L
LE920 Hardware User Guide
1vv0301026 Rev.0 - draft4 – 2013-05-
Reproduction forbidden without written authorization from Telit Communications S.p.A. - All Rights Reserved.
Page 52 of 76
9.2.
Modem Serial Port 2
Serial port 2 on the LE920 is a +1.8V UART with only the RX and TX signals.
The signals of the LE920 serial port are:
PAD
Signal
I/O
Function
Type
COMMENT
AB19
TXD_AUX O
Auxiliary UART (TX Data to DTE) 1.8V
AD19
RXD_AUX I
Auxiliary UART (RX Data to DTE) 1.8V
NOTE:
In order to avoid a back powering effect it is recommended to avoid having any HIGH logic
level signal applied to the digital pins of the module when is powered OFF or during an
ON/OFF transition.
9.3.
RS232 Level Translation
In order to interface the Telit LE920 with a PC com port or a RS232 (EIA/TIA-232)
application a level translator is required. This level translator must:
x
Invert the electrical signal in both directions;
x
Change the level from 0/1.8V to +15/-15V.
Actually, the RS232 UART 16450, 16550, 16650 & 16750 chipsets accept signals with lower
levels on the RS232 side (EIA/TIA-562), allowing a lower voltage-multiplying ratio on the
level translator. Note that the negative signal voltage must be less than 0V and hence some
sort of level translation is always required.
The simplest way to translate the levels and invert the signal is by using a single chip level
translator. There are a multitude of them, differing in the number of drivers and receivers and
in the levels (be sure to get a true RS232 level translator not a RS485 or other standards).
By convention the driver is the level translator from the 0-1.8V UART to the RS232 level.
The receiver is the translator from the RS232 level to 0-1.8V UART.
In order to translate the whole set of control lines of the UART you will need:
x
5 drivers
x
3 receivers
NOTE:
The digital input lines working at 1.8V CMOS have an absolute maximum input voltage of
2.7V; therefore the level translator IC shall not be powered by the +3.8V supply of the
module. Instead, it must be powered from a +1.8V (dedicated) power supply.
This is because in this way the level translator IC outputs on the module side (i.e. LE920
inputs) will work at +3.8V interface levels, damaging the module inputs.
