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voltage converter;
3G module;
SMA connector for external antenna;
microcontroller (MC);
Mini USB;
RTC module battery;
9-pin RS232 (DB-9);
4-pin terminal connector (GPIO);
SIM1 and SIM2 holder;
an operating SIM selection module (SIM selection);
LEDs;
hard reset and signal strength detection button (Firmware key).
Fig. 2.1
ATM functional diagram