www.rhydolabz.com
SIM900 GSM/GPRS TTL UART Modem - User Manual
W
e
b
r
i
n
g
t
h
e
w
o
r
l
d
t
o
y
o
u
…
…
.
.
Page 13
AT+CMGL=”REC UNREAD”
Read all received unread messages
Debug port
•
Null modem port
•
Only contain Data lines TXD and RXD
•
Debug Port used for debugging and upgrading firmware. It cannot be used for CSD call, FAX call. And the
Debug port can not use multiplexing function. It does not support autobauding function.
•
Debug port supports the communication rates is 115200bps
2.1 Software Upgrade and Software Debug
The DBG_TXD, DBG_RXD and GND must be connected to the IO connector when user need to upgrade software
and debug software, the DBG_TXD, DBG_RXD should be used for software upgrade and for software debugging.
The TXD and RXD also should be connected to the IO connector, if user wants to send AT command or data stream
to GSM MODEM. The PWRKEY pin is recommended to connect to the IO connector. The user also can add a switch
between the PWRKEY and the GND. The PWRKEY should be connected to the GND when GSM MODEM is
upgrading software. Please refer to the following figure.
1K
Serial port I/O Connector
GSM MODEM(DCE)
DBG_TXD
DBG_RXD
GND
PWRKEY
TXD
RXD
GND
PWRKEY
