Hardware Design V1.01(091226)

SIM900 Hardware Design
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 Min Max Unit
V
IL
0 0.15*VDD_EXT V
V
IH
0.85 *VDD_EXT VDD_EXT V
V
OL
0 0.1 V
V
OH
VDD_EXT -0.1 VDD_EXT V
Figure 20: Connection
of the serial interfaces
SIM900_HD_V1.01 26.12.2009
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