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SIM800L not answering CSD call
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SIM800L not answering CSD call

by Fuzzcraft on Wed May 16, 2018 9:28 am

This is not directly FONA related, but this seems to be a large-ish forum with experienced people. I'm developing a new version of a cellular modem that used to employ a SIM900B module. The updated design is for a SIM800L (or H) module, like the FONA. The modem uses circuit switched data (CSD) with an appropriate SIM to handle incoming M2M calls. This has worked flawlessly over the course of 18 years and several hardware iterations of the device. Not so with the SIM800L. It does everything I want it to do, except auto-answer an incoming data call. It auto-answers a voice call, but never a CSD call. The specifications say it supports CSD.

I'm scratching my head. I know CSD is an ancient protocol, meant as a stop gap measure in the early days of mobile communications, and very few people nowadays use it. It is, however, still being supported by service providers, and it's specified as being supported in the SIM800L.

Does anybody know a way to get this working?

Fuzzcraft
 
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Re: SIM800L not answering CSD call

by adafruit_support_mike on Wed May 16, 2018 11:50 pm

Try using AT+GSMBUSY=2

That disables incoming voice calls but allows CSD calls.

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Re: SIM800L not answering CSD call

by Fuzzcraft on Thu May 17, 2018 4:29 am

Thank you for your reply, and for allowing this question.

I tried AT+GSMBUSY=2 (hadn't found this in the AT manual yet), but unfortunately no luck.

Also tried AT+CSNS=4, which seems to be related to CSD as well.

The SIM800L responds, draws a few dozen mA for a few seconds, then falls back asleep (less than 1 mA), without answering. A voice call simply connects immediately, but obviously no data transfer is possible, only silence.

What I find strange is that, although listed in the AT command spec, AT+CBST (bearer service type) isn't supported, and always returns ERROR. This is for outgoing data calls, but still I find it worrying. Would it be possible that SIMCOM simply omitted the functionality and failed to reflect this in the specs and documentation? It wouldn't be the first time that docs aren't accurate.

Fuzzcraft
 
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Re: SIM800L not answering CSD call

by adafruit_support_mike on Fri May 18, 2018 12:23 am

That's possible. The datasheets and command reference are terse at best, and don't describe the larger concepts of working with the device at all. It's all 'enter this command, get something from this list of responses.' There also seems to be a healthy dose of Your Mileage May Vary.

We haven't worked with CSD, so I'm afraid we don't have any insight into that particular feature.

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Re: SIM800L not answering CSD call

by Fuzzcraft on Fri May 18, 2018 4:25 am

Thank you for your time, Mike.

CSD is hardly being used anymore, but I'm stuck with a legacy system.

Trying to talk directly to SIMCOM about this seems like a waste of time. I'm going to look for a different module.

Fuzzcraft
 
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Joined: Wed May 16, 2018 9:03 am

Please be positive and constructive with your questions and comments.