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SIM900 GSM/GPRS RS232 Modem - User Manual
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Page 8
2.0 Serial Interfaces
Name
Pin
Function
Seriel Port DB9
GND
5
Ground
CTS
8
Clear to send
RTS
7
Request to send
TXD
2
Transmit data
RXD
3
Receive data
The GSM module is designed as a DCE (Data Communication Equipment), following the traditional DCE-DTE (Data
Terminal Equipment) connection. The GSM Modem and the client (DTE) are connected through the following
signal (as following figure shows). Auto bauding supports baud rate from 1200bps to 57600bps.
Serial port
•
TXD: Send data to the RXD signal line of the DTE
•
RXD: Receive data from the TXD signal line of the DTE
serial port of the GSM engine supports auto bauding for the following baud rates: 1200, 2400, 4800, 9600,
19200,38400 and 57600bps. Factory setting is auto bauding 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 auto
bauding mode, specific attention should be paid to the following requirements:
Synchronization between DTE and DCE:
When DCE powers on with the auto bauding 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”.