S521 is a Remote terminal unit with analog inputs, digital inputs and HART modem. The device supports optional RS485 interface as well. The device supports B1 / B3 / B5 / B8 / B40 / B41 amongst the 4G/LTE bands, B1 / B2 / B5 / B6 / B8 / B19 amongst the 3G UMTS/HSPA+ bands, 850/ 900/ 1800/ 1900 MHz amongst the 2G EDGE/GSM/GPRS bands. The device comes with 3 dBi Antenna plugs and supports single SIM. The device has a Half-Duplex HART Modem Overlaying 1200bps FSK Digital Communication on Top of installed 4mA–20mA Current Loop Infrastructure. The RTU has 2 numbers of 4-20mA Analog inputs, 2 numbers of Opto Isolated Digital Inputs and has an operating temperature range of – 20 to 70 Deg C. The RTU runs on Yocto, Long-term supported Linux Kernel (LTSI).