Issue with DF Robot SIM908 chip

Hi All -

I'm building a GPS tracker that can log data coordinates using an Arduino MEGA 2560 and a GPS/GPRS/GSM shield from DFRobot.

http://www.dfrobot.com/index.php?route=product/product&filter_name=GPS&product_id=673#.UnkcdvlJMZ4

We are having problems with the board that are hard to reproduce, so I thought I will describe the circumstances and hopefully you can suggest a way to fix it, or a way to gather more information.

RIght now for testing, It's being powered from an AC-to-DC converter at 7.7 volts. We've measured Vin on the shield at 7 volts.

We're using the code hosted here: http://www.cooking-hacks.com/documentation/tutorials/geolocation-tracker-gprs-gps-geoposition-sim908-arduino-raspberry-pi

In the section "Realtime geolocation tracking". We have modified it a bit, but not much.

Most recently, we've been having problems with the section of the code that establishes a GPRS connection to upload the GPS coordinates to a webserver. When the arduino sends this command:

AT+SAPBR=3,1,"Contype","GPRS'

The shield returns the following:

AT+SAPBR=3,1,"Contype","GPRS"

SIM900 R11.0

ERROR

This behavior is intermittent. Sometimes it works, then a day later it doesn't. We can't figure out what the difference is, because we're not changing hardware or software. We also sometimes have problems with these commands:

AT+SAPBR=1,1 AT+HTTPINIT

Sometimes, the failure is because the shield returned "ERROR". Other times, the shield returns nonsense strings (i.e. ÿÿÿÿÿÿÿÿÿÿ).

Any ideas about what could be causing these failures?

Hi! I don't know if you resolved this but, are you connecting the modem in hardware serial or by the software serial library? if you are using the hardware serial, in wich baudrate are you connecting it? I recently had problems setting the baudrate under the 115200 bauds/sec with hardware serial.

Hope it helps.

PD: I'm sorry for my rusty english

Charly