SD Card Failure

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fifty60

fifty60

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I have an HMI screen that has a 2GB MLC memory card. The card is not a brand name card.

I am seeing quite a few of these cards fail in the field, but never on the same screen. Can anyone offer any input on what could possibly go wrong with an SD card?

There is a USB port on the screen. The USB port is connected to an extension cable and then panel mounted to the front of the equipment. The metal housing of the USB connector is almost touching the painted metal surface of the equipment. Could this create issues with the SD card by shorting the USB housing to the enclosure of the equipment?
 
The HMI is powered from a floating 24VDC power supply. The shield of the power cable is connected to the functional ground input on the HMI on one end, and then connected to the equipment ground on the other end.

The USB itself has +5V, D+, D-, and Ground pins. This ground is floating as well, right? Or, is the USB ground connected to the case of the USB? Even if it is connected to the USB housing, it would be using a different grounding source than the equipment..

Since the HMI power cable shield is connected to the equipment ground, when the USB cable touches the cabinet it shorting the potential difference between the shield ground and the ground point where the USB case touches the equipment case?

If I do not connect the shield of the power cable to the HMI's functional ground, would that then take care of the problem?

Would an RS232 connection on the same device also cause a problem? Is the signal ground on an RS232 connected to the DB-9 case?
 
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Ground

Ground

Sounds like you're on the right track
Ground needs to be ground everywhere - same electrical point.
Then on to the actual earth ground.
0 VDC may NOT be grounded but common to all 0 VDC in the same loops.
Shields are normally connected only at one end, that being the most grounded end. Sometimes they act like antennae and drives cause much RFI.
 
Understood, but how can the metal casing of the USB connector touching the painted equipment enclosure cause my SD card to fail?
 
The power supply is Class 2 24VDC. The power inputs for the HMI touch screen are "+" "-" and "FG".

The FG should stand for functional ground. We currently use a shielded cable to supply the + and - to the HMI touchscreen. The shield on the cable is connected to the "FG" pin. The other end of the shield is then connected to the ground point on the equipment, which is ultimately bonded to the equipment grounding conductor.

Should the shield only be connected on one end (the FG end), or does it need to be connected to the FG on one end, and the equipment safety ground on the other?

I would think that the FG would prefer to be floating, so that anything else that is referenced to ground (a USB for example) that is connected to it does not have any potential for shorting through the touchscreen..
 
fifty60 said:
The power supply is Class 2 24VDC. The power inputs for the HMI touch screen are "+" "-" and "FG".

The FG should stand for functional ground. We currently use a shielded cable to supply the + and - to the HMI touchscreen. The shield on the cable is connected to the "FG" pin. The other end of the shield is then connected to the ground point on the equipment, which is ultimately bonded to the equipment grounding conductor.

Should the shield only be connected on one end (the FG end), or does it need to be connected to the FG on one end, and the equipment safety ground on the other?

I would think that the FG would prefer to be floating, so that anything else that is referenced to ground (a USB for example) that is connected to it does not have any potential for shorting through the touchscreen..
Click to expand...
I would assume FG to stand for "floating ground" rather than "functional ground", as the latter seems a bit redundant.

What does the HMI's manual say???
 
"FG" in the SMPS business means "Frame Ground", aka Chassis Ground. If your power supply has an all metal case, that is sometimes already connected to the mounting base, but there is no guarantee that this makes contact with your ground plane in the panel, especially if it is DIN rail mounted, because the DIN rail itself might not be a solid ground connection. So they expect you to use the FG terminal to make a solid connection to whatever your ground system is.

Grounding of signal cables such as those going to the HMI is to safely shunt any induced EMI / RFI to ground. It should only be done at one end, to avoid having the shield carry any difference in ground potential from one point to the other as a voltage on the shield and then be able to pick up noise that gets into your instrument (in this case the HMI). In the case of a door mounted device, the grounding of the door may end up with slightly more resistance compared to the grounding at the back panel, which can create that problem. The preferred method is to ground the end at the control panel, that way if you always do it like that, you don't worry about which end is grounded and if there is any difference in potential.
 
I deal with mainly 2 different HMI screens. They are both operated off of 24VDC power supplies. The power input to one of the HMI's is +, -, and FG. The power input to the other HMI is only + and -.

The FG is definitely connected to the internal components of the HMI. For both screens the SD card port, USB port, and Serial DB9 ports are all connected to the Chassis. So on one HMI screen, the HMI is referenced to safety ground through the FG, and the other HMI is floating since there is not a FG input.

Here is where the USB comes in. The USB is connected to a panel mounted USB extension cable. The operator connects to the usb be through the panel mounted USB jack. The only thing that is connected to the USB port is a USB drive, so no connections to computers, etc.

When connecting the USB to the panel mounted jack, the metal on the USB drive can bridge the painted metal on the panel mounted jack to the equipment enclosure. For the HMI with the FG, it is already bonded to functional ground through the FG port, so it is probably not that big of an issue. For the other HMI that is not referenced to the same ground as the enclosure, this could potentially cause a problem.

I am seeing bad SD cards in both HMI's. I am trying to determine if it is the fault of the cards, or something going on with the USB jack grounding to the painted frame of the equipment, on occasion. Any help here is much appreciated..
 
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