a 7-segment display.

I could just follow that, but I want to go  a step further and make the display output available on the USB-serial interface the arduino provides.

But, I have a problem understanding the following:

The Fluke has gnd and -5V available on the connector. If I would just put the arduino on that, no problem, because I could just use -5v as ground and gnd as 5v supply. (right?)

But if I have the arduino connected at the same time to a pc over USB, I don't understand what would happen because the USB side of the arduino would get 5V and gnd, and the Fluke side of the arduino would get 5V and gnd, but that would connect the Fluke's -5v to the gnd on the usb side.
 The Fluke seems to be floating, i.e. no path between its gnd (at that point) and the outside gnd.

I'm at a loss to understand this.

Does it even matter that the fluke -5v is connected to the same gnd as the usb side?

How can I measure voltage differentials in such a constellation?

Or in a more general case: how do I connect a device to an arduino which is also connected over USB to something else.
I can't just assume that its ok to connect gnd level together? or can I?

[jcw0752]

As long as your grounds are all floating it doesn't matter what you call them. 5 volts of potential is exactly that. The problem occurs when you start assuming that a floating ground in one location in your system is at the same potential as the floating ground in another location. From what you describe you may have problems if you try to use it to make measurements on the arduino.

John

[mavu]

As long as your grounds are all floating it doesn't matter what you call them. 5 volts of potential is exactly that. The problem occurs when you start assuming that a floating ground in one location in your system is at the same potential as the floating ground in another location. From what you describe you may have problems if you try to use it to make measurements on the arduino.

John

Ok, I'll take that as meaning its not completely insane, and just try stuff and see if any magic smoke happens

thanks

[macboy]

Be very careful when interfacing a DMM to the outside world. Remember that a DMM input is not ground referenced, it is completely floating (unlike an oscilloscope). When you connect the inputs to something to measure it, then most or all circuitry inside the DMM will be referenced to one or the other input (usually black). You must not externally connect anything else (including yourself) to these floating bits, or you could can a serious safety hazard and will likely kill something: the DMM and/or the computer and/or the thing you are probing and/or yourself.

ALL Commercial DMM interface connectors like serial, GPIB, etc. are referenced to ground, that's real earth ground from the AC input. Often with such DMMs, the entire digital side is isolated from the analog side (input and ADC circuits) using opto-couplers or transformers (but mostly optos). Occassionally most of the digital stuff is input-referenced and there is a smaller set of isolated ground-referenced digital circuitry to handle the I/O connectors.

You must check if the circuitry that the Arduino connects to is floating or referenced to an input. Don't be fooled by a "ground" symbol in schematics, it may just be the circuit's "ground" which could be connected to the negative input of the meter, but not to earth. Check for opto-couplers and earth-referenced circuits. For the 8050A, I am almost certain that there will be none, without having actually looked inside or at the schematics.  I am 99.9% sure that the Arduino will end up being ground-referenced to one or the other input (for this meter). This is a plastic-cased meter (not a grounded metal case) which has no external connections other than the input jacks. Assume everything inside is at a potentially dangerous, floating, arbitrary voltage when the meter is being used.

To do this safely, you need to isolate the external connection from the inputs. I'd recommend using a USB-UART device for the "outside" connection. Then you have a +5V power supply from the connected device, and it talks in logic levels not RS232 levels of +-15 V or so. Use a pair of optos to move the data between the Arduino and the USB-UART. Make sure that the optos are rated for at least the highest rated voltage of the meter (probably 1000V).