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I'm using a SIM900A GSM Shield to communicate between an arduino due and an API. I currently using it's default values in the multiplexer of GSM Shield. But now I'm in need of changing it's frame size to 255. When I check the current frame size, it gives following response which indicates the frame size as 127.

CMUX Read:AT+CMUX?

+CMUX: 0,0,5,127,10,3,30,10,2
OK

Then I used following AT command to change it to 255. But it gives an ERROR.

sim900_send_cmd("AT+CMUX=0[,0[,1[,255[,10[,3[,30[,10[,2]]]]]]]]\r\n");

CMUX Read:AT+CMUX=0[,0[,5[,255[,10[,3[,30[,10[,2]]]]]]]] 
ERROR

What am I doing wrong here ? am I missing a step ? Any insight will be much appreciated. Thank you

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    You should not terminate the AT command line with `\r\n`, only just `\r` is valid. Do yourself a favour and read all of chapter 5 in [V.250](http://www.itu.int/rec/T-REC-V.250-200307-I/en), that will teach you a lot about basic AT command handling. – hlovdal Aug 19 '15 at 05:50
  • I didnt know that, thank You –  Aug 19 '15 at 09:49

1 Answers1

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I'm not an expert in AT commands, but I bet you don't need all the brackets in yours. Brackets are used to indicate parameters which could be omitted. So your command should look like this:

CMUX Read:AT+CMUX=0,0,5,255,10,3,30,10,2

Maybe even a shoter version will work:

CMUX Read:AT+CMUX=0,0,5,255
Dmitry Grigoryev
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