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

Adam Nielsen a.nielsen at shikadi.net
Tue Apr 18 23:23:17 AEST 2017

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

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


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