I am having KwikStik K40 device. In that we are communicating with the SIM module(SIM900A) through UART. The SIM module has RS232 port. And we have checked using MAX232 to transmit and receiving the signals. It can transmit and receive when we loop back the TXD and RXD signals in K40. But it fails to work if we connect it to the SIM module. Whether we want to use RS232 protocol or RS485 protocol. If it is RS485 what type of level translator to be used.