Ethernet is great, but it is limited in the sence that you need a switch – so what I would like to do is to build a 3 port switch into every box as a module or permanent. This is actually quite straight forward as you replace the Ethernet Phy with a small Switch using the same space. These exist in many variants but you can get 1 internal RMII interface and two external Ethernet interfaces effectively making up a 3 port switch using the same space as an existing Phy.
The picture below show an early mock-up – just placing main components to get an idea of space usage, and it looks very possible. I am not sure about how much passive components I need on the switvh, but it looks reasonable.
Below is a block diagram of the module.
I can add this to my main board or add it as a sub-module. Sub-module will include an extra MCU and a 8Mbps link to main. 8Mbps is not 100Mbps, but hard reality is that 8Mbps full duplex is a huge traffic load for any MCU. Having an extra MCU dealing with Ethernet also is an advantage as this can then support Web config pages and FW downloads. FW Download can be stored on SPI Flash and transferred to main that either update itself or update a module.
The downside with having this as a module is added latency on messages and added cost, but in reality we should be talking about < 0.5ms extra in latency. Advantages of having a sub-module is that you can ditch Ethernet if your CAN only and need the two extra positions.
The advantage with dual ethernet and a switch is that you connect device to device in a circle and have a redundant system.
Once I am at it I can also draft a single Ethernet module as add-on just using a standard Phy. This is great for sensors like cameras that need Ethernet. It allows a single Ethernet not connected to any other ethernet as well in cases where you want redundancy through two different Ethernets Networks. In theory we can build our own switch like this, but message latency would be an issue – it would work with some extra SW in main, but be slower than a proper HW switch.
Looking at the mock-up for the dual Ethernet I am however concerned about space requirement for a single Ethernet as a module – lets see what we can acieve.
As for inserting dual Ethernet on the new main board the 3D Paint mock-up below shows that it very possible. I move MCU to right and extend the board as needed – keep in mind that I cut the board 4cm on right side – as the switch replace the current PHY I am left with plenty of space to add a trafoc and a connector. It is an option. In this case we can choose to not install Ethernet, install one or two as needed – lets think about it.
One of the key reasons I need RMII and low level MAC access is Profinet RT Class 1. Profinet uses a proprietary Layer 2 for so-called faster Ethernet messages. And Yes the messages are faster and have less overhead, but Profinet/Profibus design is based on a request-answer system. This means you send a cyclic message roud-trip. I will implement ProfiNet for compability with PLS world, but I will not use that as the core communication protocol. Also – I think it is time the world implement an open source standard for communication between devices.