How to handle a modem that stops responding...

Jessy Exum jessy.diamondman at
Wed Oct 28 00:18:22 UTC 2020

Hello everyone,

I am working with some Cinterion modems (PLAS9) and have seen them lock up
completely (requiring a hard reset as the AT channel does not even accept
commands anymore).

The reason I am posting about this here is that ModemManager doesn't
realize the modem is gone. Every message sent to the modem will fail to
send or timeout, but ModemManager will continue showing the last state of
the modem before it stopped responding.

I am happy to do the work to make ModemManager respond more sanely to this
situation, but I want to make sure that whatever I do follows agreed upon
best practices, and that I don't introduce too strange behavior for a
single family of modems. If the agreed upon fix can be done to some of the
modem base classes, that would be even better (consistency).

My initial thought is detecting if the AT Channel is unreachable for a
time, to drop and re-enumerate the device. If there is a failure to
reenumerate, that at least shows that the modem is not just sitting there
in a connected state, or whatever it was before the crash.

I would love to hear what everyone thinks about this problem, what people
think we should about it, etc.

Thank you for your time,
Jessy Diamond Exum
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