5i25 daughter boards 5v com connected to chassis ground ???

I was working on system tonight checking how I wanted to setup my power supplies with the mesa cards whether I use separate power supplies for VIN and field checking the ground and com for noise when the servo's move. I found that the common of the 5v logic power for the boards were tied to ground. That took my floating 5vdc power supply and the com was at chassis ground.

After scratching my head and checking, it seems it is coming from the computer. I checked the shield of the DB25 and it was at ground also. I checked the 7i77 and the 5v com is connected to chassis also.

Is this correct?

I thought what is the point of having a floating power supply if your going to tie it to ground. All along I was checking with the continuity on my fluke. I checked resistance and it seems there is a 10 ohm resistor that I assume is the protection.

Just wondering what the story is with this or am I overthinking this?

After carefully installing separate 5V and 24V power supplies which appeared to be best practice, I added manual MPG handwheels to my 7i76e, I had to tie the grounds of the 24V and 5V sections together before the encoders would do their magic! At that point, I began to wonder why I wasted money on the separate 5V power supply. Up until that point because it was an ethernet connection, the grounds were isolated. I think everybody is out to get you in this area!

So whats the point of all this isolation stuff. It is all tied back to chassis ground anyway, so say someone swaps line and neutral on a 120vac extension then you have ac potential on your chassis ground, which is basically all your dc power supply grounds including field power.

Its not suppose to be though. I have a powersupply and has no ptential to ground until I connect into the system.

No, ground is still ground even if you swap line and neutral (just dont swap line and ground)

rodw: for the MPG you dont need to connect field power common and 5V logic common, but the MPG 5V must share the field power common.
if you want the field power isolated from logic power and use a 5V MPG you could power the MPG from a 5V regulator running on field power. (Thisis also true of the 7I76/7I77)
Because the 7I76E is connected by Ethernet, it is completely isolated from the control PC so isolation of 5 and 24V supplies is up to you.


If isolation of 7I77 logic power from the PC is important you can add a 7I77ISOL

PCW wrote:

if you want the field power isolated from logic power and use a 5V MPG you could power the MPG from a 5V regulator running on field power. (Thisis also true of the 7I76/7I77)
Because the 7I76E is connected by Ethernet, it is completely isolated from the control PC so isolation of 5 and 24V supplies is up to you.

Thanks Peter,

So given I'm using 24V for field power, are you saying to use a 24 volt to 5 volt power supply to power MPG's to maintain isolation?

I did think that Ethernet had a significant advantage in a noisy plasma environment hence my choice of card so thanks for the confirmation.

If its important to keep it the logic 5V and field power isolated you can do that
but normally there's no great advantage

Thanks for the info Peter. I think I will leave it like it is unless there is a problem. I was just wandering and couldnt understand it.