HMI Grounding Power Supply Common

[fifty60]

I am looking at an HMI screen that operates off of 24VDC. On the back connector there is a "+" a "-" and a ground symbol enclosed in a circle. When I connect all pos. neg. and a ground wire to the connector, everything operates as expected. The ground wire connected to the ground terminal at the HMI side, and then to safety ground on the panel side. The ground terminal is also internally connected to the frame of the HMI.

When I plug in a DB-9 (RS232), things get interesting. With the DB-9 installed and the ground wire installed, my power supply powering the HMI is now ground referenced instead of floating.

I can kind of see how the signal ground on the RS232 is interacting with the frame ground. But, I do not see how this is grounding the "-" of my power supply. Any thoughts on how the "-" terminal is being connected here?

 

[fifty60]

When I remove either the frame ground or the DB-9, then the "-" is no longer ground referenced. But with both plugged in, then the "-" is ground referenced..

 

[petersonra]

My guess would be that the rs232 common (usually pin 5) is internally connected to PE at the other end somehow.

I have seen this done with rs422 but not rs232.

 

[fifty60]

I can understand that, but why is it ground referencing the power supply? None of those internal pins should be internally connected to either of the power supply inputs, should they?

 

[GoldDigger]

I can understand that, but why is it ground referencing the power supply? None of those internal pins should be internally connected to either of the power supply inputs, should they?

It is not out of the question that the "signal ground" of the RS-232 in the display is connected to the - power input with the + of the RS-232 signal driver chip connected to power + and the negative signal voltage derived internally in the chip using a charge pump.
Then since the DB-9 cable does not have enough wires the signal ground and the frame ground are connected together and to the connector shell. When you plug in the cable you tie frame to power supply negative, then when you ground the frame you ground reference the incoming power.
All of this makes a good case for opto-isolators and a floating signal power supply at each end.

 

[fifty60]

Are the optoisolators for the RS232 signals? Do you think this would have been designed like this on purpose?

Is it possible the ground on the controller next to the power inputs is not meant to be connected to earth ground, but only a shield ground? Seems odd though that you would want to connect the shield to the controller frame. Why not connect the shield on the other end and send all the electrical noise away from the HMI?

 

[GoldDigger]

Are the optoisolators for the RS232 signals? Do you think this would have been designed like this on purpose?

Is it possible the ground on the controller next to the power inputs is not meant to be connected to earth ground, but only a shield ground? Seems odd though that you would want to connect the shield to the controller frame. Why not connect the shield on the other end and send all the electrical noise away from the HMI?

From experience I can tell you that expecting the designers of RS-232 interfaces that are just incidental to the product to make good engineering choices is not justified. And that goes double for the cable makers.
And I think that the ground terminal is intended to be an earth ground.
But at the same time they assume that the power supply negative is ground referenced too.

 

[fifty60]

Can you please elaborate more on what you mean by "since the DB-9 cable does not have enough wires the signal ground and the frame ground are connected together and to the connector shell"

 

[GoldDigger]

Can you please elaborate more on what you mean by "since the DB-9 cable does not have enough wires the signal ground and the frame ground are connected together and to the connector shell"

Sure:

Here is a commonly used pin description for the DB-9 connector when used for RS-232:




Note that there is only one pin labelled ground. In various application documents it is sometimes written as just "ground" but more often as "signal ground", and the latter is what it is intended to be. It is the voltage reference for the bipolar but not balanced RS-232 signal lines. The shell of the connector is designated as shield or frame ground or protective ground. But there is no place on the typical connector body to actually attach a shield wire to the connector shell.
In the DB-25 version of the interface there are separate pins for signal ground and shield.
In the DB-9 version a conservative (but dim) cable maker will, to insure a good solid protective ground connection, tie the connector shell as best he can to pin 5 and either use that wire alone in an unshielded cable OR a wire which is also connected to the shield at one or both ends OR just the shield for both functions.

You can figure out which you have with inspection (if it is not a molded connector) and/or some ohmmeter work.
But it appears that the important part is the pin 5 is tied to the connector shell which in turn is tied to frame ground.

Further complicating things are those applications that actually need other combinations of RS-232 data wires than are present normally in the DB-9 or want to also provide interface power to an external device. That road leads to chaos if you ever step outside that designer's narrow world.

 

[fifty60]

Thanks for the explanation, much appreciated!