u-blox LARA-R203 System Integration Manual page 106

Providing the TXD and RXD lines only (not using the complete V24 link)
If the functionality of the CTS, RTS, DSR, DCD, RI and DTR lines is not required in the application, or the lines are
not available:

Connect the module RTS input line to GND or to the CTS output line of the module: since the module requires
RTS active (low electrical level) if HW flow-control is enabled (AT&K3, which is the default setting).
 Connect the module DTR input to GND using a 0  series resistor, since it may be useful to set DTR active if
not specifically handled (see the u-blox AT Commands Manual [2], &D, S0, +CSGT, +CNMI AT commands)

Leave the DSR, DCD and RI lines of the module floating, with a test-point on DCD
If RS-232 compatible signal levels are needed, the Maxim MAX13234E voltage level translator can be used. This
chip translates voltage levels from 1.8 V (module side) to the RS-232 standard.
If a 1.8 V Application Processor (DTE) is used, the circuit that should be implemented as described in Figure 56:
Application Processor
(1.8V DTE)
TxD
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
Figure 56: UART interface application circuit with partial V.24 link (3-wire) in the DTE/DCE serial communication (1.8 V DTE)
If a 3.0 V Application Processor (DTE) is used, then it is recommended to connect the 1.8 V UART interface of the
module (DCE) by means of appropriate unidirectional voltage translators using the module V_INT output as 1.8 V
supply for the voltage translators on the module side, as described in Figure 57.
Application Processor
(3.0V DTE)
3V0
VCC
TxD
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
Figure 57: UART interface application circuit with partial V.24 link (3-wire) in DTE/DCE serial communication (3.0 V DTE)
UBX-16010573 - R12
0Ω
TP
0Ω TP
0Ω TP
TP
Unidirectional
1V8
Voltage Translator
VCCA VCCB
C1
C2
DIR1
0Ω TP
A1 B1
0Ω TP
A2 B2
DIR2 OE
GND
U1
0Ω
TP
TP
LARA-R2 series - System Integration Manual
LARA-R2 series
(1.8V DCE)
TXD
12
RXD
13
RTS
10
CTS
11
DTR
9
DSR
6
RI
7
DCD
8
GND
LARA-R2 series
(1.8V DCE)
TP
V_INT
4
TXD
12
RXD
13
RTS
10
CTS
11
DTR
9
DSR
6
RI
7
DCD
8
GND
Design-in
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