Raspberry pi 4B and a modem in a USB enclosure power questions

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poodad
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Raspberry pi 4B and a modem in a USB enclosure power questions

Post by poodad » Sun Jul 26, 2020 10:42 am

I'm looking at using GoldenOrb on a RPI 4B and a Sierra Wireless EM7565 modem in a USB enclosure, but I'm wondering about power requirements. The RPi can be powered by PoE, and the modem is powered over USB.

This would be mounted in an enclosure on the antenna mast.

I looked at the EM7565 data sheet and it says max current is 1.3A at 3.3V (4.29W). USB 3 can supply 900ma at 5V (4.5W). So, as long as the M.2 to USB enclosure is doing close to 100% efficient 5V to 3.3V conversion, the pi should just be able to power the modem.

The pi itself requires 3A at 5V (15W). Presumably, that includes a power budget for having at least one USB C port at 900mA.

IIRC, the "standard" 1Gb PoE injector can do 16W at 48V (333mA) (although there are a bunch of PoE standards of varying power).

So, in theory, it should all work, but that assumes the RPi is doing close to 100% efficient DC to DC conversion of 48V to 5V, and that the USB enclosure is doing 100% efficient conversion of 5V to 3.3V

In theory, the difference between theory and practice is small. In practice maybe not so much.

I'm beginning to think this power scheme just isn't feasible.

My options would seem to be:

Run 120V AC to the enclosure and use separate power supplies for the RPi and modem.

Run two ethernet cables on power injectors to the enclosure. One would be data and power to the RPi. The other would be power only via a PoE splitter to the USB enclosure (which appears to have an AUX power micro USB jack).

Mount an 120V outlet inside my attic where the cables com in and use separate RPi and modem power supplies on which I've extended the DC cables by 20' or so using 16 gauge wire.

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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by Viper67857 » Sun Jul 26, 2020 12:08 pm

Another option: don't use the cheapest possible injector... 16w is very very low-end.. 30w is still fairly cheap, and 60w exists... You can also use 2 usb cables from the pi to the enclosure (1 for power + data and 1 for extra power only).

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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by poodad » Sun Jul 26, 2020 3:36 pm

Viper67857 wrote:
Sun Jul 26, 2020 12:08 pm
Another option: don't use the cheapest possible injector... 16w is very very low-end.. 30w is still fairly cheap, and 60w exists... You can also use 2 usb cables from the pi to the enclosure (1 for power + data and 1 for extra power only).
OK, I figured this out - I think. The RPi 4 comes enabled for PoE, but does not actually do PoE. To do PoE, you need the PoE "hat" which does IEEE 802.3af-2003 PoE - which is 16 Watts (actually, only 12W guaranteed at the end device). The PoE hat specs further say that it provides 5V at 2.5A.

So, I think that my setup WILL work. But I like the idea of using two USB cables to double the power (the M.2 to USB enclosure has a USB3 data/power jack, and an aux USB for power. I need to check with the enclosure manufacturer and verify it will use power from both.

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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by poodad » Sun Jul 26, 2020 5:59 pm

More info:

According to the chart about 3/4 of the way down this page https://www.raspberrypi.org/documentation/faqs/, a pi 4B under load draws 1.25A. That, with the 900mA for my modem would be a max of 2.15A.

The PoE "hat" for the RPi says it can do 2.5A. That *should* leave a safety margin of 350mA. And that's with the RPi and modem both maxed. The RPi draws 600mA at idle, and I'm guessing a simple router would be much closer to idle than max CPU.

So the bottom line is, this should work.

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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by Didneywhorl » Thu Jul 30, 2020 12:02 pm

Mock it up and test it under load. That's the only way in my opinion. Let it get good and hot too, as it may in your normal conditions.
Run a high quality video stream then while that is running run speed tests and maybe even start a large file download.

Do this over and over. It would be the best way to be sure. Running it once or twice will not be enough. This would be better than installing it and then having to disassemble it when problems arise, though not entirely avoidable sometimes.
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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by poodad » Sat Aug 01, 2020 9:02 am

Viper67857 wrote:
Sun Jul 26, 2020 12:08 pm
Another option: don't use the cheapest possible injector... 16w is very very low-end.. 30w is still fairly cheap, and 60w exists... You can also use 2 usb cables from the pi to the enclosure (1 for power + data and 1 for extra power only).
It's not that simple. The old 100mb method of PoE was to just shoot 48V down the unused pairs in a cat5 cable with no intelligence. The new gigabit standards requires handshaking between the PoE device and injector or switch port.

There's two common standards for doing power over gigabit ethernet. IEEE 802.3af-2003 which supplies 13W and IEEE 802.3at-2009 which supplies 25W. There are other variations that suppy even higher current.

The device and power source will only negotiate the highest power supported by both ends. The RPi PoE hat is 802.3af which means that even if you plug it into a higher power injector, you aren't getting more juice to the RPi.

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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by Didneywhorl » Sat Aug 01, 2020 9:54 am

High powered gigabit injectors exist for this purpose. The standards are in place for minimums, it does not stop the device from being supplied more current if its connected devices draw it.

Instead of getting wrapped up in "standards", just try it. Just make sure the voltages match.
Were not setting up million dollar industrial networks, were playing with a $35 raspberry pi.
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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by poodad » Sat Aug 01, 2020 5:56 pm

Well, this project is just increasing in suck at every corner. I put together my final build list and started ordering. The RPi, PoE hat and aux USB 3 cable got ordered on Amazon.

I went to order the modem and USB enclosure from netshop to find that they've shut down to take inventory. No firm ETA on reopening (I'm not complaining, just saying that this is an unexpected wrench in the works).

I got my Raspberry Pi 4 and PoE injector today. I downloaded the GoldenOrb image for RPi 3 assuming that it would work (the RPi 4 specs say "backwards software compatibility"). It does not work. Google says there are several people in the same boat as me.

The ROOTer folks are apparently waiting on a stable release of OpenWRT for RPi 4 before they'll do GoldenOrb for it. The OpenWRT people have had a daily snapshot of OpenWRT for RPi 4 for a while, but they are waiting - well I have no idea of what they are waiting for to release a stable version.

So, I have a few options:

1. Maybe GoldenOrb will release for the RPi 4 before I can get the modem and enclosure in hand.
2. I can buy a RPi 3 and use it until GoldenOrb is released for the RPi 4.
3. I can install the daily build of OpenWRT (which seems to work just fine) and manually install all the stuff that ROOTer bundles.

I'm also toying with the idea of scrapping the RPi and associated software and instead use pfsense on a small x86 system. I'm already familiar with pfsense - just need to find a cat12 model that it is known to work with. But I'd also lose PoE with this option, and that was a pretty attractive feature of the original plan.

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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by Viper67857 » Sat Aug 01, 2020 10:44 pm

You should be able to compile a modemmanager build of the snapshot and get it going without full goldenorb... If you've never setup a Linux build environment, though, be prepared for some trial & error... If using a virtual machine under windows, be sure to allocate plenty of hard drive space, as it can use like 30gb to generate that little ~16mb flash image...

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Re: Raspberry pi 4B and a modem in a USB enclosure power questions

Post by poodad » Sun Aug 02, 2020 12:36 pm

poodad wrote:
Sat Aug 01, 2020 5:56 pm
...
I'm also toying with the idea of scrapping the RPi and associated software and instead use pfsense on a small x86 system....
pfsense is almost certainly out of the running. The only way FreeBSD (the OS under pfsense) supports LTE modems is via ppp. That limits performance to the 20mb/sec range.

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