The onboard two set power supply interface VCC5 5 V power supply, VCC4 interface, 3.5-4.5 V power supply, optional power on self start (default) and control start.The onboard SMA (standard) and IPXmini antenna interface, SIM900A interface reserved reset.The size of the module is 49 * 50. The computer can give early computer debugging USB module power, a very large amount of data under the condition of the recommended current more than 1A. Standby dozens of MA data can be set to provide dormancy, dormancy of 10MA low power. Support 2, mobile phone 3,4G card. The serial port circuit: support for 3.3 V single chip microcomputer. TTL serial port support.3.3 and 5 V single chip microcomputer.The sim card circuit increase the SMF05C ESD chip.Antenna circuit: guarantee short and straight, in order to improve the signal strength. PCB screen printing mark: any interface, convenient development twice, the SIM900 / ONE hardware is fully follow the design when the design manual.Two power supply interface: VCC5, 5 V DC above 1A. Computer 5 V power supply can be early computer USB. DC long data circuit over larger recommended 5V1A. VCC4, 3.5-4.5 V power supply, ibid., Suitable for lithium battery. Control pin all leads.ONE ttl level, compatible with 3.3 V and 5 V.The two antenna interface, the standard SMA straight head, connector for IPXmini antenna.A way of speech interface, the way Mike interface. The control interface of each pin description: GND-GND SIMR SIM900A RXD, TTL level, cannot be connected directly to the 232 level SIMT SIM900A TXD, TTL level, cannot be connected directly to the 232 level RST-SIM900A reset, active low VCC_MCU when the SIM900A module and 5 V TTL level communication, this pin is connected to DC 5 V; When the level of communication of SIM900A and 3.3 V TTL, this pin is connected to DC 3.3 V. VCC5-DC 5V input. VCC4 -DC3.5-4.5 input Onboard Resources: Serial port circuit (with protection) Antenna interface circuit (SMA bend port female) SIM card circuit (