Ethernet connector resistors to chassis - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: April 17, 2021, 12:16:48 pm
content: Hi,with reference to the attached schematic of a Ethernet connector with integrated magnetics, are the 75R resistors and 1,000pF capacitor needed if instead of an integrated magnetics connector you use a generic pin header?I noticed that the capacitor is connected to chassis ground (the metal enclosure of the connector itself). So I am wondering if they are used for noise/ESD or if they are part of the Ethernet Phy specs?Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 17, 2021, 06:30:01 pm
content: How would you use a pin header? Y'mean put the LEDs and transformers on the board, discrete?Yes, the resistors and cap are required. The resistors are to terminate the between-pairs modes, and the cap is to shunt ESD. Without the cap, the transformers would need to be rated 8kV or more, or need a spark gap or something. With, they only need 1.5kV. (This is easy to see comparing the cap value to the HBM ESD equivalent circuit.)Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: April 17, 2021, 07:26:33 pm
content: Thank you TimYes, just put the LEDs and the transformer on the board.

author: phil from seattle
author_rank: Super Contributor
timestamp: April 17, 2021, 07:36:02 pm
content: I'm a little unsure as to what you are trying to do. "generic pin headers" seem like a bad way to go unless you have a specific requirement. The standard RJ45 interface is commonplace and easy to use. Cables are reliable, cheap and plentiful off the shelf. If you want to use just a plain RJ45 connector (or a pin header, for that matter) you will need the proper transformers (AKA magnetics). I would look for the datasheet of the Phy chip you will be using. It should have a lot of information for using separate magnetics.To be honest, I would just use a MagJack. Often cheaper than external magnetics and far simpler to use.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 17, 2021, 07:54:03 pm
content: It can happen. I did a project last year where RJ45s would've been too bulky, and unsuitable for the environment (lots of little industrial modules in a chain). So we used small headers (housed with snap retention).And remember, signal quality is basically irrelevant, I mean look at RJ45 to begin with, they aren't even using the right pins!Quote from: phil from seattle on April 17, 2021, 07:36:02 pmYes, important to note there are different magnetics for 10/100 and 1G. AFAIK, the latter is backwards-compatible, but the trick is it requires an H-bridge driver, not push-pull. Traditional 10/100 output stage is just a pair of common source transistors, push-pull around a CT primary -- the CT is mandatory. Termination resistors are also placed there. The H-bridge is usually designed with internal termination, so all you really need is just the isolation. I've seen a few 10/100 PHYs of this design, used just for power savings I think.Usually they'll still use a CT transformer, but just recommend the CT be bypassed with a cap, no VDDA or anything.So yeah, check the datasheet, make sure it's happy, use the appropriate kind and all that.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 17, 2021, 08:12:00 pm
content: Some really cheap devices omit the capacitor and resistors and it seems to work fine but I'm sure it's far from optimal. I would suggest adding a 100k-1M or so resistor across the capacitor to prevent charge build up.

author: ricko_uk
author_rank: Super Contributor
timestamp: April 17, 2021, 09:12:08 pm
content: Thank you all,yes I normally use RJ45 but there is no space vertically in the enclosure. So need to use a external magnetics and a simple pin header. The distance tot eh Ethernet PDC is only few cm and running only at the lowest speed.

author: ricko_uk
author_rank: Super Contributor
timestamp: April 18, 2021, 11:04:12 am
content: BTW, in a couple of forums they refer to the "line" side. Which one is it? Is that the "microcontroller/IC" side or the Ethernet cable side?And what is the other side called?Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2021, 09:01:06 pm
content: Technically, "media" side is the cable. PHY side is the PHY, and (R)MII is the MCU side (then the MAC and so on, some of which may be integrated hardware, some of which may be software -- like the Wiznet chips that integrate an entire TCP/IP stack!).It's all surprisingly formal, or perhaps unsurprisingly so as "Ethernet" has gone through so many forms over so many decades.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: April 18, 2021, 10:33:46 pm
content: Thank you Tim