Does anyone have any experience with the inter-processor communication over Ethernet MDI interface. Normally, I would connect two devices with the RJ45 connectors(with RJ45 ethernet cable) and then communicate with socket. My two chips are on the same board, can I just communicate through Ethernet MDI?

The architecture looks like:

Processor0(MAC) - Ethernet PHY --- Ethernet PHY - (MAC)Processor1

Does it work. If it does not, could you let me know why.

Any suggestion will be appreciated.

3 Answers 3


Your operating system - I'll take Linux as an example, but it can well be any proprietary system - should be able to detect the physical ethernet devices, instantiate them in the kernel and start their device drivers. Eventually these devices will show up as, say, "eth0" and "eth1". From that point on, the networking stack will just work as usual (e.g. you can assign IP addresses to them). It makes no difference whether the devices are connected over RJ45 cables or by lines on the PCB or backplane.


What you are describing is "phy-less" interconnection. This connects the MII (logic level interface) directly between chips. It's a very common way to connect backplanes, interface cards (eg. an SSM in a Cisco ASA), and other closely coupled systems. Doing so reduces costs, and takes up less space and power. To the OS, there's no functional difference -- ethernet is ethernet.

[ See also: Clarification on various MII's ]

  • Thanks for your help. Some chips have MII interface to connect to external ethernet PHY chip. One of my chips integrates the Ethernet PHY into the chip, so the only interface we can connect is MDI. I think it will work :)
    – Dongguo
    Jan 25, 2018 at 21:07

You don't need a plug and socket but you may well find you need (or at least it is easiest to use) a transformer. Twisted pair ethernet uses a transformer for isolation and most phy's also use that transformer as part of the signal generation/reception. This transformer is often built-into the connectors.

It may be possible to interface two phys together without a transformer, but designing a circuit for doing so requires good knowledge of the electrical behaviour of both phys.

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