SIM900D Hardware Design
3.9 Serial Interfaces
Table10: Pin definition of the serial interfaces
Name
Pin
Function
DTR
43
Data terminal ready
RI
11
Ring indicator
DCD
42
Data carrier detection
CTS
45
Clear to send
RTS
44
Request to send
TXD
4
Transmit data
Serial port
RXD
3
Receive data
DBG_RXD
1
Receive data
Debug port
DBG_TXD
2
Transmit data
SIM900D provides two unbalanced asynchronous serial ports. One is the serial port and the other is the debug
port. The GSM module is designed as a DCE (Data Communication Equipment), following the traditional
DCE-DTE (Data Terminal Equipment) connection. The module and the client (DTE) are connected through the
following signal (as following figure shows). Autobauding supports baud rate from 1200bps to 57600bps.
When the RXD, TXD, DBG_RXD, DBU_TXD are used, it should be pull up 10k to 3V.
Serial port
z
TXD:
Transmit Data
z
RXD: Receive Data
Debug port
z
DBG_TXD: Transmit Data
z
DBG_RXD: Receive Data
The logic levels are described in following table.
NOTE: The RXD should be pulled up to 3V through a 10k resistor, if unused.
Table11: Logic levels of the serial port and debug port
Parameter
Min
Max
Unit
V
IL
0
0.4
V
V
IH
2.4
3
V
V
OL
0
0.1
V
V
OH
2.7
3
V
SIM900D_HD_V1.02
09.04.2010
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