GSM/GPRS Module Series
M85 Hardware Design
M85_Hardware_Design Confidential / Released 43 / 88
It is highly recommended to add the resistor divider circuit on the UART signal lines when the host’s level
is 3V or 3.3V. For the higher voltage level system, a level shifter IC could be used between the host and
the module. For more details about UART circuit design, please refer to
document [11]
.
The following figure shows a sketch map between module and standard RS-232 interface. Since the
electrical level of module is 2.8V, so a RS-232 level shifter must be used. Note that you should assure the
IO voltage of level shifter which connects to module is 2.8V.
TXD
RXD
RTS
CTS
DTR
RI
DCD
Module
GND
C1+
C1-
C2+
C2-
V+
VCC
GND
V-
3.3V
T1IN
T2IN
T3IN
T4IN
R1IN
R2IN
R3IN
R1OUT
R2OUT
R3OUT
T1OUT
T2OUT
T5OUT
T3OUT
T4OUT
T5IN
GND
GND
/R1OUT
1
2
3
4
5
6
7
8
9
GND
To PC Serial Port
GND
1K
1K
1K
1K
1K
5.6K
5.6K
1K
1K
5.6K
RS-232 Level Shifter
Figure 24: Sketch Map for RS-232 Interface Match
Please visit vendor website to select the suitable RS-232 level shifter IC, such as: http://www.exar.com/
and http://www.maximintegrated.com.
NOTE