Serial Interfaces - SIM Tech SIM900D HD Manual

SIM900D Hardware Design

3.9 Serial Interfaces

Table10: Pin definition of the serial interfaces
Name
DTR
RI
DCD
Serial port
CTS
RTS
TXD
RXD
DBG_RXD
Debug port
DBG_TXD
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
TXD: Transmit Data
RXD: Receive Data
Debug port
DBG_TXD: Transmit Data
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
V
IL
V
IH
V
OL
V
OH
SIM900D_HD_V1.02
Pin Function
43 Data terminal ready
11 Ring indicator
42 Data carrier detection
45 Clear to send
44 Request to send
4 Transmit data
3 Receive data
1 Receive data
2 Transmit data
Min
0
2.4
0
2.7
Max
0.4
3
0.1
3
36
Unit
V
V
V
V
09.04.2010