Using RS485 as a Serial Console

You can use the RS485 interfaces as a serial console if you are missing an HDMI connection for a screen, for example.

Required hardware

  • PC
  • Power supply unit for the power supply
  • Ethernet cable (optional)
  • USB/RS485 adapter (as available in our shop for example)

Preparation

  • Connect your router and your RevPi Flat with the Ethernet cable or
  • Set up a WLAN connection.
  • Connect your RevPi Flat to the power supply.
  • Wire the USB/RS485 adapter to the RS485 connector on the RevPi Flat. This procedure is explained in the section “Connecting Serial Devices”.
  • Optional: in “/boot/cmdline.txt” image is set to “console=tty1” by default. This allows an HDMI screen to be used as a console. We recommend that you delete this parameter if you are entering a serial console.

With the following steps you are already done:

  • Add the following to the file “/boot/cmdline.txt”
    for RS485-0:
    console=ttyAMA0,115200n8
    for RS485-1:
    console=ttyS0,115200n8
    Instead of “115200” you can also enter another baud rate of your choice.
  • Restart the RevPi Flat.
  • Connect the RevPi Flat to a PC, for example, using the adapter.
  • You can now use the serial console.

Double feature

You want to use both RS485 interfaces as a console? No problem! Note, however: you will see the boot messages of the kernel and get a login prompt on both interfaces, but the boot messages from “systemd” will only appear on the last console specified in “/boot/cmdline.txt”. This is because only this console is accessible as a so-called preferred console via “/dev/console”, where “systemd” writes its messages. It thus depends on the order of the consoles on the kernel command line.

Early riser

The boot messages of the kernel do not appear until almost 4 seconds after the boot process has started. Only then are the drivers for the RS485 interfaces fully initialized. If you want to see messages as early as possible, you can switch on an early console on RS485-0.

  • To do so, add the following to “/boot/cmdline.txt”:
    earlycon=pl011,mmio32,0x3f201000,115200n8

No early console is currently possible on RS485-1.