PoE and SoE wiring for Soekris SBCs used in wireless networking

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This page focuses mostly on the care and feeding of Soekris single board computers (SBC's), in particular the net4521, the hardware of which is optimised for wireless routing tasks.

Warning

Disclaimer

If you break anything as a result of reading this page, you get to keep the pieces. I accept no responsibility. If in doubt about anything, ask me or somebody qualified. Following these instructions incorrectly can damage your hardware, resulting in electric shock or fire hazard.

Right; that's my ass covered - herewith instructions:

PoE (Power over Ethernet) for Soekris SBC

The net4521 can accept power over ethernet (PoE) via one of its ethernet ports, eth0, and ONLY this port. This means that a single cat5 cable run can be used both to power the net4521, and communicate with it, making installations on e.g. radio masts very convenient.

Converting an existing power supply to a PoE supply (PoE injector)

This procedure involves the use of the following tools:

If you are not sure you can use the above tools correctly and safely, enlist the help of someone who can. Make sure you have read and understood the following instructions fully before beginning:
  1. Choose a power supply which outputs between 12 and 48 volts DC (the 4521 can take up to 56V, but 48V allows a little margin for error and overvoltages), at 10W or greater (for practical purposes, watts = volts times amps). Note that if you're using a linear power supply (simple transformer, rectifier, and smoothing) the wattage, or VA rating, can be as high as you like; if you're using a switch-mode power supply that can output a lot more power than you need, check that the output voltage stays within spec at low output currents, and add a load resistor in parallel with the Soekris if required. In all cases I strongly recommend measuring the following with the SBC active, with its full complement of hardware for your application: PoE voltage at supply end of cable; PoE voltage at SBC end of cable; PoE current drawn. The PoE input on the net4521 SBC is a switch mode power supply. Switch mode power supplies draw less current at higher supply voltages. Voltage drop on long cables is proportional to the current drawn, and cable length. Hence, with a higher supply voltage, there will be less voltage drop, what voltage drop there is will matter less, meaning that you can run a longer cable and the SBC will be more likely to run reliably without brownouts (main symptoms of this being general "flakiness" or random reboots for no apparent reason, especially after you've added more hardware). I have had success with 15V 1A, 19V 2A, and 24V 1A supplies.
  2. Open an RJ45 inline coupler like this one (CPC P/N CN0473366) or this one (CPC P/N TE0327366). Identify the tracks joining pins 4, 5, 7 and 8 of the two rj45 connectors to each other. If in doubt as to which pins are which, plug a patch cable into the coupler; pins 4 and 5 (the two middle pins of the 8) should correspond to the blue pair, and pins 7 and 8 (to one side) should correspond to the brown pair. Be aware that gigE crossover cables swap not only the orange and green pairs, but also the blue and brown pairs. If in doubt, use a straight-through patch cable to check pin numbers as described above.
  3. Carefully cut these four tracks with a track cutter, stanley knife or other cutting tool. Use a multimeter to check that continuity between the appropriate pins on the two connectors of the coupler has been broken (so the ethernet pass-through doesn't inadvertently get a PoE supply) and that there still is continuity on the other tracks joining pins 1, 2, 3 and 6 of the two connectors (so the ethernet pass-through works). Make a hole in the base or side of the shell in a suitable place, and feed the power supply output wires through it (if there is a connector on the output lead of the power supply, cut it off). You'll probably want to trim the lead quite short for neatness; this is up to you.
  4. Choose one of the two connectors on the coupler to be the PoE connector, and identify the positive and negative supply leads (if you power up to do this, power down again before proceeding).
  5. Strip and tin the supply leads, and on your chosen connector, carefully solder the positive wire to pins 4 and 5, and the negative wire to pins 7 and 8. This is quite easy because you're soldering to two adjacent pins in each case. Make sure that you don't short these connections to any other pins. I also insert a polyfuse inline (750mA or so for a 15V supply, less for higher voltages) to protect the supply in the event of a short on the cable; if you can get one, I suggest you do this too.
  6. With a multimeter, check that pins 1, 2, 3 and 6 still connect straight through the coupler; check that 1, 2, 3 and 6 aren't connected to each other, or to 4, 5, 7 or 8 on either connector. In particular, check that 4, 5, 7 and 8 are *not* connected from one connector to the other.
  7. Plug a patch cable into the PoE connector. Check that pins 4 and 5 on the free end of the cable shorted; likewise pins 7 and 8.
  8. Power up, and check for voltage between pins 4 and 7, 5 and 7, 4 and 8, and 5 and 8 (positive probe to 4 and 5 (blue pair), negative probe to 7 and 8 (brown pair)) on the free end of the cable. If the multimeter registers a negative voltage, the leads are swapped. If it registers no voltage, something else is wrong (possibly a short or open): power down, and check your connections carefully.
  9. If it registers a positive voltage near the nominal output of the power supply - congratulations! You appear to have a working PoE supply. Reassemble the RJ45 coupler, making sure to mark which end is the PoE connector, possibly with a marker or a piece of red insulating tape. Glue it to the power supply. The connector at the other end will serve as an ethernet pass-through port. Do not mix them up!

Using the PoE supply

  1. Make sure you have read and understood the warning and disclaimer at the top of this page in their entirety.
  2. Use a patch cable to connect eth0 on the SBC to the PoE connector on the PoE supply.
  3. Power up. The power LED on the Soekris should light up immediately and stay on; the error LED should light up for a few seconds. If this doesn't happen, the polarity may be wrong (positive and negative swapped) or you may have plugged into the wrong connector on the PoE supply: power down, and check your connections carefully.
  4. Do one of the following: The link light on eth0 on the SBC (and on the other device, if present) should light up. If you have a console connected to the SBC (with correct baud rate, handshaking options etc) you should see the memory count and suchlike taking place. If the SBC has something to boot from which launches a suitable daemon, you should be able to connect to that when booting is finished. If you haven't got anything running on the SBC yet, you may find Mike Machado's site, Running Debian on Soekris Engineering Embedded Systems, helpful. I certainly did!
  5. If the PoE lead runs outdoors, make sure the cable, PoE supply and SBC are situated in such a way that rain cannot run down the cable and into the supply or SBC.

SoE (Serial over Ethernet) for Soekris SBC

With a second run of cat 5 cable and two adaptors you can connect to both the second ethernet port and the serial port on the SBC (i.e. with two cables you can connect power, two 10/100mbit ethernet ports, and one rs232 serial port). This means that as well as communicating over ethernet and performing some routing between the ethernet ports, you can use the serial port for console access (needed to initiate PXE net booting, for example, unless of course you configure grub to PXE-boot automagically... *ponders*)

More on this later. In short, you make up adaptors to feed pins 2, 3 and 5 of the serial port down the 'spare' pins of the cat5 cable. I connect 2 to blue (cat5 pin 4), 3 to brown (cat5 pin 8) and 5 to white-blue and white-brown (pins 5 and 7). This is easy to remember because 22=4 and 23=8 :)

Links to wireless resources

Wireless LAN groups:

Related resources:

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