Ethernet connection disrupted by HDMI cable

[BarryMoon]

I've been having problems with my Pi's Ethernet connection. It has timeouts, dropped packets, etc., but also distrupts other devices on my network (even wireless connections) in the same way.

Some trial and error tracked it down to the HDMI cable. When it's unplugged, the network is fine. Plug it in to the Pi, and the entire network is disrupted.

I've seen that USB devices are a common cause of network problems, but my USB devices - keyboard and mouse - don't seem to affect it, just the HDMI cable.

The PSU, keyboard, cables, etc. were in a kit from ModMyPi, so I'm assuming they're all compatible and up to the job. The PSU is rated at 5V/2A, and the voltage across the TP points is 4.77V.

I have a brand new Pi, with the latest Raspbian Wheezy image (2012-09-18).

Does anyone have an idea about what might be going on?

[mahjongg]

Perhaps its the extra PSU load caused by the HDMI monitor (the HDI monitor consumes about 50mA) at least that is the only link between the HDMI video output and Ethernet that I can think of.

[obcd]

If other wireless devices lose connection, I would assume it's a sort of EMI. I would try a wire between the Pi GND and the mains earth. Another thing could be the shielding of the hdmi cable. If you have another one lying around, see if it works any better with that.

[mahjongg]

Yes, if you are powering the PI using a double isolated power supply (with a two prong plug) and do not connect the PI to the HDMI port the PI is in no way connected to GND, if you connect the PI through HDMI to a monitor, and the monitor isn't properly grounded itself, there is a galvanic connection to the TV, and any high voltage leakage voltages from the TV will also become present on your PI, which may indeed disturb ethernet communication (even though the ethernet is isolated with a transformer, the unused wires in ethernet are connected through 4KV capacitors to the GND of the PI). In fact its possible that you have say a hundred volt AC relative to earth on the GND of your PI in such a case, (but not harmful because it will leak away harmlessly when you touch the PI).
It is very important therefore that you ground your monitor, but grounding the PI itself may also solve the problem, as I said before the PI has no other ground connection than the TV.

[BarryMoon]

Thanks both for your replies.
I'm a total beginner with electronics and all things Pi, so I'll have to read up on some of the things you've mentioned. I wouldn't be confident wiring anything up right now!

I have tried another HDMI cable and the symptoms are the same. My TV is grounded, but the Pi itself is not (my power-supply is a "double isolated" unit). Could one solution be to find a supply that does have a ground connection?

I am puzzled about how to tell whether this is an issue with my particular Pi, or my set-up.
Presumably this is not a common problem, and most people are able to use Ethernet and a HDMI connection simultaneously?

[mahjongg]

Thanks both for your replies.
I'm a total beginner with electronics and all things Pi, so I'll have to read up on some of the things you've mentioned. I wouldn't be confident wiring anything up right now!

I have tried another HDMI cable and the symptoms are the same. My TV is grounded, but the Pi itself is not (my power-supply is a "double isolated" unit). Could one solution be to find a supply that does have a ground connection?

I am puzzled about how to tell whether this is an issue with my particular Pi, or my set-up.
Presumably this is not a common problem, and most people are able to use Ethernet and a HDMI connection simultaneously?

obviously yes.

I don't think additionally grounding the PI should matter, nobody else has the need to do so.

If you have a digital volt meter I would measure again the voltage of the input power the PI receives, as you know (bu other readers might not) to that end there are two testpoints (TP1 and TP2) on the PI, one is in between the RCA plug and the pinheader, the other one is near RG2. TP1 is 5V, and TP2 is GND. Between the two you should see something very close to 5V. Earlier you said that you measure 4.77 V, but measure with and without the HDMI cable connected, is there any difference? Also, what happens when you connect the PI to a TV using the Composite connector, do you also get ethernet problems then?

Measure AC voltage between Earth (a water tap perhaps, or the Ground prong of your mains) and the GND (TP2) of the PI, do you see a large AC voltage, what if you disconnect the HDMI cable? What if you disconnect the ethernet cable?

Did you ground the ethernet device that is at the other end of the ethernet cable? It could be that that device carries a high voltage relative to earth, and when the PI isn't grounded it simply "floats" with the high AC voltage, but if you ground the PI, it cant "float" anymore, and the high AC voltage that then exists between the ethernet cable and the PI might disrupt communication, it should not, but I have seen it happen anyway with other Ethernet sporting devices.

[BarryMoon]

Hi - I've attempted a few voltage readings following your suggestions...

Interestingly, the readings and the symptoms are exactly the same with the RCA connection as with the HDMI cable. The network is disrupted the moment I plug either TV connection into the Pi.

Earth, i.e. a radiator pipe, to Pi TP2, with no connection to the TV is +/-0.05V. With a connection to the TV, the reading is +/-0.07V. (Apologies if I'm using the wrong terminology - I mean the reading slowly oscillates between +0.07V and -0.07V).

The readings across TP1 and TP2 are:

• 4.85v - nothing plugged in
  4.84v - TV (HDMI or RCA) connected
  4.81v - TV and USB connected
  4.78v - Ethernet only
  4.77v - Ethernet and TV connected
  4.75v - Ethernet, TV and USB connected

The device at the other end of the ethernet cable is my Netgear router, which is also double-isolated.

