User's Manual

GE864- QUAD Har dwar e User Guide

1vv0300872 Rev.1 2010-04-02

9.2. RS232 Level Translation

In order to inter face the Telit GE864 with a PC com port or a RS232 (EIA/TIA-232)

application a level translator is required. This level translator must

invert the electr ical signal in both directions

change the level from 0/3V 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 for 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, differ ing in the number of driver and

receiver and in the levels (be sur e to get a true RS232 level translator not a RS485 or

other standards).

By convention the dr iver is the level translator from the 0-3V UART level to the RS232

level, while the receiver is the translator from RS232 level to 0-3V UART.

In order to translate the whole set of control lines of the UART you will need:

5 driver

3 receiver

NOTE:

The digital input lines working at 2.8VCMOS have an absolute maximum input voltage

of 3,75V; therefor e the level translator IC shall not be power ed by the +3.8V supply of

the module. Instead it shall be powered from a +2.8V / +3.0V (dedicated) power

supply.

This is because in this way the level translator IC outputs on the module side (i.e.

GE864 inputs) will wor k at +3.8V inter face levels, stressing the module inputs at its

maximum input voltage.

This can be acceptable for evaluation pur poses, but not on production devices.

NOTE:

In order to be able to do in circuit repr ogramming of the GE864 firmware, the ser ial

por t on the Telit GE864 shall be available for translation into RS232 and either it is

contr olling device shall be placed into tristate, disconnected or as a gateway for the

ser ial data when module repr ogramming occurs.

Only RXD, TXD, GND and the On/off module turn on pad are r equired to the

repr ogramming of the module, the other lines are unused.

All applicator shall include in their design such a way of reprogr amming the GE864.