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.
