Smart Machine Smart Decision MODULE( DCE) CUSTOMER (DTE) Serial port Serial port TXD TXD RXD RTS CTS UART _ DTR UART _ DCD RXD RTS CTS DTR DCD UART1_RI RING Figure 19: Connection of the serial interfaces If the voltage of UART is 3.3V, the following reference circuits are recommended. If the voltage is 3.0V, please change the resistors in the following figure from 5.6K to 14K. Figure 20: Resistor matching circuit If the voltage of UART is 3V or3.3V, the following reference circuits are recommended: SIM800L_Hardware_Design_V1.00 31 2013-08-20