Splitting a differential signal?

[FreshmanNewbie]

Can we split a differential signal into 2?

Assume the below image:

I'm using a 1000BASE-T Ethernet. In that case, can I split the ethernet differential signal pairs into 2?

Only one set of 4 tx and Rx pair will be coming out of the PHY right? In that case, can we take one more set of signal pairs from the original differential signal coming out from the PHY, and then provide it to each RJ45 connector individually?

Will it work?

At any point in time, only one of the RJ45 connector will be connected and the other will be inactive. So, in that case, can I split the differential signals into 2?

 

[barry]

How are you physically going to do this? Two connectors side by side? From your drawing, it looks like you're going to have a cable that splits in the middle. DON'T DO THAT!

What you should REALLY do is use an Ethernet switch. The problem with your approach is that you are going to have a stub, which can be bad or really bad, depending on how you do it. Stubs cause reflections and signal degradation. You might be able to do this if you've got two connectors close together (they actually do sell splitters that do that), but it's not perfect.

 

[KlausST]

Hi,

There are several "ethernet standards" ... which exactly are you talking about?

Theoretically you could "split" a differential signal.
But not ethernet. Ethernet is a peer to peer interface. And there is'nt a signle pair but several pairs with signal flow different directions.
... and maybe power supply (POE) that needs to be communicated between both (2) partners

In case you did not read how ethernet works .. then please do this.


Klaus

 

[FreshmanNewbie]

Hi,

Thank you for the answer. Yes. I am using 1000BASE-T with RGMII interface on the MAC side.

Could you please tell me when can we not split the differential signal and when we can ?
Like, what could go wrong when we split differential signal and what should we take care for it to work?

There are several "ethernet standards" ... which exactly are you talking about?

Theoretically you could "split" a differential signal.
But not ethernet. Ethernet is a peer to peer interface. And there is'nt a signle pair but several pairs with signal flow different directions.
... and maybe power supply (POE) that needs to be communicated between both (2) partners

In case you did not read how ethernet works .. then please do this.


Klaus

--- Updated Apr 30, 2022 ---

Thank you for the answer.

There are 2 boards having the 2 separate connectors.

How close should we keep the connectors together for it to work? Can you please tell me the threshold value and how will it work only till the threshold and what factors would be affected once the threshold is crossed?

How are you physically going to do this? Two connectors side by side? From your drawing, it looks like you're going to have a cable that splits in the middle. DON'T DO THAT!

What you should REALLY do is use an Ethernet switch. The problem with your approach is that you are going to have a stub, which can be bad or really bad, depending on how you do it. Stubs cause reflections and signal degradation. You might be able to do this if you've got two connectors close together (they actually do sell splitters that do that), but it's not perfect.

 

[barry]

I don’t understand your statement about two separate boards. I thought you were splitting the signal from one source. A drawing would help.

There is no “threshold value”. It’s a signal integrity issue. You need to do an analysis of your system to determine how much your signal will degrade. It depends on your cable, pc layout, etc. As I said there are passive splitters that supposedly work, and those have the connectors right next to each other.

 

[KlausST]

Hi,

Could you please tell me when can we not split the differential signal and when we can ?

For a discussion it's urgent that you have at least an idea about 1000BASE-T
* signal pairs function
* signal pair direction
* protocol
Thus I strongly recommend to read ... just the basics.
I guess this already answers your questions.

In short: it simply is not designed (neither hardware, nor protocol) to have 3 communication partners on one line.

Klaus

 

[barry]

I THINK what OP is trying to say is they have one box that they want to connect two cables to, and they will only connect the opposite end of one of the two cables at a time. I dont think this will work having a second unterminated cable just hanging there, specially for 1000base t.