GE865 Hardware User Guide
1vv0300799 Rev.9 – 27-07-2009
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 39 of 70
9.2 RS232 level translation
In order to interface the GE865 with a PC com port or a RS232 (EIA/TIA-232)
application a level translator is required. This level translator must:
•
invert the electrical signal in both directions;
•
change the level from 0/2.8V 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 0-2.8V UART to the RS232
level. The receiver is the translator from the RS232 level to 0-2.8V UART.
In order to translate the whole set of control lines of the UART you will need:
•
5 drivers
•
3 receivers
NOTE:
The digital input lines working at 2.8V CMOS have an absolute maximum input voltage
of 3.0V; therefore the level translator IC shall not be powered by the +3.8V supply of
the module. Instead, it must be powered from a +2.7V / +2.9V (dedicated) power
supply.
This is because in this way the level translator IC outputs on the module side (i.e.
GE865 inputs) will work at +3.8V interface levels, damaging the module inputs.
