SIM900 Hardware Design
3.8 Serial Interfaces
Table 7: Pin definition of the serial interfaces
Name
Pin
Function
DTR
3
Data terminal ready
RI
4
Ring indicator
DCD
5
Data carrier detection
DSR
6
Serial port
CTS
7
Date set ready
Clear to send
RTS
8
Request to send
TXD
9
Transmit data
RXD
10
Receive data
Debug port
DBG_RXD 28
DBG_TXD 27
Receive data
Transmit data
SIM900A 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 115200bps.
Serial port
z TXD: Send data to the RXD signal line of the DTE
z RXD: Receive data from the TXD signal line of the DTE
Debug port
z DBG_TXD: Send data to the RXD signal line of the DTE
z DBG_RXD: Receive data from the TXD signal line of the DTE
The logic levels are described in following table.
Table 8: Logic levels of the serial port and debug port
Parameter
VIL
VIH
VOL
VOH
Min
Max
Unit
0
0.15*VDD_EXT
V
0.85 *VDD_EXT
VDD_EXT
V
0
0.1
V
VDD_EXT -0.1
VDD_EXT
V
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26.12.2009