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At my work 20 years ago we lacked an RJ45 port. This was solved by plugging in a RJ45-doubler.

It was a passive adapter (so it was neither a switch nor a hub) and looked visually similar to:

The doubler had the effect that A and B both could reach the internet, but they could not reach each other, and IIRC it also limited the speed to 10 Mbps.

I can find many adapters that merge two RJ45 onto a single cable and then splits these at the other end of the cable with a similar adapter (so you would always buy these in pairs). This is not what I am looking for.

I am now in a similar situation: I have a router that includes a small switch that is just one port short, but I can easily find 2 cables that will never need to communicate, and where 10 Mbps would be sufficient even if shared and half-duplex (e.g. my printer, my VOIP-adapter, and possibly even my TV-box. None of these would ever communicate directly).

I can find products like this and this : "This Ethernet splitter allows three computers to share one Ethernet line one at a time, but it doesn't support three computers to connect onto the internet simultaneously."

20 years ago we really had multiple (2) computers using the same adapter simultaneously; the only limitation was that these computers could not connect to each other.

What should I search for to find such an adapter?

(Why not buy an additional switch? I want to try to avoid pollution. I have a 24 port old switch, but it uses 18 watts even when there is no traffic, and I would like to turn that off. A new switch might save on the power, but would cause pollution in production.)

Ole Tange
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5 Answers5

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This is called passive ethernet hub. The schematic is quite simple, but only works for three devices (two computers and one upstream switch):

(EEWeb: Building a Passive Ethernet Hub)

In practice these are quite fiddly even at 10 Mbps, and have been superseded by either powered switches or those pair splitters that you put at both ends and that work up to 100 Mbps.

jpa
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I have a 24 port old switch, but it uses 18 watts even when there is no traffic

Newer devices tend to consume less power than older ones.

I measured consumption of my D-Link 8-port Gigabit switch DGS-108. It consumes about 1W with a few devices connected.

With disconnected devices it consumes slightly less.

I also measured similar 5-port version DGS-105, and it consumed only slightly less than DGS-108.

One more thing to think about is opportunity cost (alternative cost). I like the idea to see our time as life currency. If you want you can spend hours choosing most efficient switch or one with least CO2 impact. Or you can spend hours planting trees. Or you can spends hours working more and then donating extra earned money to foundation which for example fights with climate change.

But you can't do all things, because a day has only 24h. Which means that by doing something you won't do something else in the same time.

I don't know which way is best, but I think after some experience you will notice that some ways to help planet may be more effective than others. And you may or may not come to conclusion, that it may be better to choose any switch now and spend this time doing something with more impact.

Karol Zlot
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I have had an encounter with such a setup--although hardwired--some years ago. (Don't let electricians near a network wire!!) Despite low loads from the two computers involved it caused noticeable network lag.

Loren Pechtel
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I have my doubts that the RJ45 doubler you remember was only at one end. I don't see how it could do all these things with no power supply and nothing at the other end. I doubt, but I am not going to say it's impossible, I simply have never seen such a thing for Ethernet, and knowing what I know about Ethernet I can't imagine how it's possible. Anyways, you could go the hard-switch route. That makes it literal knob you must switch to share the one port. Good luck.

Paco Hope
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These can work by exploiting the fact that the cable has eight wires and only four are needed. So within the cable it splits onto the unused pairs. But they are double ended.

You say you are 100% sure that it was single ended. But in an answer you give your reason for being sure was that you replaced it with a normal ethernet cable. That is not proof it was single ended, because that would have used the default four wires, which are one of the two sets of four connected at the server end.

And of course that applies pre Cat 6 cable. Cat 6 uses all 8 wires anyway.

vijila
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