I was hooking up some 10/100BaseT Ethernet cables at a customer site and I noticed that the Voice Over IP phone on a desk had an Ethernet input and an additional RJ45 that serves as a pass-through connection. This was handy when there was just 1 CAT5 cable coming into the cubicle. The incoming cable goes to the phone, and then the pass through jack was connected to their PC via another CAT5 cable.

Do you know what circuit is behind these 2 RJ45 jacks?

I would love to have taken a screwdriver to the phone, but that was beyond the scope of my work plan :)

Do you think it is a transformer? Just a couple RJ45s in parallel (yuk!)? Certainly there are not 2 MAC/PHYS, that seems cost prohibitive for a cheap VoIP phone.

How would one design such a pass though to allow several devices to share a single incoming CAT5 cable?

I've seen this question: Ethernet Without a Switch?

that has a link to this circuit: http://www.zen22142.zen.co.uk/Circuits/Interface/pethhub.htm

Perhaps this is what is inside the VoIP phone?

Cheers, David

  • 3
    \$\begingroup\$ You'd be surprised how cheap a couple of MACs and PHYs can be... \$\endgroup\$ Oct 18, 2014 at 15:22
  • 2
    \$\begingroup\$ It will have, at the very least, an ethernet hub, or better (in the more expensive ones), an ethernet switch. \$\endgroup\$
    – Majenko
    Oct 18, 2014 at 15:24
  • 1
    \$\begingroup\$ 99% there's a single-chip Ethernet switch inside. It's only a few dollars in large quantities... digikey.com/product-detail/en/KSZ8864CNXCA/576-4512-ND/4794838 \$\endgroup\$ Oct 18, 2014 at 16:57
  • \$\begingroup\$ Do note that when the phone reboots the attached computer will loose network connectivity too. Always a fun gotcha. \$\endgroup\$
    – Criggie
    Jul 28, 2017 at 11:01

1 Answer 1


Do you know what circuit is behind these 2 RJ45 jacks?

You can buy single chip ethernet switches with two twisted pair ports (i.e.. integrated phys) and a MII port designed for exactly this application. e.g.


They are pretty cheap, especially when you consider that they are used instead of a phy chip. I would bet that such a switch chip is what is in the phone.

that has a link to this circuit: http://www.zen22142.zen.co.uk/Circuits/Interface/pethhub.htm"

I highly doubt any reputable phone vendor would put a hack like that in their product. It's a clever trick but it is a major specification violation, is likely to cause major compatibility issues and only works for 10BASE-T.


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