RevPi Flat S - RS485 Serial Console - documented approach not working.

[kieranvrt]

Our team have been trying to setup a serial console interface on the RevPi Flat S. We want the backup connectivity for when we need to do on site debugging and there's not network connection to the unit. Ideally we want all of the boot messages and a console login.

I have followed what I can from the documented approach here:
https://revolutionpi.com/documentation/ ... ices-rs485

After some investigation I found that the documented /dev/tty interfaces did not matched our installation from Kunbus.

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vrtadmin@RevPi67383:/boot $ readlink /dev/ttyRS485-1
ttyAMA4
vrtadmin@RevPi67383:/boot $ readlink /dev/ttyRS485-0
ttyAMA3

I suspect the above was after a hardware or image revision.

After establishing this I have had success using the interfaces for various purposes, including readings devices and passing through to node-red for Modbus reads.

I have been able to get the serial console with login prompt using the serial-getty service directly.

I have read through other articles on the forum looking at RevPi Core and Connect attempts at the same thing viewtopic.php?p=15498&hilit=serial+console#p15498 and experimented with boot command updates such as:

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dwc_otg.lpm_enable=0 console=ttyAMA4,115200n8 root=/dev/mmcblk0p2 rootfstype=ext4 fsck.repair=yes rootwait

Note the use of ttyAMA4 here per earlier mention of ttyRS485-1.

With this change to

/boot/cmdline.txt

I receive boot logs but the system starts locking up and then fails to boot into OS. I get no terminal and cannot ping or SSH into the unit.

Any advice where I might be going wrong or may have misinterpreted a capability? Happy to provide any logs under any configuration.

[dirk]

Hello kieranvrt,
thank you very much for your comprehensive analysis and your patience in this matter.

I have passed things on internally to the documentation and hope that we will soon have a change here with the correct settings.

I have created a test setup with a RevPi Flat S and a standard RS485 USB stick (COM10).
I used the latest RevPi Bookworm 64-bit OS from January 2025 and used the following boot.txt, with the RS485-0 interface:

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dwc_otg.lpm_enable=0 console=ttyAMA3,115200n8 root=/dev/mmcblk0p2 rootfstype=ext4 fsck.repair=yes rootwait nosplash plymouth.ignore-serial-consoles

It worked straight away, and I was able to log in, and the boot process was successful.

At the same time, I was also capable of connecting via SSH.

Please verify that you have installed all the updates. This can be done as follows

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sudo apt update
sudo apt upgrade
sudo apt reboot

It may be far-fetched, but are there other interfering influences from USB devices or things that you can unplug as a test?

[kieranvrt]

Thank you for your test.

My setup was on the Kunbus Bullseye image. I will see if I can test it on the Bookworm image and replicate your results.

I did make sure no other services were configured by our team to use the serial interfaces at the time of testing and no USB devices were connected.

[kieranvrt]

Updating this issue as it took me some time to run a test.

On fresh install of 250430-revpi-bookworm-arm64-default on the RevPi Flat S I was able to run the serial console on RS485-1 as expected.

After first time setup was confirmed, I ran:
sudo apt update
sudo apt upgrade
sudo reboot

I update /boot/firmware/cmdline.txt to the below

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dwc_otg.lpm_enable=0 console=ttyAMA4,115200n8 root=/dev/mmcblk0p2 rootfstype=ext4 fsck.repair=yes rootwait nosplash plymouth.ignore-serial-consoles

confirming ttyAMA4 was RS485-1 on my unit from

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readlink /dev/ttyRS485-1

Noting on 250430-revpi-bookworm-64-default the boot commands are at /boot/firmware/cmdline.txt not /boot/boot.txt or /boot/cmdline.txt.

After the reboot the serial console works as expected. Did not appear to be any additional delay to the boot procedure, able to login with 20 seconds.