We did the firmware upgrade of a RB600 from version 3.30 to 4.11. We went back to 3.30 and upgraded again to 4.11, but now we can no longer access to the RB. The RB starts and hangs at the login screen. After a few seconds it reboot.

I thought about upgrading the RB through Netinstall, however, it was in a tower and received electrical surge on LAN ports, burning two of them (ether2 and PoE). But it worked fine before the upgrade even with only one LAN.

What can I do to save this RB?
Is there way to send RouterOS through the serial port?
And if we use a CF with linux to copy the package for RouterOS Nand RB?

Thank you.

It is not possible to install RouterOs via serial port.

But then is there any other possibility?

It is not possible to install RouterOs via serial port.

Not?
What am I doing when I connect with nullmodem and send ROS package with Xmodem to router and after that reboot and router starts up with new ROS?
I had to do this a couple of times since the ether ports are dead thus upgrade over ether net was not possible.

(I hoped the eth ports came back after a new install. It did in one instance, two others not....)

It is not possible to install RouterOs via serial port.

Not?
What am I doing when I connect with nullmodem and send ROS package with Xmodem to router and after that reboot and router starts up with new ROS?
I had to do this a couple of times since the ether ports are dead thus upgrade over ether net was not possible.

(I hoped the eth ports came back after a new install. It did in one instance, two others not....)

hi...

would you mind to share on this?

Well, connect to the router with a nulmodem cable and start a telnet session to it. Like you do when you do a netinstall.
Then, in the booter menu (press before 2 secs) you can opt for an upgrade. Here you can choose between upgrade over ether (by netinstall) or via serial. Choose this last. The console shows now the routerboard is waiting for the server to receive the file and one of the manu options in telnet is to send a file. Pick the package you want to send to the router. A "system" package takes about half an hour to transfer this way but it works. After that you reboot (power cycle) the router and the new package gets installed and rb boots from it.

This is done out of my head, so some procedures or commands might be a bit different.

Probably you had uploaded packages to your router previously and forgot about it.

From bootloader you can upgrade only firmware (.fwf files). When file is uploaded via xmodem, bootloader checks if it is fwf file, if not then file is ignored. If it is fwf file then it is written in bootloaders partition which is too small for system package.