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We've been using a quad RS485 setup for some time. No problems.

Now we'd like to increase the speed over what we can get away with on RS485 with the micro we're using. Essentially, we've been implementing an SPI port using four RS485 parts.

I've started wondering about using an Ethernet PHY and transformer as a physical layer and sort of "bit-banging" the MII interface to it. Is this a common technique, or am I missing something critical?

Goals: keep the differential benefits, better isolation, increase speed considerably.

Distance would never be greater than 10 meters.

Thanks in advance!

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  • \$\begingroup\$ hm. You'd definitely want to go for the reduced complexity version of MII (RMII), I guess. How much resources does your microcontroller have to spare? I feel that if you need more speed than RS485 can do over 10m, then you really have relatively beefy on hand and could actually do Ethernet, with all the practical benefits of doing so. \$\endgroup\$
    – Marcus Müller
    Commented Feb 8, 2021 at 23:53
  • \$\begingroup\$ All this micro does is sample some inputs and drive them out the RS485. Likewise, it receives samples and drives them locally to outputs. \$\endgroup\$
    – GaryLa
    Commented Feb 8, 2021 at 23:59
  • \$\begingroup\$ yeah, so how much resources does it have left after doing that? WHat rates are we talking here? What kind of MCU? Note that either way, you'll need to be able to supply a 50 MHz data clock from the MCU (RMII) or deal with externally clocked data at 25 MHz (MII)! \$\endgroup\$
    – Marcus Müller
    Commented Feb 8, 2021 at 23:59
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    \$\begingroup\$ It's overkill. I have a SAMD51 running at 120 MHz. Very busy just looping as: read inputs, drive data, read data, drive outputs, repeat. \$\endgroup\$
    – GaryLa
    Commented Feb 9, 2021 at 0:00
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    \$\begingroup\$ You don't bit bang Ethernet on a microcontroller. Well, unless you want to have any cycles left over to do anything other than bit banging. Get a part with an actual Ethernet MAC if you want to use Ethernet. \$\endgroup\$
    – alex.forencich
    Commented Feb 9, 2021 at 1:23

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