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MikroTik RB960PGS hEX PoE powering PoE devices: ensure you get a 48V power supply

Posted by jpluimers on 2021/08/24

By default, the [WayBack] MikroTik RB960PGS hEX PoE comes with a 24V power supply.

Most PoE capable devices cannot be powered by 24V but need 48V. I wrote about this before in the midst of the long post Linus Torvalds – Google+: Working gadgets: Ubiquiti UniFi collection (and a whole bunch of Unifi/Ubiquiti/Ubtn links)

So now I re-mention it in a much smaller post so it easier to find back, and a few links to Power over Ethernet – Wikipedia, where especially these bits are relevant:

  • The PoE Standard implementation for 802.3af (802.3at Type 1) “PoE” requires DC 44.0–57.0 V.
  • Of the PoE Non-standard_implementations, some common Passive specifications include:
    • 24VDC 0.5A 100 Mbit/s or 1 Gbit/s
    • 24VDC 1.0A 100 Mbit/s or 1 Gbit/s
    • 48VDC 1.0A 100 Mbit/s or 1 Gbit/s
    • 56VDC 1.0A and 2.0A 1 Gbit/s (used for 45W+ load point to point microwave and millimeter band radios

The 24V is what MikroTik sticks to with their default power supply.

48V enables real 802.3af (802.3at Type 1) PoE.

Realising this helped me to remote control the power to:

PoE distances

I had a hard time finding out maximum distances for PoE cable length, as the information is disparate.

From the links below, I think the conclusion is that the maximum PoE length (or reach) is not limited by PoE, but by the network standard.

This reach seems 100 meters or 328 feet for active PoE:

Note that passive PoE has much shorter distances: Power over Ethernet: Passive – Wikipedia mentions 15 feet (4.6 meters) 5V because of the voltage drop.

Furthermore, for long cables, prefer CAT 5/E over CAT 6/6A:

[WayBack] Unexpected PoE / Cable length limitations with UniFi Switch-16-150W & UniFi AC-LR Access Points | Ubiquiti Community

What you are likely seeing here is something that will become more of a problem down the road – CAT6/6A cables are being made with 26 gauge cable.

The biggest problem with 10Gb ethernet is crosstalk between the wires/pairs in the cable.   THere’s just one way to lessen this – move the wires physically farther apart.   One way the cable mfgrs have found to accomplish this while not making the cable too large – there is actually a spec for the maximum diameter of ethernet cable – is to make the copper wire itself smaller, so the insulation is thicker, so there’s more space between conductors.   This actually works ( and they love it because there’s less copper which saves them money, but they can charge more for the cable because, well, it’s spec’d higher, right?) , but it causes the problem you are seeing – the smaller conductor has higher resistance and therefore more voltage drop than the standard cat5e 24 gauge wire.   So with longer cat6 cables ther PoE limitation needs to be accounted for.   This is why I never use Cat6/6A cables except for very specific things, and never ever for PoE.

Change out your cable to cat5e and the problems will go away.   And at gigabit speeds it will work just fine up to 100 meters, even at 24V PoE.

Permanent “on”

You might think plugging in a 48V power supply is enough, but that might not cut it.

The autonegation of PoE does not always work, which in this case did not.

Setting the “PoE Out” option of a MikroTik network interface from the default “auto on” to “forced on” then gets going.

At start-up, in text mode, the RaspBerry Pi+ with an attached Chromecast first generation draws about 3.8 Watt:

With a full blown graphic UI at 1920×1200 with moving graphics in Firefox draws slightly more, somewhere between 5.2 and 5.9 Watt (usually closer to 5.2):

–jeroen

 

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