The SIM900A GSM modem can communicate with controlling devices through UART, I2C, or SPI protocols, providing flexible interface options for different applications. This cellular module functions as a complete mobile equipment unit with its own IMEI number, capable of handling
Wiring RS-485 and Using MAX485 with Arduino for Long-Distance Communication π

RS-485 is a versatile serial communication standard that enables reliable data transmission over long distances (up to 1,200 meters) and in electrically noisy environments. Unlike common protocols like UART (which uses TX/RX pins), RS-485 operates on a differential signaling method, transmitting data over two wires