Ethernet Termination Resistors

[edofbrighton]

Hi,

Can anyone tell me why the transmit Ethernet lines have 50 ohm termination resistors fitted at the source? My understanding is that you only need to have a termination at the end of a signal line. There should be a 50 source impedance, a 50 ohm characteristic impedance of the transmission line, and a 50 ohm termination resistance. Is it because Ethernet is a differential pair you need a bi-directional 100 ohm differential termination at both ends (2x 50 ohms).

Thanks in advance

 

[rjenkinsgb]

There are two different approaches to driving an ethernet cable.
See the schematics for the different concepts.

A transmission line needs impedance matched termination at both ends to avoid reflections and distortion.

Current mode using two open collector / open drain devices with current limiting and power fed via the centre tap of the isolating transformer.
That needs termination across the line as one or the other end of the transformer winding is otherwise floating and a mismatch.

Voltage mode, using two push-pull drivers. They always have low impedance so series resistors are used to match the line impedance.

**broken link removed**

 

[edofbrighton]

Ok, so are you saying that the reference schematics I've been looking at are for a current-mode line drivers and the 50 ohm RC termination seen in the left schematic is the equivalent of the series termination.

Many thanks
Ed

 

[rjenkinsgb]

You did not post any schematics or links, so I'm not sure what you have been looking at..

If you men the 2 x 50 Ohm part, that is to help maintain balance and minimise EMI on the cable, whilst matching 100 Ohms.

 

[KeepItSimpleStupid]

The characteristic Z of Ethernet cable is 100 ohms. https://support.bioconnect.com/hc/en-us/articles/115007714348-RS-485-Wiring-Guide

There should be a 50 source impedance, a 50 ohm characteristic impedance of the transmission line

That;s what you have wrong. Twisted pair can have all sorts of characteristic Z's, but Ethernet cable is 100 ohms.

 

[kubeek]

The characteristic Z of Ethernet cable is 100 ohms. https://support.bioconnect.com/hc/en-us/articles/115007714348-RS-485-Wiring-Guide



That;s what you have wrong. Twisted pair can have all sorts of characteristic Z's, but Ethernet cable is 100 ohms.

That is 100 ohms differential characteristic Z. But since you have two dirvers out of phase each needs to see 50ohms, so that you get 100 ohms differential.

 

[kubeek]

Also edofbrighton are you looking at 100Mbit or 1Gbit connection? Gbit is bidirectional so will have different termination than 100Mbit.