u-blox LARA-R203 System Integration Manual page 104

Providing the TXD, RXD, RTS, CTS and DTR lines only (not using the complete V.24 link)
If the functionality of the DSR, DCD and RI lines is not required, or the lines are not available:

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, two different external voltage translators (e.g. Maxim MAX3237E
and Texas Instruments SN74AVC4T774) can be used. The Texas Instruments chips provide the translation from 1.8
V to 3.3 V, while the Maxim chip provides the translation from 3.3 V to RS-232 compatible signal level.
Figure 52 illustrates the circuit that should be implemented as if a 1.8 V Application Processor (DTE) is used, given
that the DTE will behave properly regardless of the DSR input setting.
Application Processor
(1.8V DTE)
TxD
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
Figure 52: UART interface application circuit with partial V.24 link (6-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 a
1.8 V supply for the voltage translators on the module side, as described in Figure 53, given that the DTE will
behave properly regardless of the DSR input setting.
Application Processor
(3.0V DTE)
3V0
VCC
TxD
RxD
RTS
CTS
3V0
DTR
DSR
RI
DCD
GND
Figure 53: UART interface application circuit with partial V.24 link (6-wire) in DTE/DCE serial communication (3.0 V DTE)
Reference Description
C1, C2, C3, C4 100 nF Capacitor Ceramic X7R 0402 10% 16 V
U1 Unidirectional Voltage Translator
U2 Unidirectional Voltage Translator
Table 41: Component for UART application circuit with partial V.24 link (6-wire) in DTE/DCE serial communication (3.0 V DTE)
20 Voltage translator providing partial power down feature so that the DTE 3.0 V supply can be also ramped up before V_INT 1.8 V supply
UBX-16010573 - R12
TP
0 Ω
0 Ω TP
TP
0 Ω
0 Ω TP
Unidirectional
1V8
Voltage Translator
VCCA VCCB
C1
C2
DIR1
DIR3
TP
0 Ω
A1 B1
0 Ω TP
A2 B2
TP
0 Ω
A3 B3
TP
0 Ω
A4 B4
DIR2 OE
DIR4 GND
U1
Unidirectional
1V8
Voltage Translator
VCCA VCCB
C3 C4
DIR2
DIR1
A1 B1
A2 B2
OE
GND
U2
LARA-R2 series - System Integration Manual
LARA-R2 series
(1.8V DCE)
TXD
15
16 RXD
13 RTS
14 CTS
12 DTR
9 DSR
10 RI
11 DCD
GND
LARA-R2 series
(1.8V DCE)
V_INT
4
TXD
15
RXD
16
RTS
13
CTS
14
DTR
12
DSR
9
10 RI
DCD
11
GND
Part Number - Manufacturer
GRM155R61A104KA01 - Murata
SN74AVC4T774
SN74AVC2T245
20
- Texas Instruments
20
- Texas Instruments
Page 104 of 157
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