2022-02-18                         from the editor of ~insom
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   My current technical obsession has been Ethernet:
   specifically 10BASE-T1S, which is one of the "single pair
   Ethernet" standards.

   This lets you run 10Mbit/s up to 15m in either a
   point-to-point config (with power on the same two wires!) or
   in a multi-drop mode (with no power) with ~8 devices.

   I don't have a real world need for this or anything, it just
   seemed like a chance to play with something slightly unusual
   (as industrial gear usually is) and maybe there would be a
   stretch goal to fit in either some kernel or microcontroller
   development along the way. 10Mbit/s is slow enough that I
   stand a chance of debugging by oscilloscope or even with an
   affordable signal analyser.

   An Ethernet connection from a computer is made up of two
   parts: a MAC and a PHY. The MAC is basically the CPU-facing
   part and the PHY is where any analog interfacing with the
   real-world happens. 10BASE-T1S is specified so that you can
   use a standard MAC in combination with a new fancy PHY such
   as the Microchip LAN8670.

   Connections between MAC and PHY use MII or RMII. RMII uses
   fewer pins but is specified for much faster speeds -- so I'd
   rather use MII which runs at 2.5MHz and transmits/receives a
   nibble at a time.

   (Fun fact: 100BASE-TX uses 4b5b encoding which means each
   four bits of data from the computer results in 5 bits of
   data on the network, so your 100Mbit/s link is running at
   125Mbaud)

   I don't know where I'm going to go with this, maybe I won't
   even build anything out -- but it turns out that Ben Eater
   has a series of videos explaining Ethernet at a very low
   level (all the way down to signals on the pair and
   Manchester encoding) so that was a treat (as his video
   series always are).

   It looks like development boards which expose a MAC are
   _very_ expensive, which might make this whole thing moot --
   given that I don't have a real use case, I don't want to
   spend serious money. I just thought that connecting a couple
   of embedded Linux devices together with a new-ish networking
   tech would be cool, and that there might be a small amount
   of PCB design or breadboarding involved.