Common ground between 2 different USB hubs?

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martin_k
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Common ground between 2 different USB hubs?

Post by martin_k »

New user encountered a problem. I have a work-around but would like advice.

I am using a Picoscope 2204A to examine a signal on a shield mounted on an Arduino UNO. In use the Arduino is connected to and powered from a Raspberry Pi by USB cable. I am running Picosope on a separate Intel NUC, 'scope therefore connected to NUC by USB. This meant I had two computer systems in operation, each independently powered.

1. Raspberry Pi - USB -Arduino UNO
2. Intel NUC - USB - Picoscope

When connecting the ground clip of the oscilloscope probe to the ground rail on the UNO / shield the monitor on my NUC blanked out into powersave mode. Moving the mouse woke it up. More strange, I observed that 'auto reset' on the UNO no longer worked, i.e. the Arduino sketch did not reboot when making a serial connection to the UNO. A cold start of the UNO (unplugging - replugging) fixed that. It appears the Arduino USB interface took a non destructive 'shock'.

Guessing there must have been a current flowing between probe and UNO I put a voltmeter between probe ground clip and UNO GND terminal. Meter shows fluctuation between 5-6 Volts and 0 Volts.

As a work-around for testing I have connected the UNO to the NUC, eleminating the Raspberry Pi and thus having only 1 USB interface. Sure enough, meter now shows 0 Volts between probe ground clip and UNO GND.

For my future reference;

1. Can someone explain what the path to ground from the Picoscope probe ground clip is?
2. How do you use Picoscope on one PC to examine a USB connected device on a different PC?

martin_k
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Re: Common ground between 2 different USB hubs?

Post by martin_k »

An update.

How could Picoscope affect the PC (NUC) monitor?
The monitor (BENQ) itself has a 4 port USB hub, to which my keyboard and mouse are connected. The monitor USB is connected to NUC USB. Picoscope connected to NUC.

Arduino UNO auto reset
Even though there is now no potential difference between scope probe GND and UNO GND, connecting the scope probe still 'knocks out' auto reset on the Arduino UNO serial interface. It is intermittent but fairly repeatable.

What is GND?
The power supplies to the Intel NUC, RPi, Pi LCD monitor, and my Arduino shield are all 2 pin, no earth. UK 13A mains plug with plastic dummy earth pin. All small, light, low output voltage, DC devices. I assume switched mode power supplies. The BENQ monitor is the only device actually connected to household 'earth'.

Further question for Pico technical support
When Picoscope is connected to a PC with no earth, what does probe GND mean?

Regards,
Martin.

bennog
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Re: Common ground between 2 different USB hubs?

Post by bennog »

if you set the meter to AC you will probably measure 50V AC or more
that is why you always need to connect the shield of the pc to earth on de wall outlet. (unless you know exactly what you are doing and know the risks)
Not grounded power can blow up your electronics.

Benno

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Re: Common ground between 2 different USB hubs?

Post by martin_k »

Thanks Benno,

I have taken two lessons from this.

1. As you say, always check before connecting anything.
2. Although USB is a mature technology you do not know what you are getting. I now regard every USB hub as a 'black box'. You do not know what is inside.

bennog
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Re: Common ground between 2 different USB hubs?

Post by bennog »

for your last question the pico GND is connected to the chassis of the PC you are using it on.
So if you use the scope on a not earthed PC then you can possibly get in a lot of trouble.

Benno

martin_k
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Re: Common ground between 2 different USB hubs?

Post by martin_k »

USB Cable
Does the shielding of the USB cable play a significant part in the operation of the Pico oscilloscope?

In the PicoScope 2000 Series Quick Start Guide it says;
"CAUTION
To prevent measurement errors caused by poor grounding, always use the high-quality
USB cable supplied with the oscilloscope."

In addition to the cable that came with the oscilloscope I have a second USB 2.0 Type-A to Type-B cable, the one connecting to the Arduino UNO.

If I use the continuity test on my digital multimeter between the metal shell on the ends of the cables I see;
Arduino cable - buzzer sounds constantly, auto ranging quickly settles at about 1 Ohm.

Pico supplied cable - buzzer intermittent, auto ranging takes a long time to settle at about 40 Ohms by which time the buzzer is off.

I assume the shield screen in the Pico cable is not well connected to the cable ends. Does that meet the specification of a high quality USB cable?

bennog
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Re: Common ground between 2 different USB hubs?

Post by bennog »

Measured my pico cable and it was 15 ohm (variating) so I guess a capacitor also involved.

High quality 5M usb3 cable was about 1.11 ohm also a bit on the high side even for 5M.
It can be the original pico cable has some blowout resistor in the shielding to prevent high current if you hook up the ground clip to e.g. +12V

Benno

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Re: Common ground between 2 different USB hubs?

Post by martin_k »

Benno,
Thanks very much for testing your cables.

Regards,
Martin.

Martyn
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Re: Common ground between 2 different USB hubs?

Post by Martyn »

There are no components in our cables, the only differences to standard USB cables are :-

- The ground and shield are connected at the PC end. This has been part of our cable design for many years and was added to overcome deficiencies with some desktop PC's, particularly the front USB ports which can have a port quality cable inside connecting them to the motherboard.

- We use better quality cables to ensure the quality of the ground return path and to prevent voltage drop across the cable which would be prevent our devices from operating.

It does sound as if you may have a faulty cable so please email support@picotech.com and we can arrange to get a replacement sent out to you.
Martyn
Technical Support Manager

martin_k
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Re: Common ground between 2 different USB hubs?

Post by martin_k »

Martyn,

Thanks for clarifying the specification of the Pico USB cable. Email to support will follow shortly.

Regards,
Martin.
(p.s. regarding spamhaus blacklist. As discussed in another thread posts are accepted after cycles of Preview/Submit but it typically takes me 5-6 attempts. I am in that cycle now).

bennog
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Re: Common ground between 2 different USB hubs?

Post by bennog »

I have tested 2 different Pico USB3 cables and 1 cable had about 10 ohm and the other 25 ohm between the outer shielding of the USB connectors.

For me it is not a problem, will check the ground of the scope BNC and the ground of the PC and outlet later today.

Benno

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