Function Of Serial Port And Debug Port - SimCom SIM900D Hardware Design

If only RXD and TXD are used in user's application, other serial pins should be kept open. Please refer to
following figure.
Note: if sleep mode is need in this situation, the user need to connect the DTR signal as well, or only sleep mode2 can be used.
For details, please refer to document [7].
4.6.1

Function of Serial Port and Debug Port

Serial port:
Full modem device.
Contains data lines TXD and RXD, hardware flow control lines RTS and CTS, status lines DTR, DCD and
RI.
Serial port can be used for CSD FAX, GPRS service and AT communication. It can also be used for
multiplexing function. For details about multiplexing function, please refer to document [7].
Serial port supports the following baud rates:
1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200bps
Autobauding only supports the following baud rates:
1200, 2400, 4800, 9600, 19200, 38400 and 57600bps
The default setting is autobauding.
Autobauding allows SIM900D to automatically detect the baud rate of the host device. Pay more attention to the
following requirements:
Synchronization between DTE and DCE:
When DCE powers on with autobauding enabled, user must firstly send character "A" to synchronize the
baud rate. It is recommended to send "AT" until DTE receives the "OK" response, which means DTE and
DCE are correctly synchronized. For more information please refer to the AT command "AT+IPR".
Restrictions of autobauding operation:
The DTE serial port must be set at 8 data bits, no parity and 1 stop bit.
The URC such as "RDY", "+CFUN: 1" and "+CPIN: READY" will not be reported.
Note: User can use AT command "AT+IPR=x" to set a fixed baud rate and the setting will be saved to non-volatile flash
memory automatically. After the configuration is set as fixed baud rate, the URC such as "RDY", "+CFUN: 1" and "+CPIN:
READY" will be reported when SIM900D is powered on.
Debug port:
SIM900D_Hardware Design_V2.02
MODULE (DCE)
Serial port
TXD
RXD
RTS
CTS
DTR
DCD
RI
Debug port
DBG_TX
DBG_RX
Figure 22: Connection of RXD and TXD only
CUSTOMER (DTE)
Serial port1
TXD
RXD
RTS
CTS
DTR
DCD
RING
Serial port2
TXD
RXD
30
Smart Machine Smart Decision
2012-05-07