User's Manual

GE865 Hardware User Guide

1vv0300799 Rev.6 - 04/06/09

8.2 RS232 level translation

In order to interface the Telit 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

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.