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Chassis Gnd/Signal Ground - Any Views?

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David Porter

Jan 1, 1970
0
I am developing a USB peripheral using the FTDI 245 and wonder if anyone has
any suggestion of grounding.
The USB Spec stipulates that the shield of the USB cable should only be
connected at the host end. If my peripheral is mounted in a metal enclosure,
should the case be connected to the USB shield and signal ground left
floating or should the ground layer from the PCB be connected to the metal
case via mounting screws and the USB shield isolated.

My device may well also be mounted in another metal earthed enclosure.

Any ideas/thoughts would be greatly apprecaiated.
 
D

DarkMatter

Jan 1, 1970
0
I am developing a USB peripheral using the FTDI 245 and wonder if anyone has
any suggestion of grounding.
The USB Spec stipulates that the shield of the USB cable should only be
connected at the host end. If my peripheral is mounted in a metal enclosure,
should the case be connected to the USB shield and signal ground left
floating or should the ground layer from the PCB be connected to the metal
case via mounting screws and the USB shield isolated.

My device may well also be mounted in another metal earthed enclosure.

Any ideas/thoughts would be greatly apprecaiated.


As long as you keep the case isolated from the circuit, you are
alright as the case would then become an integral portion of the
"shield". The host end is connected as specified, but leave any
circuit grounding off that line, and not attached to the case.
That includes circuit board ground rail and planes et al.
 
D

David Porter

Jan 1, 1970
0
As long as you keep the case isolated from the circuit, you are
alright as the case would then become an integral portion of the
"shield". The host end is connected as specified, but leave any
circuit grounding off that line, and not attached to the case.
That includes circuit board ground rail and planes et al.

Thanks for that. However, the device is likely to be fitted into another
enclosure where the signal grounds would be connected to other equipment. If
any of those items of equipment joined chassis gound with signal ground we
would end up with the shielding on the USB being inaffective.
 
D

DarkMatter

Jan 1, 1970
0
Thanks for that. However, the device is likely to be fitted into another
enclosure where the signal grounds would be connected to other equipment. If
any of those items of equipment joined chassis gound with signal ground we
would end up with the shielding on the USB being inaffective.

I guess it all comes down to what level of communications you are
making. Opto isolate all of your inputs to the device from the other
chassis. I know... I know... COM Cost Of Manufacture.

What do you want, good grammar or good taste... errr...

What do you want, good signals or cheap costs. :]
 
D

DarkMatter

Jan 1, 1970
0
Would it be better to isolate USB shield from chassis on peripheral and use
signal ground to shield case. Regardless of what is then done it will not
join chassis ground and signal ground as far as the USB host is concerned.


Stopping the USB cable's shield at the case connector without
connecting it would be no different than connecting it, and not using
the case in any way at the circuit or other connectors. In fact, it
would complete the faraday cage around the circuit pretty good.
 
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