Hello,

I would like to use an 10/100/1000Mbps Ethernet transceiver for 
switching between 1000Mbps Ethernet and 100Mbps Ethernet.

I am planning a redesign of an i.MX6 board which offers various 
interfaces (I2C, SPI, USB, 100Mbps Ethernet, ...) over a 40-pin 
connector to another board which has the magnetics and the connector for 
Ethernet assembled.

During the redesign process Gigabit Ethernet shall be added (because the 
processor can deliver it). However, I don't have enough lines left on 
the connector to add this.

But on the new version the same 40-pin connector will get assembled a 
second time (as an assembly variant) for optional additional interfaces. 
This way the new version stays compatible to older boards. My idea was 
that I will route the 4 differential pairs for Gigabit-Ethernet to the 
new connector and the 2 already used pairs also to the old existing 
connector. So these 2 pairs will get routed to both connectors. My idea 
was to use 0 Ohm resistors to switch between the variants.

On the boards with the connector I would also use 0 Ohm resistors to 
select the right mode and then route the differential pairs to the 
magnetics and the RJ45 jack (maybe I will choose a RJ45 jack with 
integrated magnetics).

This way I could stay compatible to already existing boards by using the 
100Mbps.
And if 1000Mbps are wanted, I could assemble the new connector and 
switch the 0 Ohm resistors to get Gigabit-Ethernet.

Do you understand my question or would a small sketch be helpful?
Do you think it could work this way? Do you think the 0 Ohm resistors 
are a good idea or that this idea could work?

I am thankful for all input!
Cheers,
Egon