User's Manual
Table Of Contents
- Contents
- Version history
- 1 Introduction
- 2 SIM900B Overview
- 3 Application Interface
- 3.1 SIM900B Pin Description
- 3.2 Operating Modes
- 3.3 Power Supply
- 3.4 Power Up and Power Down Scenarios
- 3.5 Power Saving
- 3.6 Summary of State Transitions (except SLEEP mode)
- 3.7 RTC Backup
- 3.8 Serial Interfaces
- 3.9 Audio Interfaces
- 3.10 Buzzer
- 3.11 SIM Card Interface
- 3.12 LCD Display Interface
- 3.13 Keypad Interface
- 3.14 ADC
- 3.15 Behaviors of the RI
- 3.16 Network Status Indication
- 3.17 General Purpose Input Output (GPIO)
- 4 Antenna Interface
- 5 Electrical, Reliability and Radio Characteristics
- 6 Mechanics
SIM900B Hardware Design
TXD
RXD
RTS
CTS
DTR
DCD
RI
TXD
RXD
RTS
CTS
DTR
DCD
RING
MODULE (DCE)
CUSTOMER (DTE)
Serial port1Serial port
Debug port
Serial port2
DBG_RX
DBG_TX
TXD
RXD
Figure 16: Connection
of the serial interfaces
3.8.1 Function of serial port & debug port supporting
Serial port
z Modem device.
z Contains data lines TXD and RXD, State lines RTS and CTS, Control lines DTR, DCD and RI.
z Serial port can be used for CSD FAX, GPRS service and send AT command of controlling module. Also
serial port can be used for multiplexing function. SIM900B supports only basic mode of multiplexing so far.
z Serial port supports the communication rates as following:
1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200bps.
z Autobauding supports baud rates as following:
1200, 2400, 4800, 9600, 19200, 38400, and 57600bps，the default setting is autobauding。
Autobauding allows the GSM engine to automatically detect the baud rate configured in the host application. The
serial port of the GSM engine supports autobauding for the following baud rates: 1200, 2400, 4800, 9600, 19200,
38400, 57600bps. Factory setting is autobauding enabled. This gives you the flexibility to put the GSM engine
into operation no matter what baud rate your host application is configured to. To take advantage of autobauding
mode, specific attention should be paid to the following requirements:
Synchronization between DTE and DCE:
When DCE powers on with the autobauding enabled, user must first send “A” to synchronize the baud rate. It is
recommended to wait 2 to 3 seconds before sending “AT” character. After receiving the “OK” response, DTE
and DCE are correctly synchronized. The more information please refer to the AT command “AT+IPR”.
Restrictions on autobauding operation
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