Feasibility of short range ethernet with single ended connection - Page 1
Forum section: electronics / projects

author: qwaarjet
author_rank: Regular Contributor
timestamp: June 29, 2018, 03:22:28 am
content: Ok this might be a stupid idea. I want to connect a series of nodes together via ethernet. Each node would have a 3port ethernet switch like the kind found in many IP phones. The nodes connect together closely so the actual signal length is only a dozen mm or so. However I'm limited in the number of contacts I have available so instead of using 4 contacts for a normal ethernet PHY I only can budget 2+ a power ground per port. is there a good/ known technique to do this? primary reason for this odd solution is to leverage existing software that is design to work with ethernet devices.Tl;DR how to use standard ethernet stack with only 2 hardware contacts.

author: ignator
author_rank: Regular Contributor
timestamp: June 29, 2018, 04:11:07 am
content: The ethernet PHY requires a differential input. This could be as simple as creating this via a transistor circuit. Probably will work for 10BaseT, but at 100BaseT, the transmitter is sending out FIR filter pulses for the receiver to adjust the phase filter. I'm at the edge of just knowing this is done, but not how it works exactly.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 29, 2018, 04:27:15 am
content: Ethernet is transformer coupled. Normally the center tap is bypassed to ground, but you might get away with instead connecting the '-' output to ground and transmitting the + signal as a single-ended signal. That will force the return current through the ground connection which isn't great for signal integrity, but for Fast Ethernet over a few cm it will probably work fine. Is this a good idea? No. But it might work.

author: a59d1
author_rank: Regular Contributor
timestamp: June 29, 2018, 04:44:40 am
content: AFAIK there exists a two-contact Ethernet standard:https://standards.ieee.org/findstds/standard/802.3bp-2016.html

author: Berni
author_rank: Super Contributor
timestamp: June 29, 2018, 05:14:20 am
content: Yep there is a half duplex form of ethernet that requires a single twisted pair. But as far as i know its not supported by simply any network card you find.Another option is one of those ethernet over coax or ethernet over power line devices.

author: Marco
author_rank: Super Contributor
timestamp: June 29, 2018, 06:54:05 am
content: Quote from: ejeffrey on June 29, 2018, 04:27:15 amI'd try capacitively coupling the receiver to ground and +.All the manufacturers have slightly different recommendations for how to transformerless couple (resistors to ground or not and what size resistors) so follow their recommendation apart from coupling the - input to ground.

author: Daixiwen
author_rank: Frequent Contributor
timestamp: June 29, 2018, 07:16:06 am
content: Most of the phy chip datasheets I've been through mention that they support half duplex 100-Base T mode. They always lack the information about how to do this in practise though. I don't know if it is as simple as connecting the two pairs together but it may be worth a try.Which phys are you using?

author: Berni
author_rank: Super Contributor
timestamp: June 29, 2018, 07:28:22 am
content: You have to be careful about it because half duplex BASE100T can also mean that its using both pairs but never at the same time. This has to do with the way hubs work where they are not capable of taking in data when someone is already sending it out.

author: forrestc
author_rank: Unknown
timestamp: June 29, 2018, 08:48:35 am
content: Quote from: qwaarjet on June 29, 2018, 03:22:28 amCan you clarify how many pins you actually have? Is it 2 for ethernet + 1 for power + 1 for ground?If that's the case, you might be actually be able to use all 4 for ethernet and then use a standard poe pinning for the power. Ethernet uses magnetics, and Power over Ethernet (in one wiring arrangement) uses the center tap of the two pairs to provide power. In this way you use the TX pair for both TX data and say positive power, and the RX pair for both RX data and negative power.