[Linux-aus] Grant Application 2017 - TP-Link PoE switches

Robert Manietta robert.manietta at gmail.com
Wed Apr 19 06:12:56 AEST 2017

Hi Adam,

Thank you for your comments, We were aware of this.

That being said it is something we will need to ensure we have noted, so no
one damages other equipment.


Robert Manietta

On Tue, Apr 18, 2017 at 11:23 PM, Adam Nielsen <a.nielsen at shikadi.net>

> Hi Robert,
> > Please see our grant application for constructive comment and review.
> This might not be relevant for your purposes, but are you aware that
> the TP-Link TL-SG1008P switch you are proposing is a non-isolated PoE
> switch?
> This means that any device it powers cannot be connected to any other
> earthed device, or it will short out the switch's power supply.  The
> reason is that (no doubt for cost cutting measures), any PoE devices
> connected to this switch have their negative supply (GND) connected to
> -48VDC with respect to earth, rather than the more usual direct
> connection to earth itself.
> The end result of this is that, should you power your Pi via PoE, all
> the connector shields on the Pi are sitting at -48VDC.  So when you
> (for example) plug in a HDMI cable connected to a TV, the 0V shield on
> the cable gets connected to -48VDC on the Pi, short circuiting the
> brick powering the TP-Link switch.
> I had this exact problem and connecting a HDMI cable to the Pi produces
> a spark and shuts off the switch, although the switch does reboot once
> the cable is removed again.  Although I didn't leave it connected for
> long enough to see if the power supply exploded or caught fire...
> If you will only ever be powering PoE devices that do not plug into any
> other powered device (e.g. VoIP phones, IP cameras, etc.) then you can
> probably get away with it, but I am a bit wary of my TP-Link switch now
> after this experience.  I have to remember that any PoE device
> connected to this switch absolutely cannot have certain things plugged
> into it - HDMI monitors, powered USB hard drives, headphone cable to
> an amplifier (48 volts on an amplifier's audio input, ouch!)
> I would recommend looking at eBay for a second hand Cisco switch
> instead, which is what I ended up getting to replace the TP-Link when
> I eventually needed more than eight ports.  You can get a 48-port PoE
> switch for $16 more than one TP-Link unit if you are willing to settle
> for 100Mbps on all switch ports, or if you want gigabit then a 48-port
> Cisco is ~$7 less than the three TP-Link 8-port switches in your
> proposal - not bad for double the capacity.
> I was a bit fearful of using a Cisco switch, thinking I'd have to take
> a course to figure out how to use it, but setting it up is one of the
> easiest things I've ever done.  Installing Linux is way harder.  It
> doesn't actually need any configuration at all if you want to use it
> like an unmanaged switch.  I love how the configuration is just one big
> text file, which you pretty much just copy onto the device and it
> immediately starts functioning exactly as described.  OpenWRT and the
> other router distros could learn a thing or two from that!
> Anyway, I just thought I had better point out one of the limitations
> with the TP-Link in case it affects what you were intending to do with
> it.
> Cheers,
> Adam.



*Robert Manietta*

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