treczoks
Posts: 36
Joined: Tue Jan 03, 2012 9:55 am

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Tue Apr 03, 2012 7:28 am

Well, the RPi is using 100Mbit Ethernet, which in turn only utilizes pins 1, 2, 3 and 6 of the TP ethernet cable.

Power-over-Ethernet (PoE) uses pins 4, 5, 7 and 8. While putting PoE on a standard RPi is to expensive in parts and PCB space, just routing the pins to four additonal pins in the expansion port (or even placing a seperate, four-hole port right next to the ethernet jack) might be feasible, so one could add a PoE solution on a daughterboard.

BTW: Which polyswitch does the RPi use, and how would it cope with power coming from the "inside", i.e. from the expansion port?

Yours, Christian

User avatar
nick.mccloud
Posts: 804
Joined: Sat Feb 04, 2012 4:18 pm

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Tue Apr 03, 2012 8:20 am

The general consensus is that there won't be changes to the PCB for months to come, indeed unlikely even in 2013!

You could have an interim adapter that plugs in to the ethernet jack that picks out the PoE for you.

treczoks
Posts: 36
Joined: Tue Jan 03, 2012 9:55 am

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Tue Apr 03, 2012 9:45 am

nmcc said:


The general consensus is that there won't be changes to the PCB for months to come, indeed unlikely even in 2013!


I would not dare to speculate about devices that popular to go unchanged in such a timeframe. It might be a goal, but that can change e.g. if there is a  simple way to make things cheaper. But thats all speculation.

nmcc also said:


You could have an interim adapter that plugs in to the ethernet jack that picks out the PoE for you.


No, thats not the way to do it. I will probably just solder the pins from the jack to matching pins on the daughter board.

mobeyduck
Posts: 173
Joined: Tue Nov 29, 2011 6:39 pm

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Tue Apr 03, 2012 1:09 pm

Its easyer to do that from the cable and only wire 4 to the rj45 and 4 to mabey an rj11 easy to use correct size feasable to put on a expension board, or straigt to the board but that isnt really flexible.

I would take a rj45 cable extension like to put inbetween the cable and the pi+board so you can still use the same cable for future products and it makes the power solution more portable

Neil
Posts: 98
Joined: Thu Sep 29, 2011 7:10 am
Contact: Website

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Wed Apr 04, 2012 7:37 pm

treczoks said:

Power-over-Ethernet (PoE) uses pins 4, 5, 7 and 8. While putting PoE on a standard RPi is to expensive in parts and PCB space, just routing the pins to four additonal pins in the expansion port (or even placing a seperate, four-hole port right next to the ethernet jack) might be feasible, so one could add a PoE solution on a daughterboard.
Not quite.  The PoE standard defines two ways to run PoE over Ethernet cables.  One way is as you say, on pins 4, 5, 7 and 8.  The other way is as a DC bias applied to the Tx and Rx pairs.  This is one reason you need the isolation transformers on the Ethernet port (somehow I doubt the PHY would like -48V applied to its pins!).

Your approach would work when connected to some PoE hubs, but not all hubs.

Cheers,

Neil

treczoks
Posts: 36
Joined: Tue Jan 03, 2012 9:55 am

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Thu Apr 05, 2012 12:48 pm

~Neil~ said:


treczoks said:


Power-over-Ethernet (PoE) uses pins 4, 5, 7 and 8. [...]


Not quite.  The PoE standard defines two ways to run PoE over Ethernet cables.


Yes, I am well aware that using 4/5/7/8 is only one way. Actually, I was not aware that there were ONLY two standards here (Last time I dealt with PoE, each manufacturer had its very own ideas about it)


One way is as you say, on pins 4, 5, 7 and 8.  The other way is as a DC bias applied to the Tx and Rx pairs.  This is one reason you need the isolation transformers on the Ethernet port (somehow I doubt the PHY would like -48V applied to its pins!).


Yeah, I know about the DC bias method. But I won't go that way, as it is to expensive for my use case, and not necessary, either.


Your approach would work when connected to some PoE hubs, but not all hubs.


Who said something about "PoE Hubs"? I'm just going to use a cable adapter to feed the 48V into the unused pairs. I know: crude. But it works. The Power is sufficient good & clean not to disturb the ethernet pairs, and power fluctuations can be dealt with capacitors and friends.

All in all it is about getting power and connectivity to a RPi with as little cabling effort as possible.

Neil
Posts: 98
Joined: Thu Sep 29, 2011 7:10 am
Contact: Website

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Thu Apr 05, 2012 4:04 pm

Well OK so that might work in *your* *specific* application.  But the RaspPi is designed, made and sold to a much wider audience, and your proposed change would potentially expose a dangerous voltage to users who plug their RasPis into PoE hubs that supply power on the unused pairs.

(I say potentially as I know there is the analogue protocol to sniff what's at the load end before applying full power.  But that relies on the hub being compliant as well.)

Neil

redman684
Posts: 48
Joined: Fri Dec 09, 2011 5:25 pm

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Fri Apr 06, 2012 12:06 pm

The same could apply to the USB connector. USB on a pin header would be nice in some situations. And the USB power connector would definitely be something I would like to see replaced by a pin header. And of course the LED's, so you can properly mount them in your case. And and and. I guess there are as many desired variations as there are projects.

I can imagine one of the first next versions of the Pi having no connectors at all, only pin headers, or even just soldering holes. All these connectors are of little use when you want to combine the Pi with other hardware into a single device. Just like the arduino universe has the Boarduino  and Arduino nano which are designed for use on a breadboard.

arm2
Posts: 253
Joined: Thu Dec 15, 2011 3:46 pm

Re: Routing Ethernet pins 4,5,7,8 to the expansion port?

Sun Apr 08, 2012 9:01 pm

<Advert mode ON>

For 'Spare Wires POE' you could adapt: http://www.cjemicros.co.uk/mic.....RPI-POE-IS

Full kit: http://www.cjemicros.co.uk/mic.....POE-IS-DIY

Or ready made: http://www.cjemicros.co.uk/mic.....I-POE-IS-C

Or 'Proper POE' http://www.cjemicros.co.uk/mic.....+ADAPT-I-S though that also needs a DC 2.1mm inline socket to microUSB adaptor.

Return to “General discussion”