GSM AT command response in between downlink data

Question

I have a weird problem that I can't figure out in SIMCOM900A, I am receiving downlink data of about 1080 bytes every 5 seconds, meanwhile doing other things like sending data to server. However, sometimes AT command response comes in between downlink data. Like below:- @@ to ## is my downlink packet but I receive AT+CSQ response in between!

@@10500000110483&A8813&B182&C1027&D~OTA_S|True|InProcess|182|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

+CSQ: 10,0

OK

OQsf0M+kYo3xJFvKF65jI8rWMbrlW1y3XnYVu6WfS~##

Any suggestions as to why this is happening or how to fix it are welcome! Thanks in advance.

Answer 1

This is a configurational issue. Basically with SIMCOM900A you are limited to two serial interfaces if my memory serves me right. One for AT commands and modem control, the second for debugging.

As you have setup a data connection via the AT command interface it is effectively sharing the interface for incoming/outgoing data communication and modem control commands. What you are seeing with the +CSQ messages are unsolicited (URC) result codes.

You basically have two options:

1) Make use of multiplexing via GSM 07.10 which your modem supports. These then connects to your modem and configures using AT+CMUX so that you will have effectively two ways of connecting to the same serial device.

Thereafter you can use one for managing the modem and sending/receiving AT commands and responses. And the second for receiving/sending pure data (you can also execute AT commands in this channel but there are limitations).

2) Modify your host program/script to cope with this situation and make decisions on what to do based on whether it is data or AT command responses / unsolicited response codes.

NOTE: A word of warning regarding multiplexing. By multiplexing you will end up degrading the performance of communication between the modem and host.

NOTE: It makes real sense always to have a good read through of your modems manual regarding interfacing and AT commands. Manufacturers are renowned for implementing slightly differently compared to the GSM specs :)