I have RB411U with ZTE MF-112 usb 3g modem connected. Everything works fine.
But sometimes modem hangs, i get "could not acquire serial port" messages in log.

"/port print" shows that port became in "inactive" state. It happens in about few hours, randomly. Re-plugging the modem helps. Searching the web about this problem, i found that it is a common problem with RB's and 3g modem's.

Also i found that rebooting RB also helps in that situation, so setting up watchdog was a real solution.

But rebooting a whole router is too slow! I checked that usb power is not switching off during RB reboot, so modem not resetting. So what's happening during reboot, causing modem to work again? Only usb become inactive and active again? Maybe there is a faster way to do this than rebooting the whole RB?

I have RB411U with ZTE MF-112 usb 3g modem connected. Everything works fine.
But sometimes modem hangs, i get "could not acquire serial port" messages in log.

"/port print" shows that port became in "inactive" state. It happens in about few hours, randomly. Re-plugging the modem helps. Searching the web about this problem, i found that it is a common problem with RB's and 3g modem's.

Also i found that rebooting RB also helps in that situation, so setting up watchdog was a real solution.

But rebooting a whole router is too slow! I checked that usb power is not switching off during RB reboot, so modem not resetting. So what's happening during reboot, causing modem to work again? Only usb become inactive and active again? Maybe there is a faster way to do this than rebooting the whole RB?

Try disable/enable instead of reboot. Also, make sure you are using the newest version 5.7, as there have been various fixes recently.

Try disable/enable instead of reboot. Also, make sure you are using the newest version 5.7, as there have been various fixes recently.

Disable and enable what?

I am using 5.2, and will try to upgrade to 5.7, thanks!

Try disable/enable instead of reboot. Also, make sure you are using the newest version 5.7, as there have been various fixes recently.

Disable and enable what?

I am using 5.2, and will try to upgrade to 5.7, thanks!

The modem ppp interface.

The modem ppp interface.

Just upgraded to 5.7. 10 hours uptime, still no "inactive port" problem, will see what happens tomorrow.
Restarting ppp client doesnt help when serial bort became inactive But rebooting do.

Had an simular issue with ZTE MF820D 4G modem. I contacted support and got the following answer from Sergejs...


"Next version will have 3G support for the particular modem. We are working on 4G
support, it will need some time to get 4G working."

RGDS

5.7 still having "could not acquire serial port". It's no wonder because there's no word about USB in the RouterOS changelog from 5.2 till 5.7.
So i set up watchdog to ping google.com