ModemManager fails to utilize Modem, when it is not ready yet

Christian Schneider cschneider at
Tue Apr 4 15:41:47 UTC 2023

Hi, I have a SIMCOM_SIM7600E-H modem.

When the Modem is freshly "activated" (eg. after powering on the whole 
device, toggling the MPCIE reset pin, unplugging/plugging the MPCIE 
card), ModemManager is not able to utilize the modem, it ends in state
   Status   |             state: failed
            |     failed reason: sim-missing
            |       power state: on

at this point, when I restart ModemManager (I don't do anything with the 
modem itself), it works, the modem is usable, calls, sms, internet 
connection work.

I've looked at the debug output of ModemManager and found this: In the 
"bad" case, some of the AT commands fail, where the final straw for 
ModemManager seems to be "AT+CPIN?" that is answered with "+CME ERROR: 
10" which is interpreted as "SIM not inserted" by ModemManager.
In the "good" case the answer to "AT+CPIN?" is "+CPIN: READY" and 
ModemManager is happy.

Using the AT interface manually, I can notice this behavior:
Directly after starting the modem, the answer to "AT+CPIN?" is "+CME 
ERROR: 10", only after some 30 seconds, the answer is "+CPIN: READY"
I don't now the standards good enough, to judge if such behavior is 
allowed, but I'm pretty sure, that this behavior is the problem for 
ModemManager. When the answer to "AT+CPIN?" is "+CME ERROR: 10" once, it 
marks the modem as failed without retrying.

I attach a complete log of ModemManager Debug output. ModemManager 
starts, when the device is "hot", eg running for a while, so everything 
is working. When the device is fully recoginzed by ModemManager, I 
toggle the Reset pin of the modem which causes the restart of the modem, 
the device disappeares from ModemManager, and after a while, when the 
/dev/ttyUSBx devices appear again, ModemManager starts talking to the 
modem, but get's errors, as it is too "early".

As I said, I don't know, if this behavior of the modem is acceptable 
from the standard. Can someone comment about the behavior of the modem? 
Should I get to the manufacturer, and tell them, they do something wrong 
(and hope they fix it) or should ModemManager handle such a situation, 
eg by retrying?

BR, Christian

Newtonstr. 18
12489 Berlin


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