Grounding issue when connecting GPIB-USB-HS to a laptop?

[MeisterS]

Hi,

The installation guide for the GPIB-USB-HS asks to "ensure that the computer and the GPIB devices are at the same ground potential."  I was wondering if there could be any problems with connecting laptops (because they are floating)?

What would/could happen if the computer was at a different ground potential?  Shorting?

In case the ground potential is different, the installation guide also mentions to use a GPIB bus expander/isolator or an isolated USB hub.  I am guessing that the laptop USB ports are "isolated" and should therefore be fine.  Is that true?

Thanks! 

[Mr.F]

Hi Meister,

If the laptop is on a different ground plane that the instrument it is possible that a ground loop will be set up with could interject noise into your signal and possible make communication difficult. I don’t know about the laptop ports being isolated, but there should not be a problem with shorting using your device in a normal environment.

[JÞB · Accepted solution]

Meister,

Because your laptop is "floating" it's referance (chassis ground) has no relation to your equipment referance (Earth ground) unless you take some steps to tie the Laptop chassis to Earth potential.  Attaching a GPIB cable to your laptop and your equipment will provide a path to discharge the potential differance between your laptop chassis and earth through the GND signal on the GBIP bus.  The GPIB bus is not intended to carry large currents (discharge large potentials) and you may overstress the input protection circuits on either the laptop or the equipment when you connect the GPIB cable.  Its kind of like touching a doorknob on a dry day after walking across wool carpet- you will get a "static" shock when you make initial contact.  Do this to your equipment and laptop often enough and you will damage them.

A worse case exists when the laptop chassis is tied to a different potential than the equipments Earth ground (usually due to building wiring errors.)  In this case there must be a constant current flowing through the GBIP GND signal commonly known as a ground loop.  Noise and / or damage will be observed depending on the potintial differance.

The cure is to ALWAYS connect the laptop chassis to the equipment chassis through an external wire BEFORE connecting the GPIB Bus. 

I hope this clarifies the reasoning of "Warning" that you see in the installation guide.  Kudos to you for reading your guide and paying attention to the warnings!  Never be afraid to ask when you don't understand the warnings

[JÞB]

Matt,

I am surprised by your response.  The warnings in manufacturer's installation guides should never be dismissed in a cavilier manner.  In this case its even a warning every similar NI product guide carries.  Please edit your post.

[MeisterS]

Jeff,

Thanks a lot for the reply.  That helped me a lot in understanding the warning and issues. Here is a quick follow up question:

Since my laptop chassis is made out of plastic, I am looking for a way to connect to it. Playing around with an Ohmmeter, I noticed that the thing most resembling a chassis (or common ground) is the outer metal part for any connectors (i.e. the metal shell that surrounds the monitor pins is connected to the metal shell that surrounds the USB connectos, etc).  Similar things could be said for some of the electronics I am connecting to.

If I just connect the GPIB-USB-HS to the laptop and the instruments, an initial static discharge current could flow through the ground (outer metal shell) of the USB/GPIB port.  I am surprised that you say that this current could hurt anything, because I thought it would just go through the shell of the wire (I assumed that it would bypass any electronics).  So would you suggest to use a separate cable and touch it briefly between the laptop ground (i.e. a USB port ground) and the chassis of the instrument to get rid of any static electricity and then hook up the GPIB-USB-HS?  I am guessing that once a connection is established, there can be no further problems.  So it appears that I don’t need a permanent connection between the two chassis.  Or is there a better way to ground a laptop permanently?

Sorry if I am beating this to death, but I am new to using GPIB and would like to understand any issues.  Thanks again!

Stefan 

[JÞB]

MeisterS-

You aren't beatting it to death at all!  It sounds to me like you want to learn the best way to aviod common mistakes (pardon the pun)

The return's and shields of the various periphrials (serial, gpib, usb etc..) are not intended to carry current.  If I HAD to use a laptop I would try one of the following methods.

1) use a powered docking station that is plugged into the same electrical source as the equipment.

2) attach the laptop chassis to equipment ground through the charging port (this connection is designed to carry current and is usually well protected from surges)

3) Depending on the model, the chargeing adaptor may provide a connection from chassis to building ground.  If the charger is grounded you can plug in the laptop and depend on this connection.

 4) Use a bus isolator such as an NI GPIB-120B (sorry if I sound like a sales rep- CCI's NI aliance membership was announced at NI Week and we are quite proud of it)

[sth]

Jeff is right about the isolation requirements.  The USB ports in general have a grounded shield.  That makes your laptop not floating but grounded to the chassis of your GPIB device.  This is both good and bad.  It is good that your laptop is grounded.  It is bad that you have laptop digital noise (lots of high speed pulsing clocks) connected to the sensitive instrument.  Lastly *if* you plug in your laptop to a power source then you have a ground loop where the chassis of the instrument and the chassis of the laptop are grounded separately and also connected.

To break this loop and isolate the computer noise we use a GPIB isolator 120C which NI no longer makes.  They were expensive but very nice.  I have been thinking about USB isolators with a new power supply in a powered hub to run the GPIB converter.   I don't know much about the new NI GPIB isolators and their characteristics.

Almost all peripherals from your laptop carry a ground as well as signal.  The ethernet port is probably the exception where all the signals are inductively coupled to the laptop. 

[Mr.F]

Hi Jeff,

Sorry if my earlier post was unclear. I was addressing Meister’s original concern of shorting caused by a floating laptop connected to a grounded device or being connected to a different ground plane. I did not mean to imply anything that would dismiss the installation guideline.  Your dialogue with Meister went more in-depth that I thought his original question was, and those steps are good to follow to minimize stresses on your laptop and/or sensitive equipment which could lead to damage. I agree with you that Meister did a good job making sure that he understood everything in the installation guide, seeking help when he was unclear and it looks like now he has all the information he needs!

[Steven_T]

Hello Scott,

I just wanted to let you and the other know that we still have the GPIB expanders/isolators available.  You are correct that the old isolators are no longer sold, but we do have new revisions available.

The older isolator was the GPIB-120A.

The new one that replaces it is the GPIB-120B.

Thanks,

Steven T.

[sth]

Steven,

You are right.  In fact in looking at the specs the new 120B also isolates both GPIB from the power supply which is an additional feature.  They are not cheap but they are invaluable for ultimate low level signal measurement.

I see that you still promote these as completely transparent to the user software, which wasn't quite true for the 120A and I presume is still true for the 120B.

The isolator presents all addresses as listeners to the controller.  Thus if you are scanning a bus looking for listeners you get a positive on all addresses if there is an isolator rather than only those with a device if the isolator is not there.  Is this still true for the 120B?

I think this is inherent in the design of an isolator.  So I am not complaining that it has this change by putting an isolator in the bus but rather that the specifications are over stated that it is not completely transparent.  I had to rewrite one of my VIs when I started using one of the isolators.