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  • Author Author: colporteur
  • Date Created: Date Created
  • Views 4300 views
  • Likes 3 likes
  • Comments 17 comments
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The Road to Raspberry Pi4B/ PoE Hat RoadTest Review (PoE insight needed)

colporteur

********************ISSUE RESOLVED. Credit to for identifying that a networking device was the issue.  ***********************

 

I would like the assistance of members that have PoE experience in reviewing my findings to help troubleshoot Ethernet failure under PoE.

 

I powered up the Pi4B & PoE Hat using a PoE power supply and managed to ssh over the wired Ethernet connection. Satisfied it worked, I command lined "sudo shutdown -h now" so I could teach go teach a class. When I cycled the PoE power supply on my return, the configuration failed to work. I determined Ethernet network was not being detected under PoE.

 

To isolate the issue, I ran a number of scenarios. I found it almost impossible to describe in writing the testing scenarios. Being a visual learner, pictures of each scenario seemed easier.  The attached drawing shows the hardware, connections and power status for each scenario. My interpretation of the results suggest the PoE power supply and PoE Hat are not at fault. The issue is with the Pi4B. Not working also means no indicator lights on the Pi RJ45 Ethernet Jack.

  • Compare scenario B & G: PoE power supply not at fault.
  • Compare scenario A & E & I: PoE Hat not at fault.
  • When you rule everything else out, what do you have left Pi4B

The PoE Hat controls the Pi Ethernet interface via the PoE headers. Would the PoE Hat have changed something (i.e. firmware/software) in the Pi4B when it detected PoE? It did work. I confirmed by checking logs. But only once.

 

An O/S re-install did nothing to resolve the issue.

 

If you look at the scenario's you will note the final scenario "J" using a Pi3B+ has not been ran. The scenario would use the PoE Hat to power the Pi3B+. The Pi3B+ is a personal device, not provided as part of the RoadTest. I am concerned running this scenario may render the Pi3B+ in the same state as the Pi4B.

 

Is the fault with the PoE Hat? Is it with the PoE power supply? Maybe it is the Pi4B? I am looking for individuals to review my findings and provide their insight.

image

  • in reply to shabaz

    Absolutely awesome! Thank you for chiming in. The resident switch will not negotiate 100M connection with the Pi4B. It does negotiate a 1G connection. When I use an replacement switch the Pi4B negotiates 100M on the 4wire (modified cable) and 1G on the eight wire.

     

    The resident switch negotiated 100M & 1G connections with the Pi3B+. The funny part is the resident switch is newer than the replacement switch. The resident switch is a Netgear and the replacement switch is a D-Link.

     

    The new switch enables the PoE power supply to be installed and provide 100M Ethernet connection to the Pi4B.

     

    These are the kind of findings I wanted to discover. I have no PoE experience. I'm sceptical when integrating technology. This demonstrates that integrating new and old may introduce problems.Your insight has help immensely at identify the issues.

     

    I have to write a note to informing him the issue has been resolved.  Again thank you.

  • in reply to colporteur

    Hi Sean,

     

    Just as a quick test, (I don't have a PoE HAT or a power injector), I tried a 4-wire cable (i.e. 1,2,3,6 straight wired) on my Pi 4 connected to my home switch, and the link does come up at 100Mbps as expected. Strange issue. I guess it might be a faulty Pi 4 if all else is ruled out. Hard to know. Maybe a transformer got burned out due to the PoE injector if the wiring was initially faulty or something? I'm out of ideas! : ( With a real switch with PoE, the power never get turned on until it is requested (due to certain resistance between pins) but I don't know if the PoE injector is behaving like that or just internally wired to permanently supplying 48V.

    You could check that with a multimeter, with half a cable (i.e. bare wires on one end, and RJ45 connector on the other, connected to the PoE injector. If the injector is compliant, then there should not be approx 48V available (or at any substantial current capability), because there is no special resistance between the pins. If you see 48V then that's not truly 802.3af compliant.

     

    Anyway, the equipment I used was not very recent, the switch is 10 years since date of manufacture.

    This is the output I see when typing ethtool eth0

     

    root@raspberrypi:/home/pi# ethtool eth0
Settings for eth0:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Half 1000baseT/Full 
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Half 1000baseT/Full 
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Link partner advertised link modes:  10baseT/Half 10baseT/Full 
                                             100baseT/Half 100baseT/Full 
        Link partner advertised pause frame use: No
        Link partner advertised auto-negotiation: Yes
        Link partner advertised FEC modes: Not reported
        Speed: 100Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: gs
        Wake-on: d
        SecureOn password: 00:00:00:00:00:00
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: yes
root@raspberrypi:/home/pi#

  • in reply to colporteur
    Ethernet did work with the cable before the modifications.

     

     

    I see.. since it works with the 8-wire connection in that case, and since your 4-wire cable works with the Pi 3, then it's likely either:

    (a) extremely faulty Pi 4 linux image

    (b) an issue with the device it is connected to (hub or switch?)

     

    The cable is directly mapping for the four connected wires, right?

    i.e. 1, 3,5,6 connected to 1,3,5,6 on each end respectively.

     

    I'm wondering if the issue could be a very old switch or hub. If you have the chance, it could be good to connect it directly into the modem/switch (i.e. the device where your DSL/Cable connection comes in, to its Ethernet connection) (in case that equipment is newer or different).

     

    You could also try wiring pins 1,2 to 3,6 respectively  at the other end, and vice-versa. So, pins 1,2,3,6 to map to 3,6,1,2. That's a long shot though. Also, please don't blame me if things go worse, so you may not wish to attempt this. It is all just guesswork from me.

     

    My analysis of the Ethernet circuit doesn't show any active components circuits for the PoE pins

    I think that's not important though currently, since it's curious why 100 Mbps isn't working with the 4 wires. The Pi should be able to establish a 100Mbps connection with just the 4 wires. If that doesn't work, then it will never work with your PoE power injector, because that will only expose 4 connections for the data (it is a power injector intended for 100Mbps and not 1Gbps) according to the PDF on the Newark product page for it. So, it's important to get it running with the 4 wires first, if you intend to use it with the power injector.

  • in reply to shabaz

    I cut the wires attached to pins 4, 5, 7, & 8 and the Pi4B Ethernet did not work. Ethernet did work with the cable before the modifications.

     

    Ethernet worked on the Pi3B with the modified cable.

     

    Worked means I could establish an SSH connection and network indicator lights were flashing.

     

    Following a thread from your suggestion I found a schematic for the Pi4B. https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/rpi_SCH_4b_4p0_reduced.pdf  My analysis of the Ethernet circuit doesn't show any active components circuits for the PoE pins. The TAP pins appear to route to the PoE header. Other than what appears to be a cap and resistor I don't see anything. Stumps me at what the Pi4B is not seeing when the wires are cut.

    image