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22
GPIO5
I/O
1.8
V
On
0
V
Off
This
I/O
line
goes
directly
to
the
Huawei
GPIO5
line.
See
Huawei
Hardware
Guide.
23
RESIN_IN_NOT
I
1.8V
to
7V
High
0V
to
.6V
Low
This
signal
reaches
the
Huawei
609
through
an
inverter.
See
Huawei
Hardware
Guide.
24
VIN_ENABLE
I
Normally
enabled,
since
resistor
pullup
to
VIN.
Low
disables.
Consider
using
an
N
‐
channel
MOSFET’s
output
to
disable
VIN.
25
USB
D
‐
I/O
‐
This
connects
to
the
Huawei
module.
26
GROUND
27
DTR
I
1.8V
to
25V
High
‐
25V
to
.6V
Low
High
means
Data
Terminal
Ready
28
TXD
I
1.8V
to
25V
High
‐
25V
to
.6V
Low
This
is
the
RS
‐
232
Transmit
Data
line.
29
RXD
O
High
impedance
for
High.
0V
for
Low
This
logic
‐
level
signal
is
the
output
of
an
open
‐
drain
N
‐
channel
MOSFET.
Pull
this
up
to
your
logic’s
plus
power
supply.
30
DCD#
O
0
V
On
1.8
V
Off
0
V
means
Data
Carrier
Detect
Serial Port Lines
The 8 serial port lines typically interface to logic.
The header’s
RXD
line is the Huawei module’s
TX
after it goes through an open-drain
Mosfet N-channel inverter. Equipment using this line should pull the
RXD
line up to the
equipment’s logic supply. When no data is flowing, the
RXD
line is at 0 volts. The start
bit causes the Mosfet to go to a no-current state, so the pullup resistor raises the input
to the supply voltage of whatever is receiving the data from the modem.
The other 4 outputs (
CTS#
,
DSR#
,
DCD#
,
RING#
) come directly out of the Huawei
module and must be inverted to logic levels, NOT the RS-232 levels found on the DE-9.
The outputs go between 0 and 1.8 volts nominal. When level conversion is needed,
consider using an N-channel Mosfet circuit.
47