Electronic – Transit between RS485 and CAN

More than likely it switches into transmit mode when TXD becomes active and a diode and capacitor "latch" that first transmit edge and hold that voltage "active" for longer than at least one byte.

I see a little surface mount diode on the PCB (D1) and quite possibly this is what I'm referring to. I have seen this trick done before and the only disadvantage is that when the TX session ends there might be a few milliseconds to wait before the RX direction is restored (due to the capacitor discharging).

I found this on the web as a partial circuit: -

When receiving, TX0 is active high thus forcing a low on DE (as expected). When TX0 goes low it rapidly discharges C0 and sets up the direction port for transmitting. When transmitting and TX data changes state, C0 remains largely discharged although ultimately R3 will slowly (relatively speaking) re-charge it but not until some time after the last transmit byte has been sent. Values shown i.e. 22k and 1000pF are dependent on baud rate.

The RS422 and RS485 standards only specify the electrical requirements. Both can be full-duplex but isn't a requirement of either specification. They are also similar enough that you can interoperate them in some situations.

Now the big difference, which you already noted is that RS485 uses a tri-state system. This means you can have multiple transmitters sharing the single two wire bus, switching individual nodes as needed. In this configuration full-duplex probably isn't necessary or desirable.

In comparison RS422 can only have one driver per wire pair, but the specifications allows for 10 receivers.

If you're only using them for point-to-point communication, there really isn't much difference.