i have this weired problem connecting an arduino uno to sim900 via softwareserial.
if i hook up rx and tx of the sim900 to the hardware serial of arduino (pin0,pin1)and i wire reset of the board to gnd , and use Hterm to communicate to sim900 : averything is good , i can make calls send sms , receive sms and feedback from sim900.
now , if i hook up sim900 to softwareserial , let say 9 and 10 ,and i use a code to send AT commands to sim900 via softwareserial ,let say : ATD06xxxxxxxx; , the first commands execute fine , and i have a feedback , but the second one is bad .
here is an exemple :
assume i send ATD06xxxxxxxx;
(i hang up i receive this instead of BUSY)
i have to power off/on the sim900 to accept communication , and it seem that the second command get scrambled before it is send to the sim900.
now if i use arduino to communicate to the sim900 via hardware serial ( usb unplugged) it work fine , i can execute as many AT commands as i want.but i can not debug via serial.print
i use arduino 1.8.9