##Problem description The serial port __/dev/ttyS1__ (used on the Terra-M to link the GPRS modem) doesn't work using the A4 microSD with Kernel 3.11.6: ##Solution Download and save the [at91-ariag25.dtb](http://terzo.acmesystems.it/download/microsd/A4/kernel/at91-ariag25.dtb) file in the first microSD partition and reboot. This error is due to a wrong device tree used on the first microSD with Kernel 3.11. Here is the right definition: [at91-ariag25.dts](http://terzo.acmesystems.it/download/microsd/A4/at91-ariag25.dts)