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Losing PPP when send SMS with CMUX | Telit Cinterion IoT Developer Community

February 26, 2020 - 4:09pm, 1750 views

I am talking to a PLS62-W over the UART interface at 921600 bits/s.

I can set up a PPP interface and it seems reliable. But I would like to receive and send SMS at the same time. The way forward seem to be to use CMUX mode, and the PPP link seems reliable on a MUX channel.

But often when I then send an SMS on another MUX channel, shortly afterwards I get a Modem Status Command on the MUX control channel saying that the PPP-channel's V24 octet has changed to 0x06 (FC asserted, CTS dropped). Shortly afterwards it returns to 0x0c (FC negated; DSR and CTS set).

Then I get a UIH message on the PPP channel that my PPP stack interprets as "LCP Terminated by Peer". And the PPP link is dropped. I observe that the TX activity pin stays low from here on (and I find it difficult to reset the PLS62 to restore normal operation).

What information should I capture to help identify whether this is a bug in my code, something in the PLS62 module, or something in the network? I should point out that the SMS itself does get through as intended, but it seems a coincidence that I lose PPP by sending a text.

I expect many most people who want to keep a PPP link up at the same time as sending an SMS do so over the USB interface. That is not an option for me.