[mahjongg]

Interesting:

That the connection method doesn't matter points to that simply connecting the ground of the PI to the ground of the TV creates the disturbance, and also that the disturbance is not caused by a collapse of the PI's power supply volate (that would be difficult to achieve with a composite connection, but possible with a HDMI connection.

Your measurements to GND are puzzling! did you use the AC setting, was the PI grounded through the HDMI cable and the TV? I still suspect some HF noise entering the system through a faulty ground connection. If there is a high voltage signal on GND it would be maybe a hundred volt or something, not a fraction of a volt.

[obcd]

If you only connect the GND connection of the RCA cable, does the same happen? (It's the outer ring)
You say that other equipment drops their connection as well. Are those wireless or wired connections?
What's the distance between your Pi and your router?

[BarryMoon]

Yes, it seems the network is affected even if I just connect the outer-rings of the RCA cable.

I've been testing by pinging google.com. I noticed today:

• 1. When the TV is connected, I can still successfully ping the Pi itself using its IP address (my router assigns it a static IP), even though pings to google and other addresses fail.
  2. When fully connecting the RCA cable, the ping's intiial DNS query usually fails. But when I tried connecting only the ground, the ping got a little futher - it located the DNS entry, but got no response. (This was generally consistent but not 100%)

I think this implies that it's the external network, rather than the router itself, that's somehow being affected - maybe even the phone line? (It's ASDL broadband).

If my voltage measurements are puzzling then I'm probably reading them wrong - I don't really know what I'm doing! I don't think my multimeter has a setting specifically for AC (the guide just says the same thing for AC and DC).
What I see on the 20V range, with the red probe on ground (I tried a ground pin this time) and the black probe on TP2, is a reading that slowly oscillates between -0.05 to +0.05 (through zero) over a period of about 4 seconds. The range increases to +-0.07 when the Pi is grounded with the TV connection.

When you say HF noise, would that be noise on the mains circuit or the network itself? I do have HomePlugs on my network, although I'm not using them here. Would they affect anything?

Thanks again - I appreciate your help...

[BarryMoon]

I just turned off all the HomePlugs and the symptoms were the same, so they're not part of the problem.

[Burngate]

You don't say where you are. If you're in the UK this won't help, but elsewhere ...
Are you're PSUs two-pin ones? And if so, can you turn one or more over? This should invert any interference, but also may change any ground voltages.

[BarryMoon]

Sorry obcd, I forgot to say - both wired and wireless connections are affected, and the network cable between the Pi and my router is about 2 metres.

Hi Burngate - yes, I am in the UK!

[mahjongg]

Yes, it seems the network is affected even if I just connect the outer-rings of the RCA cable.

This means that either:
1) Grounding the PI through the ground of your TV is causing the problem
or
2) Some interference voltage is on the "ground" of your TV and it is injected into the PI when you connect the ground of your TV to the PI.

It would be useful at this stage to see what happens if you ground the PI to a reliable earth connection, obviously without the TV connected to the PI in any way, to determine which of the two above scenario's is true.

What I'm wondering, how do you determine the ethernet connection of your PI goes bad without a display?

[BarryMoon]

Hi. I've just tried connecting the Pi directly to ground - exactly the same symptoms occur as when connected to the TV. So it's not the TV per-se, but rather the connection to ground.

Good question re. testing the Pi's connection with no display. I've mostly been checking other devices' connections - because I know that when the Pi's connection is broken, other devices' connections break too. But actually I can prove this because logging in to the Pi remotely does work - whether connected to the TV or not, and using an SSH session I can test the connection with ping.

I'd missed this. It's only just occurred to me (sorry) that what's actually being disrupted is the connection to the Internet , i.e. external addresses. The Pi can ping other local devices on the network just fine, and vice-versa. So what I've been saying about "Ethernet" is misleading - it's only the Internet connection, from any machine on my network, that's actually disrupted when the Pi is grounded.

[mahjongg]

If you can SSH to your PI, the ethernet connection is NOT disturbed, it does however look like DHCP fails, that you URL isn't translated into an IP address.

[obcd]

The issue is clearly the adsl connection failing when the Pi is running and when it's grounded.
I think we had someone else with the same issue already. His solution was placing the adsl modem further away from the Pi.
Sometimes it's the phone signal entering the modem that is already weak.
A little noise caused by the Pi could just make it fail.
If you disconnect the Pi / modem-router cable, do the other computers get their internet again?
If you disconnect your adsl modem from the power and reconnect it while the Pi is running, does it work like that afterwards?

[BarryMoon]

Thanks for your patience both, I appreciate your help. Apologies for it taking so long to get to the actual issue.

It might not be solely DHCP that's failing. An external ping (to google.com, say) will mostly find the IP, but often the look-up will time-out. When the IP look-up does succeed, then it will occasionally get a ping response from the server - but mostly the ping will time-out.

I restarted the router with everything connected (while Pi is grounded) but that was no different; in fact the router apparently had trouble connecting until I pulled the TV cable out of the Pi when all was well again.

But when I disconnected the network cable from the Pi, leaving the TV connected, I was extremely surprised to see that the symptoms remained. As long as the Pi is turned on, and grounded, the Internet connection is affected - even when the Pi is not actually connected to the network!

[mahjongg]

So you have only connected the power and the HDMI cable to the PI?

Sounds like electromagnetic interference, which may be generated by the PSU, and somehow is emitted through the ground loop closed by the HDMI cable, perhaps a ferrite clamp filter in the HDMI (and/or power cable) may help.