Ethernet PHY -- PHY interconnect on same PCB. - Page 1
Forum section: electronics / projects

author: mrpackethead
author_rank: Super Contributor
timestamp: December 29, 2015, 06:58:50 pm
content: Im considering how to connect two Ethernet PHYs between devices on teh same PCB. They will be less than 100mm apart.Options:(1) Connect them together using two transformers, just replacing the "cable" with pcb tracks.(2) Capactively couple them. This requires the use of 14 R&C between the two devices.. the R and C, are cheap parts ( 49R9 resistors and 100nF caps ). Placement of parts is the expensive line item.(3) Use a single transformer, and connect one phy to one side and the other to the other side.. transformers are not so expensive, and id only need to place 3 parts per connection. ( my phys have internal termination and biasing )...I see lots of applicaiton notes for (2), a few people doing (1) but can't find any use-case of (3).. It seems to me it *should* work, but i'm worried i'm missing something.

author: daqq
author_rank: Super Contributor
timestamp: December 29, 2015, 09:08:42 pm
content: QuoteA single transformer shouldn't work - Ethernet to the best of my knowledge (100 MBit) uses a dedicated RX and TX line - as such you need to transfer two signals - so two transformers, or one dual.If placing parts is a problem, you save a bit using resistor arrays - I've seen up to eight resistors per single package. I used a 4x 100 R resistor array for an ethernet PHY my self - combine two 100R parrallel, you get 50R. Also, you should be OK with less parts - 1 coupling cap per line (RX+, RX-, TX+, TX-), two termination resistors and depending on your PHY a bias resistor. A lot of those components will be present even with a transformer.I can't imagine a transformer being cheaper than a few pick and place operations though.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 29, 2015, 09:26:04 pm
content: I really hate it when people here question the premise of whatever the OP is asking or proposing, but I nevertheless have to ask why in the world you would want to use ethernet for intra-board communication? Why not high speed SPI or the like?That said, see TI document SLLA310 for more info on various non-standard methods of ethernet interconnects.

author: CM800
author_rank: Frequent Contributor
timestamp: December 29, 2015, 09:44:00 pm
content: Quote from: MagicSmoker on December 29, 2015, 09:26:04 pmI've had to do something similar to this before (all though the job got canceled before we got to the build stage, where several servo drives were mounted on a PCB not unlike these:http://www.elmomc.com/products/gold-twitter-servo-drive.htm

author: nctnico
author_rank: Super Contributor
timestamp: December 29, 2015, 10:25:03 pm
content: In a similar situation I left the PHYs out and connected RMI(I) master to RMI(I) slave. If the PHYs must be used then you can probably connect the RX and TX pairs with a capacitor. I don't see the need for many extra capacitors. For a short length the impedance matching isn't necessary because the lines are too short to cause reflections. 100Mbit ethernet has a spectrum up to 120MHz.

author: dmg
author_rank: Regular Contributor
timestamp: December 29, 2015, 11:51:50 pm
content: If you can leave out the PHY's and just connect (R)GMII to (R)GMII then you can do it that way. At least in gigabit ethernet there might be some trace delays to take account for while doing this, depending on how flexible your MII's are. You'll need termination near each receiver side.If you have to link the things through ethernet and want to do via capacitive coupling you can do so only if your PHY is ready for that. There are some PHY's out there that are ready for this, and some that can only do magnetic coupling. I've designed boards for backplanes following standards that link the cards through ethernet over PCB and there were several appnotes for this as far as I remember.If you use a phy that's mangetic-only you can do away with a single transformer per pair. Find the simplest magnetics you can, with simple 1:1 center tapped transformers and no internal termination, and terminate each side of the transformer as you would on the chip side, and you should be good to go. In backplane applications you need to add magnetics to each board but in your case it shouldn't be neccesary.

author: mrpackethead
author_rank: Super Contributor
timestamp: December 30, 2015, 01:54:06 am
content: A single transformer shouldn't work - Ethernet to the best of my knowledge (100 MBit) uses a dedicated RX and TX line - as such you need to transfer two signals - so two transformers, or one dual.Sorry for not being clear. Yes, its two transformers, one for the TX and for the RX. You can make it run half duplex, but thats a differnet story.I can't imagine a transformer being cheaper than a few pick and place operations though.$0.39 for the transformer. 0.12 for each PNP

author: mrpackethead
author_rank: Super Contributor
timestamp: December 30, 2015, 01:55:29 am
content: Quote from: MagicSmoker on December 29, 2015, 09:26:04 pmBecuase i have to move ethernet packets between devices. In this case, its two ethernet switch fabrics that are being connected together.

author: mrpackethead
author_rank: Super Contributor
timestamp: December 30, 2015, 02:00:11 am
content: Quote from: nctnico on December 29, 2015, 10:25:03 pmThat would be awesome if i could pull that off.. I just need a switch fabric IC that has 4 RMII' and 1 GRMII, and costs about $3 - $3.50. The switch fabric i'm using has inbuilt PHYs on it 4 x 100Mb ports, and GRMII on the gigabit port. ( which im attaching to an upstream Gig Phy ).Three downstream switches are attached to this, each with a 100M uplink to the 'top' switch.The bandwidth requirements are very small in this applicaiton, the reason for doing this, is all about form-factor and a function that i just can't buy! ( why else would you build a 12 port ethernet switch! )

author: mrpackethead
author_rank: Super Contributor
timestamp: December 30, 2015, 02:03:49 am
content: Quote from: dmg on December 29, 2015, 11:51:50 pmThis is my thinking, that a single transformer per pair should work.. Its my fall back position if the capactive coupling does'nt work. I have some concerns because the Switch Fabric Chips have internal termination and biasing.. And there is no notes as to how that is configured internally...Waiting waiting for Micrel/Microchip to respond.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 30, 2015, 06:03:23 pm
content: Quote from: mrpackethead on December 30, 2015, 02:00:11 amNot sure if it's going to end up cheaper, but you could use a gigabit transformer module for two ports. You'll also be able to use the 4th port on the inner switch by routing it to another connector. (Just leave the spot blank if you don't need it.)If the transceiver uses center tapped transformers like most do, capacitive coupling may or may not work. If it is one of the few that does not require center tap, it will probably work.