Hi,

again I'm writing because 2G/3G communication is not working as intended. Lets forget about all the USB and miciPCI-e 2G/3G problems that were already heavily documented but not solved. Because of this fact I started to test 2G serial modems. I put together a RB411 with a serial port and disabled the console on serial0. The router connected in an instant and was working for 2 weeks until this night when it disconnected. I have one more router (RB411UAHR) on-site, that uses 3G and is connected to the test router, so I was able to access it. The thing I saw was, I wasn't able to communicate with the modem via serial port. I couldn't enter anything (/system serial-terminal serial0) as if the modem was inaccessible on serial0. The modem uses standard AT commands. I tried to reboot the router to no effect. So I drove to the site and changed the test router and connected it back to the serial modem... and it worked. Be advised I did not power cycle the serial modem, only reconnect it to the other Mikrotik.

serial0:

baud: 115200
parity: none
data bits: 8
stop bit: 1
flow control: none

I am quite confident now, that the problem is in Mikrotik and not compatibily with 3rd pary hardware, because this modem uses standard AT commands and doesn't need any drivers.

I really need help with this one, because I can understand that if you have a 3G router at home it is no problem to reset it once a week, but once you use 300+ routers scattered around EU, then you see problems and patterns emerging.

i cant see the relation between disconnecting and reconnecting the modem, as its a standard serial interface, and rebooting it would serve the same purpose, its not as if the slave modem can have any "sense" if it connected or not, nor is the tik capable of tracking if there is something connected, so besides some floating ground issue that could be creeping up i cant see any relation?

you should be using just the 3 pins on the cable, you can try grounding it better, with pin 5 also to the side of the connector, but this is an extreme long shot if they are on the same DC power supply.

i know this is not directly related to your problem, but do keep this in mind with serial communication, and that
https://www.scantool.net/forum/index.php?topic=731.0

Thank you for suggestions CyberT. I will try and report back. Grounding could be a problem.