Integration Manual

SARA-R4/N4 series - System Integration Manual
UBX-16029218 - R11 Design-in Page 102 of 157
Providing the TXD and RXD lines only
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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, then:
 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 line to GND using a 0  series resistor, because it is useful to set DTR
active if not specifically handled, in particular to have URCs presented over the UART interface (see
SARA-R4/N4 series AT Commands Manual [1], &D, S0, +CNMI AT commands)
 Leave 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 in Figure 46.
TxD
Application Processor
(1.8V DTE)
RxD
RTS
CTS
DTR
DSR
RI
DCD
GND
SARA-R4/N4
(1.8V DCE)
12
TXD
9
DTR
13
RXD
10
RTS
11
CTS
6
DSR
7
RI
8
DCD
GND
0Ω
TP
0Ω
TP
0Ω
TP
TP
Figure 46: UART interface application circuit with a 3-wire link in the DTE/DCE serial communication (1.8V 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 in Figure 47.
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Flow control is not supported by ‘00’, ‘01’ and SARA-R410M-02B product versions, but the RTS input must be set low to use the UART on
‘00’ and ‘01’ versions. The DTR input must be set low to have URCs presented over UART on ‘00’, ‘01’ and ‘x2’ product versions.